levels. For instance, many premenopausal women experiencing hyperprolactinemia do not experience galactorrhea and only some women who experience galactorrhea will be diagnosed with hyperprolactinemia. Thus, galactorrhea may be observed in individuals with normal prolactin levels and does not necessarily indicate hyperprolactinemia. This phenomenon is likely due to galactorrhea requiring adequate levels of progesterone or estrogen to prepare the breast tissue. Additionally, some women may also experience loss of libido and breast pain, particularly when prolactin levels rise initially, as the hormone promotes tissue changes in the breast. In men, the most common symptoms of hyperprolactinemia are decreased libido, sexual dysfunction, erectile dysfunction/impotence, infertility, and gynecomastia. Unlike women, men do not experience reliable indicators of elevated prolactin such as menstruation to prompt immediate medical consultation. As a result, the early signs of hyperprolactinemia are generally more difficult to detect and may go unnoticed until more severe symptoms are present. For instance, symptoms such as loss of libido and sexual dysfunction are subtle, arise gradually, and may falsely indicate a differential cause. Many men with pituitary tumor-associated hyperprolactinemia may forego clinical help until they begin to experience serious endocrine and vision complications, such as major headaches or eye problems. Long-term hyperprolactinaemia can lead to detrimental changes in bone metabolism as a result of hypoestrogenism and hypoandrogenism. Studies have shown that chronically elevated prolactin levels lead to increased bone resorption and suppression of bone formation, leading to reduced bone density, increased risk of fractures, and increased risk of osteoporosis. The chronic presence of hyperprolactinemia can lead to hypogonadism and osteolysis in men. Causes Prolactin secretion is regulated by both stimulatory and inhibitory mechanisms. Dopamine acts on pituitary lactotroph D2 receptors to inhibit prolactin secretion while other peptides and hormones, such as thyrotropin releasing hormone (TRH), stimulate prolactin secretion. As a result, hyperprolactinemia may be caused by disinhibition (e.g., compression of the pituitary stalk or reduced dopamine levels) or excess production. The most common cause of hyperprolactinemia is prolactinoma (a type of pituitary adenoma). A blood serum prolactin level of 1000–5000 mIU/L (47-235 ng/mL) may arise from either mechanism, however levels >5000 mIU/L (>235 ng/mL) is likely due to the activity of an adenoma. Prolactin blood levels are typically correlated to the size the tumors. Pituitary tumors smaller than 10 mm in diameter, or microadenomas, tend to have prolactin levels <200 ng/mL. Macroadenomas larger than 10 mm in diameter possess prolactin >1000 ng/mL. Hyperprolactinemia