The first descriptions about disturbances in menstrual cycle are found in Papyrus Ebres [named after the Egyptologist Georg M. Ebers (1837-1898)], from New Kingdom period (1450-1550 B.C.). Amenorrhea may be classified according to etiology into three subtypes, including primary amenorrhea, secondary amenorrhea, and functional amenorrhea. Primary amenorrhea is basically refers to a young girl who have not experienced menarche, at all. Secondary amenorrhea reflects a woman who had ordinary menstruation cycles, experiencing at least 3 months of absence of menstruation cycle. Functional amenorrhea is a subtype of the amenorrhea caused by exaggerated different lifestyles. Primary and secondary amenorrhea are distinguished solely based on history. Mainly the pathophysiology of amenorrhea is described in many categories, include hypothalamic, pituitary, thyroid, adrenal, ovarian, uterine, and vaginal pathogenesis. About 25 various genes, in 3 different group of Kallmann syndrome related genes, hypothalamus–pituitary–gonadal (HPG) axis related genes, and obesity related genes, play role in amenorrhea. Common causes of amenorrhea include breastfeeding, pregnancy, menopause, and stress. Primary amenorrhea must be differentiated from other diseases that cause lack of menstrual cycle, such as Mullerian agenesis, 3-beta-hydroxysteroid dehydrogenase type 2 deficiency, androgen insensitivity syndrome, Kallmann syndrome, Turner syndrome, and 17-alpha-hydroxylase deficiency. In contrast, secondary amenorrhea must be differentiated from other diseases that cause menstrual cycle arrest, such as primary ovarian insufficiency, hypothyroidism, hyperprolactinemia, polycystic ovary syndrome, and Asherman’s syndrome. The prevalence of amenorrhea is approximately 3,000 to 4,000 per 100,000 individuals worldwide. The most common risk factor in the development of primary amenorrhea is chromosomal disorder and the most common risk factor in the development of secondary amenorrhea is breastfeeding. If left untreated, all of patients with amenorrhea may progress to develop infertility and osteoporosis. Common complications of amenorrhea are based on the background disease that induced it. Prognosis is generally excellent and the mortality rate of patients with amenorrhea is approximately less than 1%, generally in amenorrhea due to brain lesions. The initial laboratory tests for evaluating amenorrhea are pregnancy test, thyroid stimulating hormone (TSH), follicle stimulating hormone (FSH), and prolactin (PRL). Second line laboratory tests include free and total testosterone, dehydroepiandrosterone sulfate (DHEAS), and also progesterone challenge test. There are no echocardiography findings associated with amenorrhea. However, an echocardiography may be helpful in the diagnosis of the diseases that can cause amenorrhea, such as Turner syndrome. Findings on an echocardiography suggestive of Turner syndrome include bicuspid aortic valve, elongation of transverse aortic arch, coarctation of aorta, and partial anomalous pulmonary venous return (PAPVR). There are no ultrasound findings associated with amenorrhea. However, an ultrasound may be helpful in the diagnosis of the diseases that can cause amenorrhea, such as polycystic ovary syndrome (PCOS), premature ovarian insufficiency, androgen insensitivity syndrome, 17-alpha hydroxylase deficiency, and also anatomic genital defects. Pharmacologic medical therapy is recommended among patients with hypothalamic causes, pituitary causes, ovarian insufficiency, and chronic anovulation. The general principle of the treatment in amenorrhea is sex hormone replacement therapy, with suitable forms of estrogen and progesterone. The mainstay of treatment for amenorrhea is medical therapy. Surgery is usually reserved for patients with either hypothalamus or pituitary tumors, Turner syndrome, and genital anatomical defects (imperforate hymen or transverse vaginal septum). The surgical treatment for hypothalamus or pituitary tumors is tumor resection via endoscopic transsphenoidal surgery. Ovaries have to be excised in Turner syndrome to prevent malignant transformation. Small incision is the main surgery for imperforate hymen and septal excision is the main treatment for transverse vaginal septum.

The ancient Egyptian belief honored “menstrual blood” as life-giving. The Mesopotamian mother goddess, named Ninhursag, is believed to have created mankind from loam and her “blood of life“. The first descriptions about disturbances in menstrual cycle are found in Papyrus Ebres [named after the Egyptologist Georg M. Ebers (1837-1898)], from New Kingdom period (1450-1550 B.C.E). They described the patients as a “women who suffers from the side of her pubic region as an irregularity of her menstruation“. In 1907, British Medical Journal, released an article about different types of treatments (mostly herbal and conservative) for amenorrhea. In 1911, some researchers evaluate the therapeutic methods presented 4 years ago and make some suggestions to manage amenorrhea better. The term amenorrhea is derived from Greek language [a = negative, men = month, rhoia = flow], means lack of menstruation cycle in a woman.

Amenorrhea may be classified according to etiology into three subtypes, including primary amenorrhea, secondary amenorrhea, and functional amenorrhea. Primary amenorrhea is basically refers to a young girl who have not experienced menarche, at all, classified as hypergonadotropic hypogonadism, hypogonadotropic hypogonadism, and eugonadotropic state. Secondary amenorrhea reflects a woman who had normal menstruation cycles, experiencing at least 3 months of absence of menstruation cycle. It is classified as polycystic ovary syndrome, hypothalamic–pituitary dysfunction, hypothalamic–pituitary failure, and ovarian failure. Functional amenorrhea is a subtype of the amenorrhea caused by exaggerated different lifestyles, classified as stress, weight loss, and exercise related groups.

Amenorrhea is defined as absence of menstrual cycle. The pathophysiology of amenorrhea include hypothalamic, pituitary, thyroid, adrenal, ovarian, uterine, and vaginal causes. About 25 different genes are involved in the pathogenesis of amenorrhea such as 3 different groups of Kallmann syndrome related genes, hypothalamus–pituitary–gonadal (HPG) axis related genes, and obesity related genes. On gross pathology, normal endometrium is the characteristic findings of amenorrhea. Patients of amenorrhea from craniopharyngioma have cystic mass filled with motor oil-like fluid on gross pathology. On microscopic histopathological analysis, craniopharyngioma presents as trabecular squamous epithelium surrounded by palisaded columnar epithelium, small-to-medium sized cells with moderate amount of basophilic cytoplasm, bland nuclei, and calcifications. On microscopic histopathological analysis, pituitary adenoma as a cause of amenorrhea presents as loss of fibrous stroma and nested cells of normal anterior pituitary (based on the type of adenoma).

Common causes of amenorrhea are breastfeeding, pregnancy, menopause, and stress. Common causes of primary amenorrhea are craniopharyngioma, idiopathic gonadotropin deficiency, Kallmann’s syndrome, Mayer-Rokitansky-Hauser Syndrome, Mullerian dysgenesis, and outflow tract disorders. Common causes of secondary amenorrhea are craniocerebral trauma, curettage, Cushing’s syndrome, depression, diabetes mellitus, and drug side effects. Common causes of functional amenorrhea are stress, rapid weight loss, and excessive exercise.  

As amenorrhea manifests in a variety of clinical forms, differentiation must be established in accordance with the particular subtype. Primary amenorrhea must be differentiated from other diseases that cause lack of menstrual cycle, such as Mullerian agenesis, 3-beta-hydroxysteroid dehydrogenase type 2 deficiency, androgen insensitivity syndrome, Kallmann syndrome, Turner syndrome, and 17-alpha-hydroxylase deficiency. In contrast, secondary amenorrhea must be differentiated from other diseases that cause menstrual cycle arrest, such as primary ovarian insufficiency, hypothyroidism, hyperprolactinemia, polycystic ovary syndrome, and Asherman’s syndrome.

The incidence of primary amenorrhea is approximately 3,000 cases per 100,000 individuals, mostly due to hypothalamic amenorrhea. The incidence of secondary amenorrhea is approximately 3,300 per 100,000 individuals. The prevalence of amenorrhea is approximately 3,000 to 4,000 per 100,000 individuals worldwide. The prevalence of amenorrhea was estimated to be 13,400 cases per 100,000 female athletes. The mortality rate of amenorrhea is less than 1%, and seen in patients of pituitary macroadenomas or generally brain lesions which cause amenorrhea. Primary amenorrhea is initially diagnosed among adolescents, 16 years of age. There is no racial predilection for amenorrhea. Commonly, females in developed countries experience puberty and menarche earlier than females of developing countries. This can be attributed to nutritional and socioeconomic situation but since the age of diagnosis of primary amenorrhea is based on the society’s mean age of puberty onset and menarche, there is not any difference between developing and developed countries in terms of prevalence of amenorrhea.

The most common risk factor in the development of primary amenorrhea is chromosomal disorder and the most common risk factor in the development of secondary amenorrhea is breastfeeding. Common risk factors in the development of amenorrhea include risk factors related to hypothalamus, pituitary, ovaries, and also functional amenorrhea. Most common hypothalamic risk factors are Kallmann syndrome and chronic disorders. Most common pituitary risk factors are hyperprolactinemia and pituitary microadenoma. 

According to the US Preventive Services Task Force (USPSTF), there is insufficient evidence to recommend routine screening for amenorrhea.

If left untreated, all of patients with amenorrhea may progress to develop infertility and osteoporosis. Common complications of amenorrhea are based on the background disease that induced it. Prognosis is generally excellent and the mortality rate of patients with amenorrhea is approximately less than 1%, generally in brain lesions.

There are no established criteria for the diagnosis of amenorrhea. Diagnosis is based on delayed or absent menstrual cycle.

The hallmark of primary amenorrhea is lack of menarche 15 years of age, while other secondary sexual characteristics are already appeared; or lack of menarche after 5 years of thelarche, if it is occurred before 10 years of age. The hallmark of secondary amenorrhea is menstrual cycle interruption for at least 3 months, however was regular before; or menstrual cycle interruption for at least 6 months, however was irregular before.

Physical examination of patients with amenorrhea is based on underlying disease. The presence of hirsutism and acne on physical examination is diagnostic of polycystic ovary disease. The presence of galactorrhea and vision loss on physical examination is diagnostic of hyperprolactinemia (prolactinoma). The presence of bulging in vulva and imperforated hymen on physical examination is highly suggestive of imperforate hymen.

The initial laboratory tests for evaluating amenorrhea are pregnancy test, thyroid stimulating hormone (TSH), follicle stimulating hormone (FSH), and prolactin (PRL). Second line laboratory tests include free and total testosterone, dehydroepiandrosterone sulfate (DHEAS), and also progesterone challenge test.

There are no ECG findings associated with amenorrhea.

There are no X-ray findings associated with amenorrhea, exclusively. There are no X-ray findings associated with most common causes of amenorrhea, like polycystic ovary syndrome (PCOS) and premature ovarian failure. However, an X-ray may be helpful in the diagnosis of delayed puberty.

There are no echocardiography findings associated with amenorrhea. However, an echocardiography may be helpful in the diagnosis of the diseases that can cause amenorrhea, such as Turner syndrome. Findings on an echocardiography suggestive of Turner syndrome include bicuspid aortic valve, elongation of transverse aortic arch, coarctation of aorta, and partial anomalous pulmonary venous return (PAPVR). There are no ultrasound findings associated with amenorrhea. However, an ultrasound may be helpful in the diagnosis of the diseases that can cause amenorrhea, such as polycystic ovary syndrome (PCOS), premature ovarian insufficiency, androgen insensitivity syndrome, 17-alpha hydroxylase deficiency, and also anatomic genital defects.

There are no CT scan findings associated with amenorrhea. However, a CT scan may be helpful in the diagnosis of the diseases that can cause amenorrhea, such as Turner syndrome, androgen insensitivity syndrome and also anatomic genital defects.

There are no MRI findings associated with amenorrhea. However, a MRI may be helpful in the diagnosis of the diseases that can cause amenorrhea, such as polycystic ovary syndrome (PCOS), androgen insensitivity syndrome, anatomic genital defects, and also pituitary adenoma.

Hysterosalpingography (HSG) may be helpful in the diagnosis of the anatomic defects that can cause amenorrhea. Findings on a hysterosalpingography diagnostic of Asherman syndrome include multiple irregular linear (or lacunar) filling defects showing intrauterine adhesion, inability to distend the endometrial cavity, and totally non-filled uterine mostly in severe cases. Testicular scan can diagnosis the intra-abdominal or inguinal testes in androgen insensitivity syndrome.  

Karyotyping is used to diagnose amenorrhea caused by chromosomal disorders, such as Turner syndrome. University of Pennsylvania Smell Identification Test (UPSIT), consist of microencapsulated odorants released by scratching standardized odor-impregnated questionnaires, is used to detect hyposmia or anosmia in Kallmann syndrome.

Pharmacologic medical therapy is recommended among patients with hypothalamic causes, pituitary causes, ovarian insufficiency, and chronic anovulation. The general principle of the treatment in amenorrhea is sex hormone replacement therapy, mostly with suitable forms of estrogen and progesterone.

The mainstay of treatment for amenorrhea is medical therapy. Surgery is usually reserved for patients with either hypothalamus or pituitary tumors, Turner syndrome, and genital anatomical defects (imperforate hymen or transverse vaginal septum). The surgical treatment for hypothalamus or pituitary tumors is tumor resection via endoscopic transsphenoidal surgery. Ovaries have to be excised in Turner syndrome to prevent malignant transformation. Small incision is the main surgery for imperforate hymen and septal excision is the main treatment for transverse vaginal septum.

Based on US Preventive Services Task Force (USPTSF), there are no established measures for the primary prevention of amenorrhea.

Effective measures for the secondary prevention of functional hypothalamic amenorrhea include oral contraceptive pills (OCPs), androgen therapy, recombinant insulin like growth factor 1 (IGF-1), recombinant leptin, bisphosphonates, and increasing calorie intake.

The ancient Egyptian belief honored “menstrual blood” as life-giving. The Mesopotamian mother goddess, named Ninhursag, is believed to have created mankind from loam and her “blood of life“. The first description on disturbances in menstrual cycle can be found in Papyrus Ebres [named after the Egyptologist Georg M. Ebers (1837-1898)], from New Kingdom period (1450-1550 B.C.E). They described the patients as a “women who suffers from the side of her pubic region as an irregularity of her menstruation“. In 1907, British Medical Journal, released an article about different types of treatments (mostly herbal and conservative) for amenorrhea. These treatment options were further evaluated in 1911 and led to a better approach in the management of amenorrhea. The term amenorrhea is derived from Greek language [a = negative, men = month, rhoia = flow], means lack of menstruation cycle in a woman.

Amenorrhea historical perspective

• The ancient Egyptian belief honored menstrual blood as life-giving. The Mesopotamian mother goddess, named Ninhursag, is believed to have created mankind from loam and her “blood of life“.^[1]
• The first description on disturbances in menstrual cycle can be found in Papyrus Ebres [named after the Egyptologist Georg M. Ebers (1837-1898)], from New Kingdom period (1450-1550 B.C.E). They described the patients as a “women who suffers from the side of her pubic region as an irregularity of her menstruation“.^[2][3]
• In Papyrus Edwin Smith [named after an American antiques dealer (1822-1906)], from 2900 B.C.E, first definition of amenorrhea presents as “woman suffering in her abdomen, so that the menstrual discharge can not leave her”. There are also herbal treatment options for this condition in the Papyrus.^[4]
• The term amenorrhea is derived from Greek language [a = negative, men = month, rhoia = flow]. The opposite is the normal menstrual period.
• Historically, the term amenorrhea has been often used as a euphemism for “unwanted pregnancy” and many traditional treatments for this condition were in fact abortifacients.
• In 1855, William Brinton, a British physician, presented a woman with secondary amenorrhea caused by severe cachexia due to chronic peptic ulcer disease.^[5]
• In 1885, Maury Deas, a British physician, suggested Permanganate of Potash as a treatment option for psychosis associated amenorrhea.^[6]
• In 1894, Jollye, a British physician, described a girl with secondary amenorrhea along with some symptoms reflecting brain mass lesion.^[7]
• In 1910, Crowe, a Canadian physician, was the first to describe pituitary gland as a regulator of gonadal stem cell.^[8]
• In 1912, Bernhard Aschner, an Austrian endocrinologist, found gonadal control by pituitary gland is regulated by another superior center in the brain. He suggested hypophysis, pituitary stalk, and centers superior to the medulla oblongata as the controlling centers. Their dysfunction leads to hypopituitarism as well as hypogonadism.^[9]
• In 1926, Philip Smith, an American endocrinologist, experienced and approved that implanting animals’ pituitary gland into other animals (mice, cats, and rabbits) leads to initiation of puberty and enlargement of gonads.^[10]
• In 1926, Bernhard Zondek, a Israeli gynecologist, had the same experience, injecting adult humans’ or cows’ pituitary gland into immature animals and witnessing the rapid rush of their puberty process.^[11]
• In 1929, Bernhard Zondek, a Israeli gynecologist, identified that, pituitary gland secretes two major hormones, that controls sexual life and maintains the species. He named them as “prolan A” and “prolan B“, named after the Latin word “prole” which means “descendant“.^[12]
• In 1930, Philip Smith, an American endocrinologist, suggested that excision of pituitary gland in immature animals can lead to failure of sexual maturation; whereas in mature animals excision can lead to gonadal atrophy, regression of sexual characteristics, and infertility.^[13]
• In 1930, Bernhard Zondek, a Israeli gynecologist, found prolan A is responsible for follicular growth stimulation and prolan B is responsible for ovulation induction and corpus luteum formation. Zondek also described that prolan A and B together induce the hormone “fuliculin” from gonads, while prolan B alone induce both fuliculin and “lutein” hormones.^[14]
• In 1931, Fevold, an American zoologist, named fuliculin and lutein as follicle stimulating factor and luteinizing factor respectively.^[15]

• In 1907, British Medical Journal, released an article about different types of treatments (mostly herbal and conservative) for amenorrhea.^[16]
• In 1911, researchers further evaluated the therapeutic methods (described in 1907) and suggested better management of amenorrhea.^[17]

]]

Amenorrhea can be classified on the basis of etiology into three subtypes, including primary amenorrhea, secondary amenorrhea, and functional amenorrhea. Primary amenorrhea can also be classified on the basis of HPG (hypothalamic-pituitary-gonadal) axis function into hypergonadotropic hypogonadism, hypogonadotropic hypogonadism, and eugonadotropic state. Secondary amenorrhea reflects an absence of menstrual cycle for at least 3 months in a woman with normal menstruation cycles in the past. Secondary amenorrhea can be classified based on pathology into polycystic ovary syndrome, hypothalamic–pituitary dysfunction, hypothalamic–pituitary failure, and ovarian failure. Functional (hypothalamic) amenorrhea is a subtype of the amenorrhea seen in patients with erratic lifestyle and can be classified on the basis of etiology into stress, weight loss, and exercise related amenorrhea.

• Amenorrhea may be classified according to etiology into three subtypes:
  • Primary amenorrhea
  • Secondary amenorrhea
  • Functional amenorrhea
• Each of the subtypes of amenorrhea has their own classifications, as following:

• Primary amenorrhea refers to an adolescent girl who have never experienced menarche. Primary amenorrhea is defined as one of the followings:^[1][2][3]
  • Normal secondary sexual characteristics and absence of menarche by 15 years of age.
  • Lack of secondary sexual characteristics and absence of menarche by 14 years of age.
  • Lack of menarche five years after initial breast development (thelarche).
• An important aspect of classification in patients with primary amenorrhea is the presence or absence of uterus.
  • If uterus is present, primary amenorrhea can be classified based on plasma level of follicle stimulating hormone (FSH) and luteinizing hormone (LH), into hypergonadotropic hypogonadism (elevated FSH and LH), hypogonadotropic hypogonadism (decreased FSH and LH), and eugonadotropic state (normal FSH and LH).
  • Patients with absent uterus can be classified into mullerian agenesis or androgen insensitivity.

• Secondary amenorrhea is more common than primary amenorrhea. Secondary amenorrhea is defined as one of the followings:
  • Absence of menstrual cycle for at least 3 months in a woman with normal menstrual cycle in the past.
  • Absence of menstrual cycle for at least 9 months in a woman with a history of oligomenorrhea.
• Secondary amenorrhea can be classified based on pathology into polycystic ovary syndrome, hypothalamic–pituitary dysfunction, hypothalamic–pituitary failure, and ovarian failure.^[1][4][5]

• Functional (hypothalmic) amenorrhea is a subtype of the amenorrhea seen in patients with erratic lifestyle and can be classified on the basis of etiology into stress, weight loss, and exercise related amenorrhea.^[6]
• Functional amenorrhea is defined as absence of menstrual cycle for 6 months from disturbances in hypothalamic-pituitary-ovarian axis, and without any anatomical or organic pathology.^[7]
• Generally, the primary factor that causes functional amenorrhea is lack of gonadotropin releasing hormone (GnRH) pulsatility.^[8]

Amenorrhea is defined as absence of menstrual cycle. The causes of amenorrhea include hypothalamic, pituitary, thyroid, adrenal, ovarian, uterine, and vaginal. About 25 different genes are involved in the pathogenesis of amenorrhea including 3 different groups of Kallmann syndrome related genes, hypothalamus–pituitary–gonadal (HPG) axis related genes, and obesity related genes. On gross pathology, normal endometrium is the characteristic findings of amenorrhea. Patients of amenorrhea from Craniopharyngioma as have cystic mass filled with motor oil-like fluid on gross pathology. On microscopic histopathological analysis, craniopharyngioma presents as trabecular squamous epithelium surrounded by palisaded columnar epithelium, small-to-medium sized cells with moderate amount of basophilic cytoplasm, bland nuclei, and calcifications. On microscopic histopathological analysis, pituitary adenoma as a cause of amenorrhea presents as loss of fibrous stroma and nested cells of normal anterior pituitary (based on the type of adenoma).

• The mean age for onset of menstruation is 12.43 years in US. About 80% of females experience menarche between 11 and 13.75 years of age.^[1]
• Almost all (98%) of females experience menarche by the age of 15.^[2]
• Gonadotropin releasing hormone (GnRH) is the main factor in puberty onset.
  • GnRH is secreted by the neurosecretory neurons of hypothalamus into the hypophysial portal system, from where it is transferred to anterior pituitary gland.
  • GnRH stimulates production and secretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH).
  • During puberty the amplitude and frequency of GnRH pulses is significantly increased.
  • GnRH secretion is regulated by certain neurotransmitters in brain, such as dopamine, endogenous opioids, norepinephrine, gamma amino butyric acid (GABA), and corticotropin releasing hormone (CRH). Alteration in level and function of these neurotransmitters may lead to specific types of amenorrhea. For example stress, exercise, and malnutrition affects CRH, β-endorphin, and dopamine, respectively.^[3]
• After the onset of puberty, the negative feedback on GnRH is removed.
• Pulsatile secretion of GnRH induce LH and FSH production which finally lead to ovulation.
• In the absence of fertilisation, the ovarian follicle is turned into corpus luteum. Endometrium proliferates through estrogen release from corpus luteum. Withdrawal of progesterone from estrogen-mediated proliferated endometrium results in menstrual bleeding.

• After activation of the HPO axis during second trimester of pregnancy, the level of gonadotropins rises from mid to term pregnancy. After cessation of placental hormone feedback, FSH and LH increases again slightly to mild secondary peak.
• Before puberty, negative feedback from adrenal androgens keep the gonadotropins in low plasma level.^[4]
• Right before puberty, the sensitivity of hypothalamus to negative feedback from adrenal androgens is decreased. This leads to increased production of GnRH from hypothalamus and make it possible for GnRH to be raised in magnitude and frequency to induce an increase in LH and FSH.^[5]
• The complete maturation of HPO axis takes around 5-7 years from the onset of menstruation. Generally, during the first two years of menstruation, the cycles are mostly anovulatory due to immature HPO axis.

• Amenorrhea is defined as the absence of menstrual cycle.^[3]
  • Primary amenorrhea is the absence of menstrual cycle by 16 years of age, in the presence of normal growth and secondary sexual characteristics.
  • Secondary amenorrhea reflects absence of menstrual cycle for at least 3 months in a woman with normal menstruation cycles in the past.
• The pathophysiology of amenorrhea is multifactorial and include hypothalamic, pituitary, thyroid, adrenal, ovarian, uterine, and vaginal causes.

• The most common cause of amenorrhea in adolescents are hypothalamic disorders and is known as hypothalamic amenorrhea.
• During the initial 2-3 years after the onset of menarche, the HPO axis is still under development. Immature HPO axis may lead to anovulatory cycles which can cause abnormalities in menstrual cycles.
• The most common cause of amenorrhea after 2-3 years of onset of puberty include eating disorders, excessive exercise, medications, and psychosocial stress.^[6][7]
• Leptin plays an important role in energy consumption, body composition, food intake along with sexual maturation and reproductive improvement.
  • It is assumed that leptin plays a role in the development of hypothalamic amenorrhea.
  • Leptin receptors are in close relationship with hypothalamus and it is postulated that leptin regulates GnRH production and secretion.
  • Patients with anorexia nervosa or excessive exercise have amenorrhea from down-regulation of leptin receptors, which can be elevated by refeeding.^[8][9][10]
  • Leptin level in cachexic patients will increase after gaining appropriate weight in patients without amenorrhea, but will remain low in patients with amenorrhea.^[11][12]
• It has been observed that leptin levels in amenorrheic athletes are low as compared to non-athletes women or athletes with regular menses.^[13][14]
• Administration of recombinant leptin for 3 months in women with conditions such as hypothalamic amenorrhea, excessive exercises or weight loss has been associated with an increased level of LH, FSH, and estradiol leading to ovulatory cycles.^[15]
• Antipsychotic drugs and other medications that have an inhibitory effect on dopamine D2 receptor lead to an increased level of prolactin. Higher levels of prolactin suppress pulsatile GnRH secretion and block positive feedback of estradiol on hypothalamus, leading to disruption of HPO axis.^[16]
• Stress and strenuous activities like other metabolic or cardiovascular responses are regulated through corticotropin releasing hormone (CRH), secreted by paraventricular nuclei of hypothalamus. CRH induce the release of β-endorphin, an endogenous opioid. Both CRH and β-endorphin suppress GnRH release. On the other hand, glucocorticoids suppress LH production from pituitary and also estrogen/progesterone from ovaries.^[17]
• Kallmann syndrome, a genetic disorder caused by KAL gene mutation, has disturbance in migration of olfactory nerves along with GnRH neurons. Lack of GnRH leads to absence of secondary sexual characteristics and amenorrhea.^[18]

• Prolactinoma is one of the most common anterior pituitary tumors. Prolactinoma leads to increased prolactin secretion and along with the tumor’s mass effect may cause suppression of GnRH.
• The second most common tumor in the suprasellar region is craniopharyngioma. The tumor leads to LH and FSH disturbances, which may cause amenorrhea.^[19]

• In hypothyroidism, the main mechanism that can lead to amenorrhea is the influence of thyrotropin releasing hormone (TRH) on lactotroph cells, increasing prolactin levels. Since the TRH is increased in hypothyroidism, it leads to functional hyperprolactinemia. The increase in prolactin may suppress the GnRH pulsatility and lead to amenorrhea.^[20]
• In hyperthyroidism, the mechanism of amenorrhea is not clear. It is assumed that increased level of sex hormone binding globulins (SHBGs) in hyperthyroidism may lead to increased levels of androgens and estrogen. Conclusively, the LH surge become absent and amenorrhea happens.^[21]

• Congenital adrenal hyperplasia (CAH) is a group of genetic enzyme deficiencies in adrenal gland. Most common is the defect in 21-hydroxylase enzyme, which leads to decrease in the level of aldosterone and cortisol. To overcome the hormone deficiencies, CRH production is increased by the hypothalamus. As described earlier, increased level of CRH may suppress GnRH and lead to amenorrhea.^[22]
• Cushing syndrome has an increased level of cortisol, that can directly inhibit HPO axis and lead to amenorrhea.

• In polycystic ovary syndrome (PCOS) insulin resistance leads to increased androgen production (insulin reduces the SHBG circulating in plasma, causing increased testosterone). In ovaries, increased stimulation from GnRH leads to increased production of 17-hydroxy progesterone and cytochrome P450c17 which promotes androgens biosynthesis.^[23][24] Finally, the pulsatility of GnRH will be disrupted leading to amenorrhea .^[25]
• Primary ovarian insufficiency is multifactorial and leads to ovarian failure and decrease in estrogen which leads to amenorrhea. (In galactosemia, it is assumed that galactose and its metabolites are toxic to ovarian tissue.)^[26]

• The main pathogenesis of amenorrhea in androgen insensitivity syndrome is the absence of uterus. The patient is genotypically male, 46 XY; but with absent sexual characteristics due to lack of functional effect of androgen hormones on their receptors.
• One of the acquired conditions that can lead to amenorrhea is Asherman syndrome. The basis of Asherman syndrome is any condition that can alter the normal histology of endometrium, such as scarring from surgical procedure or adhesion from severe infection.
• Mayer-Rokitansky-Kuster-Hauser syndrome is complete agenesis of uterine, blind ended vagina. The lack of uterine and endometrium is the main pathogenesis of amenorrhea. The main reason of uterine agenesis is overactivation of anti-mullerian hormone in embryogenesis period.^[27] Cervical agenesis also follow the similar process.
• Imperforated hymen, transverse vaginal septum, and vaginal agenesis are other anatomical disorders of female reproductive system that can lead to amenorrhea.^[28]

• The GPR54 gene, also called KISS1R, with Online Mendelian Inheritance in Man (OMIM) number of 604161 is on chromosome 19p13.3. The KISS1 gene, also known as kisspeptin1, with OMIM number of 603286 is on chromosome 1q32.
• Kisspeptin and related G-protein coupled receptor (KISS1R or GPR54) have key roles in the regulation of GnRH secretion. The GnRH secretion has to be pulsatile to stimulate gonadotropins. Kisspeptins are encoded by KISS1 gene, neuropeptides secreted from hypothalamus nuclei. It has been observed that patients with idiopathic hypogonadotropic hypogonadism have KISS1 receptor (GPR54) inactivating gene mutations.^[30][31]
• By the time of onset of puberty, the KISS1 genes become activated through neuroanatomical and functional changes from environmental triggers, critical for brain sexual maturation and hypothalamic–pituitary–gonadal axis (HPG axis) activation with pulsatile GnRH.^[32]
• Along HPG axis neurons, gamma-aminobutyric acid is inhibitory and glutamate is excitatory neurotransmitters. In related KNDy neurons in arcuate nucleus, the materials secreted are included kisspeptin, neurokinin B, and dynorphin A. Before puberty begins, inhibitory dynorphin A is the dominant element; decreased by stimulatory effect of neurokinin B, when puberty started. Conclusively, kisspeptin and GnRH/LH are increased.^[33]

• The KAL1 gene, also called anosmin-1, with OMIM number of 308700 is on chromosome Xp22.3, and encodes an extracellular matrix glycoprotein.
• Anosmin-1 is expressed at five weeks of gestation in forebrain near olfactory bulbs and stimulate the afferent fibers projections around it.^[34]
• X-linked Kallmann syndrome is directly associated with KAL1 deletion. It is assumed to result in an absence of olfactory fibers along with disrupted migration of GnRH neurons, that are supposed to from migrated olfactory placode.^[35]
• Male patient with KAL1 mutation would have central hypogonadism and anosmia/hyposmia. Additionally, the more diseases are assumed to relate with KAL1 gene, such as midline facial defects (cleft lip and/or cleft palate), short metacarpals, renal agenesis, sensorineural hearing loss, bimanual synkinesis, oculomotor abnormalities, and cerebellar ataxia.^[36]

• The FGFR1 gene, also called KAL2, with OMIM number of 136350 is on chromosome 8q12, encode a receptor tyrosine kinase protein. The FGF8 gene, also called KAL6, is on chromosome 10q24.
• FGFR1 pathway is assumed to be the main role in embryogenesis, homeostasis, and wound healing. FGF8 plays a critical role in the primary generation of neural tissue.^[37]
• On the other hand, interaction between FGFR1, FGF8, and heparan sulfate helps the olfactory bulb to become differentiated and developed and also facilitates GnRH neurons in migration and function.^[38]
• Dominant deletion mutation of FGFR1 gene is associated with a 30% decrease in hypothalamic GnRH neurons.^[39] Other defects related to FGFR1 includes cleft palate or lip, dental agenesis and bimanual synkinesis.^[36]

• The HS6ST1 gene with OMIM number of 604846 is on chromosome 2q21 has been found to be mutated in hypogonadism.^[40]

• The modifications of heparan sulfate polysaccharides in extracellular matrix have some role in FGFR–FGF and also anosmin1–cell membrane interactions.^[41][42]
• This gene may be mutated in both Kallmann syndrome and idiopathic hypogonadism resulting in various courses, disruption pf frequency of GnRH secretion, and/or GnRH deficiencies.^[40]

• The PROK2 and PROKR2 genes, also called KAL4 and KAL3, with OMIM numbers of 607002 and 607123 are on chromosomes 3p21.1 and 20p13, respectively. They are believed to be a cause of Kallmann syndrome.
• PROKR2 is a G protein coupled receptor (GPCR), has a major role in olfactory bulb development; and its mutation may lead to severe gonadal atrophy.^[43]
• In prokineticin system, there are two receptors (PROKR1 and PROKR2) and two ligands (PROK1 and PROK2). PROK1 and its receptor (PROKR1) have some role in gastrointestinal system motility. PROK2 and PROKR2 are parts of neuroendocrine system, located in arcuate nucleus, olfactory tract, and suprachiasmatic nucleus.^[44]
• The mutated versions of PROK2 and PROKR2 could lead to decrease GnRH production and hypogonadism. Other disorders caused by PROK2 and PROKR2 mutations include fibrous dysplasia, sleep disorder, severe obesity, synkinesis, and epilepsy.^[45]

• The TAC3 and TACR3 genes, also called neurokinin B (NKB) and neurokinin 3 receptor (NK3R), with OMIM numbers of 162330 and 152332, are on chromosomes 12q13–q21 and 4q25, respectively.^[46]
• It is postulated that the normal function of TAC3/TACR3 system is necessary for an intact HPG axis and its development during puberty. On the other hand, TAC3/TACR3 system disturbance may cause micropenis and cryptorchidism in males, showing the major role of TAC3/TACR3 in fetal gonadotropins secretion.^[47]
• TACR3 encoded protein (NK3R) is GPCR, initially produced in central nervous system. The major mechanism, through which the mutated gene may lead to neuroendocrine disturbance and delayed puberty, is not yet discovered completely.^[48]
• TAC3 encoded protein (NKB) is produced in arcuate nucleus of hypothalamus and play an important role in GnRH secretion. Kisspeptin is also produced and secreted in arcuate nucleus, whereas, both of them are inhibited by estrogen. It may be considered that kisspeptin and NKB have identical roles in diverting negative feedback from sex hormones to GnRH. Their mutation has been shown to be related with hypogonadism.

• The GnRH1 and GnRHR genes with OMIM numbers of 152760 and 138850 are on chromosomes 8p21–8p11.2 and 4q21.2, respectively.^[49]
• In HPG axis, GnRH is one of the most effective elements; therefore, a defect could directly influence the axis and slow down the progress.
• The GnRHR gene is also responsible for gonadal normal functions and its mutation could lead to hypogonadism and delayed puberty. The mutation in GnRHR has also been associated with conditions such as atrophic gonads along with low LH/FSH and sex hormones, sexual puberty disturbance, inability to conceive, and failure to impact from exogenous GnRH.^[50]
• GnRH1 and GnRHR genes have variable expression and cause a spectrum of symptoms, from fertile eunuch syndrome and partial idiopathic hypogonadotropic hypogonadism to complete GnRH resistance (i.e., characterized by cryptorchidism), microphallus, very low LH/FSH, and delayed puberty.^[51]
• The other disorders that have been found to be related to GnRH mutation are tooth maturation and biomineralization.^[52]

• The CHD7 gene, also called as KAL5, with OMIM number of 608892 is on chromosome 8q12.1.
• CHD7 gene mutation results in autosomal dominant CHARGE syndrome, which is a combination of hypogonadism and Kallmann syndrome, and includes:^[53]
  • Coloboma
  • Heart anomalies
  • Choanal Atresia
  • Retardation
  • Genital anomalies
  • Ear anomalies
• Screening for CHD7 gene mutation may be done in patients with hypogonadism or Kallmann syndrome with specific features such as semicircular canal hypoplasia or aplasia, dysmorphic ears, and deafness.

• The NELF gene with OMIM number of 608137 is on chromosome 9q34.3; present mostly in nervous tissues specifically during fetal development and may be found in olfactory bulb and pituitary LH releasing cells.
• The most common function is in olfactory axons and GnRH neurons, before and during neuron migration in the developmental process.^[54]

• The EBF2 gene with OMIM number of 609934 is on chromosome 8p21.2; mostly expressed in mice osteoblasts and osteoclast cells.^[55]
• EBF2 gene plays an effective role in HPG axis. Mutation in EBF2 gene can result in disruption of HPG axis, leading to secondary hypogonadism.^[56]

• The DAX1 gene, also called nuclear receptor 0B (NR0B), with OMIM number of 300473 is on chromosome Xp21.2, and expressed in all members of HPG axis (hypothalamus, pituitary, and gonads).^[57]
• During the spermatogenesis and steroidogenesis, both sertoli and leydig cells have increased expression of DAX1 gene. It is assumed that during puberty, the peak expression of DAX1 is observed.^[58]
• Other disease that can be caused by DAX1 mutation is congenital adrenal cortex hypoplasia.^[59]

• The SF1 gene, also called nuclear receptor 5A1 (NR5A1), with OMIM number of 184757 is on chromosome 9q33.3, has some role in reproduction, steroidogenesis, and sexual differentiation.
• It is mainly expressed in sertoli and leydig cells, and plays an important role in steroidogenesis and spermatogenesis. The SF1 is believed to have an increase in expression from childhood until adolescence, and is dominantly expressed by leydig cells in puberty.^[58]
• Other diseases that may be caused by SF1 mutation include male pseudohermaphroditism, Denys-Drash syndrome, and also hypospadias.^[60]

• The HESX1 gene, also called Rathke pouch homeobox (RPX), with OMIM number of 601802 is on chromosome 3p14.3, initially expressed during embryogenesis and help the formation of Rathke pouch and anterior pituitary.^[61]
• HESX1 gene has an important role in pituitary development and midfacial differentiation. Mutation may lead to pituitary hypoplasia and decreased level of all anterior pituitary hormones.^[62]
• Other disorders resulting from HESX1 mutation include septo optic dysplasia, reduced prosencephalon, anophthalmia, microphthalmia, defective olfactory development, Rathke pouch bifurcations, and abnormalities in the corpus callosum, hippocampus, and septum pellucidum.^[61]

• The LHX3 gene, also called LIM3, with OMIM number of 600577 is on chromosome 9q34.3, mainly expressed in developing anterior pituitary gland.^[63]
• LHX3 gene function is important in development of pituitary gland and pituitary hormones secretion. Mutation in the LHX3 gene may result in combined pituitary hormone deficiency (CPHD).^[64]
• LHX3 gene mutation may also result in neonatal hypoglycemia, short neck with limited rotation, mild sensorineural hearing loss, skin laxity, and skeletal abnormalities.^[63]

• The PROP1 gene with OMIM number of 601538 is on chromosome 5q35.3, with a role in developing anterior pituitary gland and associated cells such as gonadotrophs, thyrotrophs, somatotrophs, and lactotrophs.^[65]
• Mutated PROP1 gene can lead to deficiency of LH, FSH, GH, TSH, and prolactin. Decreased level of LH and FSH may also delay or inhibit the onset of puberty.^[66]
• Pituitary hormones have a vital role in regulating other endocrine organs via TRH, ACTH, FSH or LH and a mutation in PROP1 gene can lead to thyroid dysfunctions, growth retardation, and libido/lactation problems.

• The LEP and LEPR genes, also called OB and OBR, with OMIM numbers of 164160 and 601007 are on chromosomes 7q32.1 and 1p31.3, respectively; with a major role in modulation of body weight.
• These genes are believed to carry the message of onset of puberty. Recent studies have shown that recombinant leptin injection in female mice may result in puberty and cure their maturation (secondary sexual characteristics) problems.^[67]
• It has been observed that leptin levels increase by 50% just before the onset of puberty and during puberty.^[68]
• Mutation in LEP and LEPR genes may result in dysfunctional hematopoiesis, angiogenesis, wound healing, and the immune or inflammatory response.

• The PC1 gene, also known as neuroendocrine convertase 1 (NEC1), with OMIM number of 162150 is on chromosome 5q15, and regulates neuroendocrine pathway.
• PC1 gene has a dominant role in proopiomelanocortin (POMC) cleavage. PC1 gene also has a role in processing proinsulin and proglucagon in pancreas.^[69]
• Recent studies have shown that PC1 gene mutation and hypogonadotropic hypogonadism may result in extreme childhood obesity, abnormal glucose homeostasis, hypocortisolism, elevated plasma proinsulin, and POMC concentrations.^[70]

• Newly discovered MKRN3 gene has a role in ubiquitination and cell signaling. The gene family proteins are majorly expressed in fetal brain during development, especially in arcuate nucleus.
• The process of gene amplification is on its peak after birth, which gradually declines with time, and finally rises again with onset of puberty. Thus MKRN3 gene is believed to be one of the factors in onset of puberty, along with kisspeptins and neurokinin B.^[71]

• Estrogen receptor mutations are very rare, and were reported in a case report of delayed puberty.^[72]
• Estradiol promotes breast maturation and provides negative feedback to hypothalamus and pituitary, by means of estrogen receptor α (encoded by ESR1 gene).^[73]
• Female mice with mutated ESR1 gene have hypoplastic uterus with hemorrhage, multicystic ovary without corpus luteum; which is make them infertile.^[74]

The associated conditions that are related to amenorrhea, are as following:^[75]

• On gross pathology, normal endometrium in proliferative or luteal phases are characteristic findings of amenorrhea.
• In cases of amenorrhea which are secondary to other causes, the related gross pathology would be seen.
• Craniopharyngioma on gross pathology presents as cystic mass filled with motor oil-like fluid.^[76]
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

• On microscopic histopathological analysis, craniopharyngioma as a cause of amenorrhea will have the following features:
  • Trabecular squamous epithelium surrounded by palisaded columnar epithelium
  • Small-to-medium sized cells with moderate amount of basophilic cytoplasm
  • Bland nuclei
  • Calcifications
• On microscopic histopathological analysis, pituitary adenoma as a cause of amenorrhea will have the following features:
  • Loss of fibrous stroma
  • Nested cells of normal anterior pituitary (based on the type of adenoma)

• 
  Craniopharyngioma; with psammoma bodies – by Jensflorian source: Librepathology
• 
  Craniopharyngioma; with calcification and two types of epithelium – by Sarahkayb source: Librepathology
• 
  Craniopharyngioma; multicystic texture – by Jensflorian source: Librepathology
• 
  Pituitary adenoma of lactotroph cells (prolactin producing) – by Jensflorian source: Librepathology
• 
  Pituitary adenoma of lactotroph cells (prolactin producing) regression after treatment – by Jensflorian source: Librepathology
• 
  Pituitary adenoma of thyrotroph cells (TSH producing) – by Jensflorian source: Librepathology

Common causes of amenorrhea are breastfeeding, pregnancy, menopause, and stress. Common causes of primary amenorrhea are craniopharyngioma, idiopathic gonadotropin deficiency, Kallmann’s Syndrome, Mayer-Rokitansky-Hauser Syndrome, Mullerian dysgenesis, and outflow tract disorders. Common causes of secondary amenorrhea are craniocerebral trauma, curettage, Cushing’s Syndrome, depression, diabetes mellitus, and drug side effects. Common causes of functional amenorrhea are stress, rapid weight loss, and excessive exercise.

Life-threatening causes include conditions which may result in death or permanent disability within 24 hours if left untreated. There are no life-threatening causes of amenorrhea, however complications resulting from untreated amenorrhea are common.

• Breastfeeding
• Menopause
• Pregnancy
• Stress
• Turner syndrome
• Excessive exercise
• Dieting
• Obesity
• Hyperthyroidism
• Hypothyroidism

• Androgen insensitivity syndrome (testicular feminization)
• Autoimmune disorders
  • Autoimmune oophoritis (in myasthenia gravis, thyroiditis, or vitiligo)
• Aneurysms of the pituitary
• Cachexia

• Chronic disorders
  • Crohn disease
  • Cystic fibrosis
  • Sickle cell disease
  • Thalassemia major
• Chemotherapy (e.g., high-dose alkylating drugs)

• Congenital adrenal virilism
• Cushing syndrome
• Drug abuse
  • Alcohol
  • Cocaine
  • Marijuana
  • Opioids

• Drug-induced virilization
  • Androgens
  • Antidepressants
  • Danazol
  • High-dose progestins
• Eating disorders
  • Anorexia nervosa
  • Bulimia

• Genetic disorder
  • Congenital gonadotropin-releasing hormone deficiency
  • GnRH receptor gene mutations
  • Prader-Willi syndrome
  • Congenital thymic aplasia
  • Fragile X syndrome
  • Idiopathic accelerated ovarian follicular atresia
• Gestational trophoblastic disease
• Gonadal dysgenesis
• HIV infection
• Hyperprolactinemia
• Idiopathic hypogonadotropic hypogonadism
• Infiltrative disorders of the pituitary
  • Hemochromatosis
  • Langerhans cell granulomatosis
  • Sarcoidosis
  • TB

• Isolated gonadotropin deficiency
• Immunodeficiency
• Infiltrative disorders of the hypothalamus
  • Langerhans cell granulomatosis
  • Lymphoma
  • Sarcoidosis
  • Tuberculosis (TB)
• Irradiation to the pelvis
• Irradiation to the hypothalamus

• Kallmann syndrome
• Malnutrition
• Metabolic disorders
  • Addison disease
  • Diabetes mellitus
  • Galactosemia
• Psychiatric disorders
  • Stress
  • Depression
  • Obsessive-compulsive disorder
  • Schizophrenia
  • Psychoactive drugs

• Postpartum pituitary necrosis (Sheehan syndrome)
• Tumors of the brain
  • Meningioma
  • Craniopharyngioma
  • Gliomas
  • Pituitary microadenoma

• Traumatic brain injury
• Tumors of the hypothalamus
• True hermaphroditism

• Tumors producing androgens (ovarian or adrenal)

• Viral infections (mumps)