Prolactin

Prolactin is a hormone made by the pituitary gland, the pea-sized gland at the base of the brain, best known for its role in producing breast milk. Estrogen and some anti-androgens, especially [cyproterone](/glossary/cyproterone-acetate), can raise it, so it is often on the blood test list for people on feminizing hormones.

Where you'll meet it

On feminizing blood results, sometimes flagged in bold as high. It is most often checked in people taking cyproterone acetate, which is more strongly linked to raised prolactin than the other anti-androgens, and less consistently in people on estrogen with spironolactone, estrogen alone or a GnRH agonist. It rarely comes up on masculinizing hormones.

What it does

In everyone, prolactin plays a part in breast tissue, the immune system and reproduction. Its best-known job is milk production after pregnancy. Levels are normally held down by dopamine from the brain; estrogen loosens that brake a little, which is why levels tend to drift up on feminizing hormones, as they do during pregnancy.

Why it is checked

A modest rise is common on feminizing hormones and usually means nothing. Prescribers are watching for a level that keeps climbing, because that can occasionally point to a prolactinoma, a benign growth on the pituitary gland. These are uncommon, most are small, and most are treated with tablets rather than surgery. They are the reason the number is watched rather than ignored.

Other things raise prolactin too: stress, a recent meal or exercise, nipple stimulation, some antidepressants and antipsychotics, some anti-sickness medicines, an underactive thyroid and kidney problems. A single high result is often repeated before anyone draws a conclusion, sometimes with the blood taken at rest in the morning.

How it is measured

It is a routine blood test, usually done alongside your hormone levels. Labs report it in different units depending on the country, so, as with estradiol, check the units before comparing your number with anyone else's. The lab's reference range is based on cis people, so a flag is a prompt to ask, not a verdict. How to read your hormone blood results explains why the flags so often look wrong on hormones. How often it is checked depends on your plan; blood tests and monitoring sets out the usual schedule.

Signs worth mentioning

New or persistent headaches, changes in your vision, especially at the edges, or milky discharge from the nipples. None of these means something is wrong, but they are worth raising with your prescriber rather than waiting for the next routine test. Feminizing risks covers these alongside the other side effects.

What happens if it stays high

Usually a repeat test first. If it is persistently raised, a prescriber may look again at the anti-androgen (a lower dose of cyproterone, or a different medicine), review your other medicines, and sometimes refer you to an endocrinologist or arrange a pituitary scan. Do not stop or change anything yourself on the strength of one result.

Common questions

"High prolactin means a tumour." Almost always it does not. It is a reason to repeat the test and keep watching.

"Prolactin helps breast growth, so higher is better." There is no good evidence for that, and nobody should be aiming to raise it.

What to ask

Ask whether prolactin is on your panel, what level would make your prescriber act, and whether your anti-androgen is the likely reason for any rise.

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