Category: endocrine_axis
Thyroid hormone homeostasis loop. Hypothalamic TRH → anterior pituitary TSH → thyroid follicle cells → T4 (primary secretory product, ~80% of output) + T3 (~20%; the active hormone at TR-α/β). Peripheral conversion of T4 → T3 by deiodinases (D1 liver/kidney, D2 brain/pituitary, D3 inactivator). Negative feedback: T3 (intracellular pituitary) suppresses TSH. Therapeutic Rx: levothyroxine (T4) — long t½ (~7 d) allows daily dosing; liothyronine (T3) used selectively. Methimazole / PTU block thyroid peroxidase (organification step). Amiodarone disrupts thyroid status in 2-3 ways (iodine load + D1 inhibition + direct thyroiditis).