TSH
TSH is the pituitary's instruction to the thyroid gland: “make and release thyroid hormone.” It is the central control knob of the body's metabolic thermostat, setting how much thyroid hormone the body produces.
TSH is a glycoprotein hormone secreted by thyrotroph cells of the anterior pituitary under the influence of hypothalamic TRH. It binds to TSH receptors on thyroid follicular cells, stimulating them to trap iodide, build thyroid hormone, and release T4 and T3 into the blood.
The system is tightly governed by negative feedback. When thyroid hormone levels in the blood rise, they suppress TSH; when they fall, TSH climbs to prod the thyroid harder. This inverse relationship is the reason a high TSH usually signals an underactive thyroid, and a low TSH an overactive one.
Because TSH responds so sensitively to small changes in thyroid hormone, it is the single most useful blood test for thyroid function. It is also the marker doctors track when adjusting thyroid hormone replacement, aiming to bring TSH back into its normal range.
A high TSH usually means the thyroid is underactive (the pituitary is pushing harder), and a low TSH usually means it is overactive — the opposite of what beginners often guess.