Thyroid function tests

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Furqan M M. M.B.B.S[2]

Synonyms and keywords: TFTs

Thyroid function tests (TFTs) is a collective term for blood tests used to check the function of the thyroid. A TFT panel typically includes thyroid-stimulating hormone (TSH, thyrotropin) and thyroxine (T4), and triiodothyronine (T3) depending on local laboratory policy.TFTs may be requested if a patient is thought to suffer from hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid) or to monitor the effectiveness of either thyroid-suppression or hormone replacement therapy. It is also requested routinely in conditions linked to thyroid disease, such as atrial fibrillation. The normal adult reference range for serum TSH is 0.4-4.5 mIU/L. The adult reference range of FT4 using comparative direct methods is 0.7-1.8 ng/dL (9-23 pmol/L) and that for FT3 is 2.3-5.0 pg/mL (35-77 pmol/L). A high TSH level may indicate hypothyroidism. If the results indicate high TSH levels, a second confirmatory sample including TSH and free T4 levels should be tested prior to starting treatment. TSH should not be measured in isolation in patients with suspected hypothalamic-pituitary disease; the measurement of free T4 is essential. Free T4 testing, combined with TSH testing, is recommended for the routine assessment and diagnosis of thyroid function and following hyperthyroidism treatment and may or may not be used in cases of hypothyroidism.

Thyroid function tests serve as an important diagnostic tool in the diagnosis and follow-up of various hyper and hypothyroid states:^[1]

• The normal adult reference range for serum TSH is 0.4-4.5 mIU/L. A high TSH level may indicate hypothyroidism. If the results indicate high TSH levels, a second confirmatory sample including TSH and free T4 levels should be tested prior to starting treatment.
• The adult reference range of FT4 using comparative direct methods is 0.7-1.8 ng/dL (9-23 pmol/L) and that for FT3 is 2.3-5.0 pg/mL (35-77 pmol/L).
• TSH should not be measured in isolation in patients with suspected hypothalamic-pituitary disease; the measurement of free T4 is essential. Free T4 testing, combined with TSH testing, is recommended for the routine assessment and diagnosis of thyroid function and following hyperthyroidism treatment and may or may not be used in cases of hypothyroidism.
• Serum T3 testing has a low sensitivity and specificity for diagnosing hypothyroidism.
• Abnormal levels of total T4 and T3 often result from changes in transport proteins, not thyroid function. Total hormone testing should only be performed when there are discrepancies in the free hormone levels.
• Conditions other than thyroid disease should be considered when TSH levels are abnormal such as physiological changes associated with pregnancy, severe nonthyroidal illness, and medication use (including amiodarone, lithium, dopamine, and glucocorticoids).

• Hashimoto’s thyroiditis
• Riedel’s thyroiditis
• De Quervain’s thyroiditis

• Graves disease
• Thyrotoxicosis
  • De Quervain’s thyroiditis
  • Hashimoto’s thyroiditis
• Amiodarone

• Thyroid function tests may differentiate various causes of hypothyroidism:^{[2][3][4][5][6][7]}

(†)T3RU; Triiodothyronine Resin uptake. (^)TPOAb; Thyroid peroxidase antibodies. (*)TSH may be decreased transiently in the thyrotoxicosis. (**)TPOAb may be present in drug-induced hypo/hyperthyroidism such as Interferon-alpha, interleukin-2, and lithium.

• Thyroid function tests may differentiate various causes of hyperthyroidism:^{[2][3][4][5][6][7][8][9][10]}

(†)T3RU; Triiodothyronine Resin uptake. (^)TPOAb; Thyroid peroxidase antibodies.

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