prolactinoma
A prolactinoma is a benign tumor of the pituitary gland whose cells secrete prolactin — the hormone that normally drives milk production after childbirth — but they do so constantly and out of context. It is the most common hormone-secreting pituitary tumor, and a clear example of how a small, non-cancerous growth can cause big effects through a single hormone.
Two threads of symptoms follow. The prolactin itself disrupts the reproductive axis: by suppressing the signals that drive the ovaries or testes, it causes irregular or absent periods and infertility in women, low testosterone and reduced libido in men, and milk-like breast discharge in either. Separately, if the tumor grows large, it can press on the optic nerves crossing just above the pituitary, narrowing the field of vision from the sides inward.
Prolactinomas are notable for being one of the few tumors usually treated with pills rather than surgery. The brain naturally restrains prolactin with dopamine, so drugs that mimic dopamine can both shrink the tumor and switch off its excess secretion, often restoring fertility. Surgery is reserved for the minority that do not respond or that threaten vision.
A young woman whose periods have stopped and who notices milk leakage despite never being pregnant is found to have a high prolactin and a small pituitary tumor; a dopamine-mimicking pill normalizes her prolactin and restores her cycles.
A pituitary tumor often cured with a pill.
Prolactin is unusual because the hypothalamus controls it mainly by inhibition, through dopamine; that is why any condition that lowers dopamine — including some medications — can raise prolactin and mimic a prolactinoma.