Why we measure it

Pathological hyperprolactinaemia disrupts the hypothalamic-pituitary-gonadal axis by suppressing GnRH pulsatility. The mechanism is direct: elevated prolactin reduces the frequency and amplitude of GnRH pulses from the hypothalamus, which in turn reduces FSH and LH secretion, which reduces gonadal sex hormone production. In males, this manifests as low testosterone, reduced libido, erectile dysfunction, and infertility — a presentation virtually identical to primary hypogonadism, but arising from an entirely different cause requiring different management. In females, hyperprolactinaemia causes oligo- or amenorrhoea, anovulation, galactorrhoea (inappropriate milk production), and infertility. Because its symptoms overlap with many common conditions — depression, fatigue, low libido — hyperprolactinaemia is frequently missed for years. The most common causes are: prolactinoma (a benign anterior pituitary adenoma producing prolactin, by far the most common pituitary tumour); medications that block dopamine receptors or reduce dopaminergic tone (antipsychotics, metoclopramide, domperidone, some antidepressants, some antihypertensives); hypothyroidism (TRH stimulates prolactin secretion alongside TSH); and chronic renal disease (impaired prolactin clearance). Annual prolactin testing catches hyperprolactinaemia in its early phase, before it causes measurable hypogonadism, and directs clinical investigation toward the correct diagnosis.

Why every 6 months

Prolactin has a diurnal pattern — it is highest during sleep and in the early morning. Blood should be drawn fasting, in the morning, after at least 30 minutes of rest (stress, physical activity, and venepuncture anxiety can all transiently raise prolactin). Macroprolactin — a high-molecular-weight complex of prolactin with immunoglobulin G — can be present in some individuals and produce an elevated total prolactin with normal bioactive prolactin; laboratories can perform a polyethylene glycol (PEG) precipitation test to distinguish these. The six-month interval provides sufficient resolution to detect a rising trend or confirm persistent elevation.

What movement means

Standard laboratory reference ranges for serum prolactin in non-pregnant adults are approximately 4–23 ng/mL (86–496 mIU/L) for females and 3–15 ng/mL (65–324 mIU/L) for males, though ranges vary by assay. A single mildly elevated reading should be repeated under optimal conditions (fasting, rested, no recent exercise or sexual activity) before clinical action. Prolactinomas are classified by size: microadenomas (< 10 mm) and macroadenomas (≥ 10 mm); the latter are more likely to produce very high prolactin levels and can cause mass effects on adjacent pituitary structures.

In the protocol