PROLACTIN: A MILK-PRODUCING HORMONE. Prolactin is a hormone secreted by the pituitary gland which is located at the base of the brain. It circulates in low levels in the bloodstream of non pregnant women.During pregnancy, prolactin levels increase approximately ten-fold and stimulate milk formation.rolactin is a hormone secreted by the pituitary gland which is located at the base of the brain. It circulates in low levels in the bloodstream of non pregnant women.