Clinical meaning
Hypothyroidism results from insufficient production of thyroid hormones (T4 and T3) by the thyroid gland, affecting virtually every organ system. The hypothalamic-pituitary-thyroid (HPT) axis regulates thyroid hormone production: the hypothalamus secretes thyrotropin-releasing hormone (TRH), which stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH). TSH binds to TSH receptors on thyroid follicular cells, activating the sodium-iodide symporter (NIS) to trap iodide, which is then organified by thyroid peroxidase (TPO) and coupled to thyroglobulin to form T4 (thyroxine, ~80% of production) and T3 (triiodothyronine, ~20%). T4 is a prohormone converted to active T3 by type 1 and type 2 deiodinases in peripheral tissues (liver, kidneys, muscle). T3 binds nuclear thyroid hormone receptors (TR-α and TR-β) to regulate gene transcription controlling basal metabolic rate, thermogenesis, protein synthesis, lipid metabolism, cardiac contractility, and CNS development. In primary hypothyroidism (95% of cases), the thyroid gland itself is impaired — most commonly from Hashimoto thyroiditis (chronic lymphocytic thyroiditis), an autoimmune condition where anti-TPO antibodies and anti-thyroglobulin antibodies mediate lymphocytic infiltration and progressive destruction of thyroid follicular cells, leading to fibrosis and goiter formation (initially) followed...