Hypothalamic–pituitary–thyroid axis Hypothalamic–pituitary–thyroid axis The hypothalamic–pituitary–thyroid axis (HPT axis for short, a.k.a. thyroid homeostasis or thyrotropic feedback control) is part of the neuroendocrine system responsible for the regulation of metabolism. As its name suggests, it depends upon the hypothalamus, the pituitary gland, and the thyroid gland. The hypothalamus senses low circulating levels of thyroid hormone (Triiodothyronine (T3) and Thyroxine (T4)) and responds by releasing thyrotropin-releasing hormone (TRH). The TRH stimulates the anterior pituitary to produce thyroid-stimulating hormone (TSH). The TSH, in turn, stimulates the thyroid to produce thyroid hormone until levels in the blood return to normal. Thyroid hormone exerts negative feedback
Pituitary adenoma presented above. Tumors which cause visual difficulty are likely to be a "macroadenoma" greater than 10 mm in diameter; tumors less than 10 mm are "microadenoma". The differential diagnosis includes pituitary tuberculoma, especially in developing countries and in immumocompromised patients. The diagnosis is confirmed by testing hormone levels, and by radiographic imaging of the pituitary (for example, by CT scan or MRI). Unlike tumors of the posterior Pituitary, Pituitary adenomas are classified as endocrine tumors (not brain tumors). Pituitary adenomas are classified based upon anatomical, histological and functional criteria. Pituitary incidentalomas are pituitary tumors that are characterized as an incidental