High prolactin levels, or hyperprolactinemia, can be caused by several physiological, pathological, and pharmacological factors. The most common cause is a prolactinoma, a benign tumor of the pituitary gland that produces excess prolactin. Other causes include hypothyroidism (low thyroid hormone), chronic kidney disease, certain medications (such as antipsychotics, antidepressants, opioids, and antihypertensives), stress, pregnancy, nipple stimulation, and physical or psychological factors. Additionally, conditions that compress the pituitary stalk or interfere with dopamine signaling (which normally inhibits prolactin secretion) can elevate prolactin levels. Some systemic diseases, infiltrative disorders, and chest wall irritation may also lead to high prolactin. In many cases, no clear cause is found.