Prolactin 

Prolactin is a polypeptide hormone that's synthesized and secreted from the specialized cells of the anterior pituitary gland, the lactotrophs. Prolactin stimulates breast growth, milk output in adult females and the ovaries to regulate the release of luteinizing hormone and follicle-stimulating hormone. 

It also has many other functions, including essential roles in the maintenance of the immune system. It's a hormone created by the anterior pituitary gland, a pea-sized organ found at the base of the brain. Prolactin levels are naturally higher during sleep.

Prolactin levels are modulated by dopamine (a brain chemical), and the hormone is ordinarily present in small quantities in adult males and non-pregnant women. Dopamine , secreted by the hypothalamus and delivered via the hypothalamic-hypophysial portal system, is the primary constituent of prolactin inhibiting factor.

The prolactin test measures the amount of prolactin in the blood, there is no known normal function of prolactin in men.