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 apoplexy structures such as the optic nerve and the contents of the cavernous sinus are compressed. The raised pressure further impairs the blood supply to the pituitary hormone-producing tissue, leading to tissue death due to insufficient blood supply. It is recommended that magnetic resonance imaging (MRI) scan of the pituitary gland is performed if the diagnosis is suspected; this has a sensitivity of over 90% for detecting pituitary apoplexy; it may demonstrate infarction (tissue damage due to a decreased blood supply) or hemorrhage. Different MRI sequences can be used to establish when the apoplexy occurred, and the predominant form of damage