Prolactin’s physiological actions include breast development during pregnancy, milk production, and a decrease in GnRH for several months after pregnancy which causes a subsequent decrease in FSH and LH. This endocrine cascade following pregnancy causes the suppression of ovarian function in nursing women.
Some common causes of hyperprolactinemia are lactotroph adenomas, renal failure (delayed clearance of PRL), polycystic ovarian disease (estrogen causes hyperplasia of PRL secreting cells), primary hypothyroidism (TRH can act as a PRL releasing signal), and drugs or processes that decrease dopamine (recall that dopamine is also called prolactin inhibiting hormone).
The elevation of PRL depresses GnRH and thus decreases LH and FSH. Because of changes in these hormone levels, women with a lactotroph adenoma may get amenorrhea (no periods), galactorrhea (milky nipple discharge unrelated to breastfeeding), vaginal dryness, hirsutism (male-patterned hair growth), decreased libido (sex drive), and often have infertility issues. As mentioned, men with a lactotroph adenoma experience vision issues or headaches more often because diagnosis typically occurs after the tumor has enlarged to the point of compressing the optic chiasma. Men may also have erectile dysfunction and loss of libido, but infertility and galactorrhea are rare symptoms.
When diagnosing a patient with a lactotroph adenoma, it is important to measure levels of the thyroid endocrine axis due to the fact that the high levels of TRH can sometimes cause symptoms of hyperprolactinemia. This is because excessive TRH can stimulate the release of PRL. Normally, the thyroid hormones T3 and T4 work through negative feedback to inhibit TRH and TSH. If a person has low T3 and T4 as in hypothyroidism, then TRH and TSH are not inhibited by the negative feedback and their levels increase. Because the high levels of TRH can lead to hyperprolactinemia, it is best to do a simple blood test to check for thyroid dysfunction as an explanation of excessive PRL before doing expensive MRI tests on the brain to look for lactotroph adenomas.
The treatments of choice for a lactotroph adenoma are usually dopamine agonists like bromocriptine (Parlodel) and cabergoline (Dostinex) which inhibit the release of PRL. These medications can restore sexual function and shrink the size of the tumor to normalize prolactin levels. For more complicated cases, transsphenoidal surgery for removal of the tumor is an option.