What it is
Prolactin is a hormone made by the anterior pituitary gland, the pea-sized gland under the brain. UniProt's record for human prolactin (P01236) gives a 227-residue precursor: a 28-residue signal sequence that steers it out of the cell, then the 199-amino-acid hormone, folded and held by three disulfide bonds.
At 199 residues, prolactin is a protein, about five times the 40-residue line that US regulation uses to separate peptides from proteins (21 CFR 600.3). It is in this library because people arrive with the name from a blood test, and because it sits in the same family as growth hormone (somatropin) — UniProt classes both in the somatotropin/prolactin family. The peptide-versus-protein entry explains where the line falls.
A 2000 review in Physiological Reviews (Freeman et al., PMID 11015620) adds two things the name hides. Prolactin is not one molecule but "a multiplicity of posttranslational forms" — size variants, phosphorylated and glycosylated versions. And it is made not only in the pituitary but in the brain, the immune system, the uterus and the mammary gland.
What it does
UniProt's one-line function annotation is that prolactin acts primarily on the mammary gland by promoting lactation. The Freeman review describes the classic trigger as the suckling stimulus, then lists others that can raise it: light, sound, smell and stress.
What makes prolactin unusual among pituitary hormones is how it is controlled. The review states that dopamine of hypothalamic origin provides inhibitory control over its secretion: the brain mostly holds prolactin down, rather than switching it on. That is why the medicines used to lower a high prolactin, cabergoline and bromocriptine, are dopamine agonists: they restore the brake. Releasing signals exist as well: two 1971 studies showed that synthetic TRH, the hypothalamic tripeptide best known for thyroid control, raises serum prolactin in people (Jacobs et al., J Clin Endocrinol Metab, PMID 5002718; Bowers et al., Biochem Biophys Res Commun, PMID 5000704).
| What is described | How firm |
|---|---|
| Promotes milk production | Human physiology; UniProt annotation |
| Held down by hypothalamic dopamine | Human pharmacology; stated in the 2000 review |
| Made outside the pituitary (brain, immune cells, uterus) | Reported in the 2000 review |
| Diet as a way to lower it | No controlled human trial found |
The blood test, and why it can mislead
Most readers meet prolactin as a number. MedlinePlus's encyclopedia page (review date 2024-07-21) lists these "normal values", adding that ranges vary between laboratories:
| Group | MedlinePlus upper value |
|---|---|
| Men | less than 20 ng/mL |
| Nonpregnant women | less than 25 ng/mL |
| Pregnant women | less than 500 ng/mL |
The causes of a high result MedlinePlus lists are: a prolactinoma (a benign prolactin-making pituitary tumour, which its test page calls the most common cause), other pituitary or hypothalamic disease, hypothyroidism, kidney disease, macroprolactin, and medicines — antidepressants, butyrophenones, estrogens, H2 blockers, metoclopramide, opioids, phenothiazines, risperidone and verapamil — plus marijuana products. It notes that stress and other factors can raise the level temporarily and that a high result may be repeated in the early morning after fasting.
Macroprolactin is the trap. Some prolactin circulates bound into a large complex that laboratory tests still count in the total. A 2026 systematic review in the Journal of Clinical Endocrinology & Metabolism (Yadav, Hamrahian and Salvatori, PMID 42324990) pooled 45 studies covering 2,853 confirmed cases among 21,413 people screened for high prolactin. In those cases the median total prolactin was 61.4 ng/mL, while the median monomeric prolactin left after removing the bound form was 11.7 ng/mL. The authors' conclusion is that the post-removal value, not the percentage recovered, best separates macroprolactin alone from a true excess. The Endocrine Society's 2011 guideline on diagnosing and treating high prolactin (Melmed et al., PMID 21296991) is the standing clinical reference for that work-up.
What a reading means for any one person is for the clinician who ordered it. This entry reports what the sources list; it does not interpret results, and a medicine on the list above is not a reason to stop taking it.
Where it stands
- No prolactin drug exists. The medicines used when prolactin is too high — cabergoline and bromocriptine — are dopamine-receptor agonists, small molecules rather than peptides. They work by restoring the brain's brake.
- Published record: PubMed returns 57,363 records for prolactin, 1,732 tagged as randomised controlled trials; macroprolactin alone has 282 (all 2026-10-20). ClinicalTrials.gov lists 746 studies that mention prolactin, most measuring it as an outcome rather than testing it as a treatment.
- Diet: no controlled human trial found that tests a diet as a way to lower prolactin.
Related reading
Oxytocin is the other hormone of breastfeeding — prolactin makes the milk, oxytocin releases it. Procalcitonin is the library's other entry on a molecule most people meet as a lab result, and somatropin is its closest relative by structure.