The Prolactin (PRL) test measures the level of prolactin in the serum, a hormone produced by the pituitary gland that plays a key role in lactation and reproductive health. Elevated prolactin levels can indicate various conditions, such as prolactinoma, hypothyroidism, or other endocrine disorders. This test is often used to evaluate abnormal menstrual cycles, infertility, and galactorrhea (unexpected milk production) in both women and men.