Prolactin

Prolactin is a pituitary hormone best known for driving milk production, but it also helps regulate the male hormonal axis. In men it's usually checked when testosterone is low without an obvious cause, or when libido and erections are poor despite adequate testosterone. High prolactin can suppress the signals that drive the testes, so it's part of a thorough low-T workup.

A typical adult male reference range is roughly 4–15 ng/mL, with some variation by lab and assay. Values are usually reported in ng/mL on standard panels. Because prolactin rises transiently after sleep, meals, exercise, stress, sex, or nipple stimulation, a mildly elevated result is often repeated on a calm, rested morning draw before it's taken seriously.

Elevated prolactin can dampen the hypothalamic-pituitary-gonadal axis, lowering the drive to make testosterone, and it can blunt libido and erectile function even when testosterone itself looks fine. That's why a man with upper-range testosterone but stubbornly low libido may have his prolactin checked. Pathologic elevations — often cited as more than about twice the upper limit — can prompt pituitary imaging to look for a prolactinoma.

The most common pathologic cause is a benign pituitary tumor called a prolactinoma. Other drivers include hypothyroidism and certain medications — notably antipsychotics and opioids — as well as chronic stress and transient spikes from recent sex or nipple stimulation. Because the causes range from harmless and temporary to ones that need imaging and treatment, a persistently high result should be interpreted by a physician.

Macroprolactin is a large, largely inactive form of prolactin that some people carry in their blood. It can inflate a prolactin result on standard assays even though it isn't biologically meaningful, producing a high number in someone with no symptoms. When a high prolactin doesn't fit the clinical picture, labs can screen for macroprolactin so you and your physician don't chase an artifact.