Prolactin is secreted by the pituitary cells called lactotrophs and causes milk secretion from the breast after oestrogen and progesterone priming during pregnancy. Prolactin also inhibits the secretion of gonadotrophins (the hormone responsible for pituitary secretion of FSH and LH). Therefore, some patients with prolactin-secreting tumours can present with irregular cycles, or infertility, or low testosterone levels. Its secretion is increased by exercise, surgical and psychological stresses, and stimulation of the nipple. Secretion is increased during pregnancy, reaching a peak at the time of childbirth. Interestingly, prolactin is under an inhibitory control, i.e., dopamine. This factor secreted by the hypothalamus inhibits the secretion of prolactin. Hence, if due to a tumour the pituitary stalk is compressed, prolactin levels rise as the inhibitory signal is removed.