Prolactin (PRL)
Also known as: PRL, lactotropin, luteotropic hormone
A protein hormone from lactotrophs that prepares the breast during pregnancy and drives milk synthesis after birth; it is the only pituitary hormone under predominant inhibitory (dopamine) control.
Overview
Prolactin (199 amino acids) is structurally related to GH and acts on the prolactin receptor, a cytokine-family receptor signalling through JAK2–STAT5. During pregnancy, rising estrogen causes lactotroph hyperplasia (the pituitary nearly doubles in size) and prolactin rises ≈10-fold; together with estrogen, progesterone, cortisol, insulin and human placental lactogen it develops the lobuloalveolar system of the breast. However, high progesterone and estrogen block milk production until the placenta is delivered.
After delivery, prolactin stimulates milk synthesis (lactogenesis): transcription of casein and α-lactalbumin genes, lactose and lipid synthesis. Each episode of suckling produces a prolactin surge by inhibiting tuberoinfundibular dopamine. Prolactin also suppresses GnRH pulsatility (via kisspeptin neurons), producing lactational amenorrhoea and reduced fertility during frequent breastfeeding.
- Location
Source: lactotrophs of the anterior pituitary (also decidua). Targets: mammary alveolar epithelium; hypothalamus (GnRH suppression).
Function
- Stimulates milk synthesis (lactogenesis and galactopoiesis).
- Promotes breast development in pregnancy (with estrogen and progesterone).
- Inhibits GnRH secretion (lactational amenorrhoea).
- Physiology
Class: protein (199 aa, 23 kDa). Receptor: PRLR — cytokine receptor/JAK2–STAT5. Half-life ≈20–50 min. Big prolactin–IgG complexes ("macroprolactin") are biologically inactive but cause spuriously high assay results.
- Regulation
Inhibited by dopamine (D2) — dominant control. Stimulated by suckling, TRH, estrogen, pregnancy, sleep, stress, exercise, nipple stimulation, VIP and serotonin. Cleared by the kidney (raised in renal failure).
Clinical significance
Hyperprolactinaemia
Causes: pregnancy and breastfeeding (physiological), drugs (dopamine antagonists, oestrogens, opiates), prolactinoma, stalk compression, primary hypothyroidism, renal failure, chest wall lesions. Women: galactorrhoea, oligo/amenorrhoea, infertility. Men: low libido, erectile dysfunction, gynaecomastia, and mass effect from macroadenomas. Levels >≈5000 mU/L usually indicate a prolactinoma.
Prolactin deficiency
Failure of postpartum lactation is often the first sign of Sheehan syndrome.
Compiled from standard anatomy and physiology references — see the Endocrine System sources. For education only; not medical advice.