Chapter 47
Thyroid Disease
Too little or too much thyroid hormone
Thyroid Disease Overview
Thyroid disease consists of disorders affecting the production or secretion of thyroid hormones, resulting in an altered metabolic state. Too little hormone produces hypothyroidism; too much produces hyperthyroidism. The thyroid synthesizes, stores, and secretes thyroxine (T4) and triiodothyronine (T3), which regulate growth, development, and metabolic rate.
T4 is made only by the thyroid; T3 is made by the thyroid AND by peripheral conversion (deiodination) of T4 in the pituitary, liver, and kidneys (about 80% of T3). Only free (unbound) hormone is active; most is bound to thyroxine-binding globulin (TBG). The thyroid also secretes calcitonin (lowers blood calcium).
The Feedback Loop
Hypothalamic TRH -> pituitary TSH -> thyroid T4/T3. Rising free hormone shuts off TSH (negative feedback); falling hormone raises TSH. This is why TSH is the most sensitive screening test.
📖 “Thyroid disease: altered hormone production. Hypothyroidism (too little) vs hyperthyroidism (too much). T4 only from thyroid; T3 also from peripheral conversion (~80%). Only free hormone active; most bound to TBG. TRH -> TSH -> T4/T3 with negative feedback. Calcitonin lowers calcium.”