Pituitary Adenomas and Hypopituitarism,
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- The anterior pituitary gland secretes prolactin, growth hormone, and four trophic hormones, including corticotropin (ACTH), thyrotropin (TSH), and the gonadotropins, luteinizing hormone and follicle-stimulating hormone. Each trophic hormone stimulates a specific target gland.
- Anterior pituitary function is regulated by hypothalamic hormones that reach the pituitary via portal veins in the pituitary stalk. The predominant effect of hypothalamic regulation is to stimulate secretion of pituitary hormones, except for prolactin, which is inhibited by hypothalamic dopamine secretion.
- Secretion of trophic hormones is also regulated by negative feedback by their target gland hormone, and the normal pituitary response to target hormone deficiency is increased secretion of the appropriate trophic hormone.
- Anterior pituitary dysfunction can be caused by disorders of either the pituitary or hypothalamus.
- Pituitary adenomas are the most common pituitary disorder. They are classified by size and function.
- Microadenomas are <10 mm in diameter and cause clinical manifestations only if they produce excess hormone. They are too small to produce hypopituitarism or mass effects.
- Macroadenomas are >10 mm in diameter and may produce any combination of pituitary hormone excess, hypopituitarism, and mass effects (headache, visual field loss).
- Secretory adenomas produce prolactin, growth hormone, or ACTH.
- Nonsecretory macroadenomas may cause hypopituitarism or mass effects.
- Nonsecretory microadenomas are common incidental radiographic findings, seen in approximately 10% of the normal population, and do not require therapy.
- Other pituitary or hypothalamic disorders, such as head trauma, pituitary surgery or radiation, and postpartum pituitary infarction (Sheehan syndrome), may cause hypopituitarism. Other tumors of the pituitary or hypothalamus (e.g., craniopharyngioma, metastases) and inflammatory disorders (e.g., sarcoidosis, Langerhans cell histiocytosis, lymphocytic hypophysitis) may cause hypopituitarism or mass effects. Some immunomodulatory medications, most notably checkpoint inhibitors used for cancer treatment, can cause hypophysitis and hypopituitarism.