hormone deficiency
If hormone excess is a furnace stuck on, hormone deficiency is a furnace that won't light: the body asks for a hormone and not enough arrives, so the processes that hormone controls slow down or stall. Because hormones run things as basic as metabolism, growth, and salt balance, a deficiency can be felt throughout the body.
Deficiency can fail at any point in the chain. The gland itself may be damaged — by autoimmune attack, surgery, radiation, or simply running out of cells (primary deficiency). Or the command from above may be missing: if the pituitary or hypothalamus stops sending its signaling hormone, a healthy gland never gets the order to produce (central or secondary deficiency). Telling these apart matters, because the missing hormone and the level just upstream of it point to where the break is.
The good news is that most hormone deficiencies are among the most treatable conditions in medicine, because the hormone can often be replaced directly. A person with an underactive thyroid takes levothyroxine; someone whose adrenal cortex has failed takes glucocorticoid; a child lacking growth hormone can receive it by injection. Replacement aims to restore normal levels and let the body's own feedback resume as much as possible.
Hashimoto's thyroiditis slowly destroys thyroid tissue, leaving too little thyroid hormone; the deficiency brings fatigue, cold intolerance, and weight gain, and is corrected with a daily levothyroxine tablet.
A deficiency replaced by a single daily pill.
Distinguishing primary from central deficiency uses the upstream signal: in primary thyroid failure, TSH climbs as the pituitary shouts at a deaf gland; in central failure, TSH is low or inappropriately normal because the shout itself is missing.