Laboratory findings consistent with the diagnosis of prolactinoma include markedly elevated prolactin levels.

• Prolactin levels – Serum prolactin concentration may be markedly higher than normal (usually >200ng/ml).
• TSH levels – Thyrotropin releasing hormone (TRH) stimulates prolactin secretion. Thyroid stimulating hormone (TSH) is measured. If TSH is elevated, free thyroxine (T4) level must be checked to confirm that TSH elevation is due to elevated TRH and not due to decreased T4 (hypothyroidism).
• Pregnancy test – A urine pregnancy test (beta-hCG) is performed as pregnancy causes a physiologic rise in prolactin levels.
• Serum testosterone levels – Low levels of serum testosterone are found in males presenting with symptoms of hypogonadism.
• IGF-1 – Insulin-like growth factor-1 levels may be done in patients with features suggestive of acromegaly.
• Cortisol levels – Cortisol levels may be done in patients with features suggestive of Cushing’s syndrome.
• LFT – Liver function tests may be done in patients with appropriate history to exclude cirrhosis or viral hepatitis as the cause of elevated prolactin.
• RFT – Renal function tests may be done in patients with appropriate history to exclude chronic renal failure as the cause of elevated prolactin.

• The hook effect is a phenomenon which occurs particularly in macroprolactinoma.^[1]
• In patients with macroprolactinoma, the reading of the initial prolactin level may be normal or mildly increased.
• These false values are due a large amount of antigens, so this is also called the ‘high dose hook effect’.
• All patients with pituitary macroadenomas should undergo an immunoradiometric prolactin assay with multiple dilutions, if prolactinoma is suspected.

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