Prolactin is a hormone that is primarily involved in the production of breast milk by nursing mothers. It is one of the few examples in the endocrine system where the production of the hormone properly produces more of the hormone, allowing a nursing mother to produce more milk if the demand increases.
However, prolactin is also involved in the feeling of sexual satisfaction after orgasm as it blocks the effect of dopamine, which is responsible for sexual arousal. Very high levels of prolactin can cause impotence in males and lack of sexual desire in both sexes.
Doctors test for prolactin on a routine basis when trying to deal with sexual dysfunction. However, it can also be relevant to distinguishing different types of seizures. Decreased prolactin can also be a sign of bulemia.