prolactin
Prolactin is the hormone of milk-making. After childbirth, it drives the breast's mammary glands to produce milk, and it is the reason the body can feed a newborn. Its name literally means “for lactation.”
Prolactin is unusual among pituitary hormones in how it is controlled. Most anterior pituitary hormones are mainly switched on by their releasing factors, but prolactin is mainly held in check. The hypothalamus continually sends dopamine down the portal system to suppress prolactin; only when that brake is lifted — or when stimuli like nipple suckling override it — does prolactin rise.
Because of this dopamine brake, anything that interrupts the hypothalamic signal raises prolactin. A pituitary stalk injury, or drugs that block dopamine (such as some antipsychotics and anti-nausea medicines), can cause high prolactin, leading to unexpected milk production, irregular or absent periods, and reduced fertility.
The most common hormone-secreting pituitary tumor, a prolactinoma, oversecretes prolactin. Fittingly, it is usually treated not with surgery but with dopamine-agonist drugs that mimic the natural brake and shrink the tumor.
A young woman with no recent pregnancy develops milky nipple discharge and stops menstruating; blood tests show high prolactin, and an MRI reveals a small prolactinoma.
High prolactin both stimulates milk production and suppresses the reproductive axis.
Prolactin is unique in being under predominantly inhibitory (dopamine) control. This is why losing the hypothalamic signal makes prolactin go up, while other pituitary hormones go down.