Final answer:
Hyperpituitarism is most commonly caused by a prolactinoma - a tumor of the pituitary gland. This can lead to excessive production of prolactin resulting in various symptoms.
Step-by-step explanation:
Hyperpituitarism is most commonly caused by a prolactinoma, which is a benign tumor of the pituitary gland that leads to the hypersecretion of prolactin. Hypersecretion by an endocrine gland like the pituitary often results from such tumors. When these tumors cause excessive production of prolactin in the pituitary gland, this can lead to symptoms such as amenorrhea, galactorrhea, and breast enlargement.
While autoimmune disorders, congenital defects, and Sheehan syndrome can also affect the pituitary gland, they are less commonly the cause of hyperpituitarism compared to a prolactinoma. It is key to understand that hypersecretion of pituitary hormones like prolactin and growth hormone is often tumor-induced, and the adrenal glands may also suffer from disorders like Cushing's syndrome when there is an overproduction of hormones due to similar causes.