What Is Prolactin? The Pituitary Hormone Kept Under a Brake

Peptide science

Prolactin is the one anterior pituitary hormone that the brain spends all day holding back rather than pushing out. Every other hormone from that gland waits for a releasing signal from the hypothalamus. Prolactin is the reverse: the lactotroph cells that make it will secrete continuously unless dopamine tells them not to. That single inversion explains most of what is strange about prolactin — why a tumour that makes no prolactin at all can still raise the level, why a long list of ordinary medicines cause hyperprolactinaemia, and why the standard treatment for a prolactin-secreting tumour is a drug that mimics dopamine.

What is prolactin?

Prolactin is a protein hormone rather than a short peptide. The gene (PRL, on chromosome 6p22.3) encodes a 227-residue precursor; after the 28-residue signal sequence is removed, the mature circulating protein is 199 amino acids and roughly 23 kilodaltons, folded into a four-helix bundle and stapled by three disulfide bonds (UniProt P01236).

Structurally it belongs to the somatotropin/prolactin family, which also contains growth hormone and the placental lactogens. The three descend from a common ancestral gene and share the same four-helix architecture even though their amino-acid sequences have drifted a long way apart. If you want the broader context for how a molecule this size differs from the short chains most people mean by the word “peptide”, start with what is a peptide and what is a hormone.

Prolactin is not made only in the pituitary. The prolactin gene carries two tissue-specific promoters, and extrapituitary production has been documented in the central nervous system, the immune system, the uterus — including the decidua in pregnancy, where synthesis is independent of dopamine control — and the mammary gland itself.

The hormone that is switched off, not switched on

Dopamine neurons of the arcuate nucleus project down the pituitary stalk and release dopamine continuously onto lactotroph D2 receptors. That tonic inhibition is the default state. Prolactin also closes its own loop: circulating prolactin stimulates those same dopamine neurons, which is a short-loop negative feedback rather than the usual endocrine feedback through a target gland. There is no agreed single prolactin-releasing hormone; thyrotropin-releasing hormone, vasoactive intestinal peptide, serotonin, histamine, oxytocin and oestrogen can all act as releasing factors, but none has been established as the physiological one. Compare this with the tidy releasing-hormone architecture of the HPA axis or the HPT axis and the oddity stands out.

Two consequences follow directly. Suckling raises prolactin by cutting dopamine release, not by adding a stimulus. And any mass or lesion that compresses the pituitary stalk raises prolactin simply by preventing dopamine from arriving — the so-called stalk effect, which is why a modestly raised prolactin alongside a large pituitary tumour does not necessarily mean the tumour makes prolactin.

How prolactin signals

The prolactin receptor is a type I cytokine receptor with no kinase domain of its own. One face of the hormone binds a first receptor molecule, then a second receptor clamps onto the opposite face; that ligand-induced dimerisation brings two molecules of the tyrosine kinase JAK2 together, and the dominant downstream route is JAK2 to STAT5. Ras/Raf/MAP kinase, phosphoinositide 3-kinase and Src-family kinases are also engaged. Receptor expression is close to ubiquitous — a 1998 review observed that it was difficult to find a tissue that expresses no prolactin receptor at all.

What prolactin actually does

The obvious answer is lactation, and that is well established: prolactin drives the growth of mammary alveoli and pushes alveolar epithelial cells to synthesise lactose, casein and milk lipids. Receptor knockout work in mice is unambiguous — heterozygous females fail to lactate after a first pregnancy and homozygous females are infertile.

The less obvious answer is that prolactin does a great deal more. A landmark 1998 review counted more than 300 separate reported actions of prolactin across vertebrates, spanning water and salt balance, growth and development, metabolism, brain and behaviour, reproduction, and immune regulation. Two caveats are worth keeping in mind: those are actions reported across many species, not 300 proven human functions, and prolactin’s role in freshwater osmoregulation in fish is a whole separate literature that does not transfer directly to people.

• Reproduction. High prolactin suppresses gonadotropin-releasing hormone output and reduces kisspeptin input, which is the basis of lactational amenorrhoea — a natural spacing mechanism between pregnancies. See the HPG axis and kisspeptin for the upstream machinery.
• Behaviour. Prolactin is implicated in the onset of nurturing and maternal behaviour, a role it shares in part with oxytocin.
• Bone. In vitro work suggests prolactin excess increases osteoblast RANKL expression and reduces osteoprotegerin, which fits the reduced bone mineral density seen in chronic hyperprolactinaemia.
• Immunity and metabolism. Prolactin acts as a growth regulator for several tissues including cells of the immune system, and has documented metabolic effects.

Reading a prolactin result: two ways it misleads

About 85 per cent of circulating prolactin is the ordinary monomer. The rest is a covalent dimer (“big prolactin”) and a much larger polymeric form, which is often prolactin complexed with IgG autoantibodies. When those larger forms predominate, the sample is described as macroprolactinaemic. Macroprolactin is much less bioactive, but many commercial assays count it anyway, so the number comes back high while the person may be entirely asymptomatic. Retrospective analyses have put the proportion of hyperprolactinaemic samples affected at roughly 20 to 40 per cent depending on the series. Polyethylene glycol precipitation is the inexpensive screen; chromatography confirms.

The opposite artefact is the high-dose hook effect. In a very large prolactinoma, prolactin can be abundant enough to saturate both the capture and the signal antibody in a sandwich immunoassay, so less signal is generated, not more. The clinical trap is specific and well described: a genuinely enormous prolactin can be reported as only mildly raised, and the tumour is misclassified as non-functioning. The fix is to repeat the measurement on a diluted sample — the Endocrine Society recommends serial dilution whenever a very large tumour is paired with an unimpressive prolactin. Modern assay generations are far more resistant to this, but the check remains standard.

When prolactin is high

Causes fall into a few groups. Physiological: pregnancy, lactation, sleep, exercise, coitus, stress — including the stress of the venipuncture itself. Structural: prolactinoma, or the stalk effect from any other parasellar mass, granuloma, infiltration, trauma or irradiation. Systemic: chronic renal failure (roughly a third of patients, through reduced clearance and increased production), hypothyroidism, cirrhosis, chest wall trauma or shingles, seizures, polycystic ovary syndrome. And pharmacological, which is the most common non-tumoral cause by a wide margin.

Drug-induced hyperprolactinaemia is mostly the D2 story running in reverse: block the dopamine receptor and the brake comes off. Among people taking typical antipsychotics, 40 to 90 per cent have raised prolactin, and for risperidone the figure is 50 to 100 per cent. Most drug-induced elevations sit between 25 and 100 micrograms per litre, but metoclopramide, risperidone and phenothiazines can push values past 200.

Symptomatically, women typically present with menstrual disturbance, infertility or galactorrhoea; men, in whom galactorrhoea is uncommon, more often present late with low libido, erectile dysfunction and the mass effects of a larger tumour. Guideline practice is notably conservative about treating numbers rather than people: the Endocrine Society suggests not treating asymptomatic medication-induced hyperprolactinaemia and not treating asymptomatic microprolactinomas, while recommending dopamine agonist therapy — cabergoline in preference to bromocriptine — for symptomatic prolactin-secreting adenomas.

Frequently asked questions

Is prolactin a peptide?

It is a protein hormone. At 199 amino acids and about 23 kilodaltons, with three disulfide bonds and a four-helix bundle fold, it is an order of magnitude larger than the short chains usually described as peptides — though it is made and processed by the same secretory machinery.

Why is there no prolactin-releasing hormone?

Because prolactin does not need one. Lactotrophs secrete by default and the hypothalamus governs them by varying how hard it applies the dopamine brake. Several molecules can act as releasing factors, but none has been established as the physiological one.

Does a high prolactin mean a pituitary tumour?

Not on its own. Medication is the most common non-tumoral cause, and pregnancy, hypothyroidism, renal failure, chest wall stimulation and macroprolactinaemia all raise the measured value. A single elevated result is a starting point, not a diagnosis.

Can galactorrhoea occur with a normal prolactin?

Yes — roughly half of women with galactorrhoea have a normal prolactin level. The two findings correlate loosely rather than tightly.

References

Informational only — not medical advice. Nothing here is a treatment recommendation or a dose for any person; prescribing information for linaclotide and plecanatide is linked above and should be read in full by anyone with a clinical question. Consult a qualified healthcare professional about your own care. Intended for readers 21 and over.