Overview
Hormones Function Through Axes and Feedback
Endocrine regulation is built for adjustment, not constancy.
Endocrine regulation is built for adjustment, not constancy. A hormone signal changes metabolism, fluid balance, growth, stress response, reproduction, or calcium balance; the resulting physiologic change then feeds information back to the control system. Many important signals follow a three-level pathway: - The hypothalamus releases a signalling hormone. - The anterior pituitary releases a tropic hormone. - A peripheral gland releases the final hormone that acts on tissues. The hypothalamic-pituitary-thyroid axis follows the sequence: thyrotropin-releasing hormone (TRH) → thyroid-stimulating hormone (TSH) → thyroxine (T4) and triiodothyronine (T3). The hypothalamic-pituitary-adrenal axis follows: corticotropin-releasing hormone (CRH) → adrenocorticotropic hormone (ACTH) → cortisol. Negative feedback is the braking system. When circulating T4 rises, TSH secretion should fall. When cortisol rises, ACTH secretion should fall. This relationship is why one laboratory result rarely stands alone: the meaningful pattern is the relationship between the pituitary signal and the peripheral hormone. Not every endocrine system uses the same axis. Insulin and glucagon respond directly to blood glucose. Parathyroid hormone responds mainly to serum calcium. Aldosterone is regulated chiefly by renal perfusion, the renin-angiotensin-aldosterone system, and serum...