Prolactin: A hormone secreted by the pituitary gland. Prolactin stimulates lactation (milk production). It also has many other functions, including essential roles in the maintenance of the immune system.

Abnormally high prolactin can delay puberty, interfere with ovulation in women, decrease libido in men, and decrease fertility. Elevated prolactin (hyperprolactinemia) may be due to a benign tumor in the pituitary gland called a prolactinoma.

Abnormally low prolactin (hypoprolactinemia or prolactin deficiency) can cause menstrual disorders and lead to inadequate lactation. Low prolactin is due to impaired production by the cells in the anterior pituitary gland that secrete prolactin.