Prolactin is a hormone produced by the pituitary gland whose primary function is to stimulate breast milk production after childbirth. However, prolactin is also present in non-pregnant individuals of all sexes, where it plays roles in immune function, metabolism, and the regulation of reproductive hormones.

Elevated prolactin outside of pregnancy and breastfeeding (hyperprolactinaemia) is one of the most common hormonal causes of menstrual irregularity, absent periods, infertility, and low libido in women. In men, it can cause reduced testosterone, low libido, erectile dysfunction, and gynaecomastia. Causes include pituitary tumours (prolactinomas), hypothyroidism, certain medications (antipsychotics, antiemetics), and stress from the blood draw itself.