Prolactin is a single-chain polypeptide hormone secreted by the anterior pituitary under the control of prolactin-releasing factors. These inhibiting and releasing factors are secreted by the hypothalamus. Prolactin is also synthesized by the placenta and is present in amniotic fluid. Prolactin initiates and maintains lactation in females. It also plays a role in regulating gonadal function in both males and females. In adults, basal circulating prolactin is present in concentrations up to 30 ng/mL. During pregnancy and postpartum lactation, serum prolactin can increase 10- to 20-fold. Exercise, stress, and sleep also cause transient increases in prolactin levels. Consistently elevated serum prolactin levels (>30 ng/mL), in the absence of pregnancy and postpartum lactation, are indicative of hyperprolactinemia, which is the most common hypothalamic-pituitary dysfunction encountered in clinical endocrinology. Hyperprolactinemia often results in galactorrhea, amenorrhea, and infertility in females, and in impotence and hypogonadism in males. Renal failure, hypothyroidism, and prolactin-secreting pituitary adenomas are also common causes of abnormally elevated prolactin levels.