Chapter 32: Thyroid Gland Physiology & Hormone Effects

Thyroid Gland Physiology & Hormone Effects summary details the physiology of the thyroid gland, a critical endocrine organ responsible for regulating basal metabolic rate, growth, and neural development. The text begins by outlining the functional anatomy of the thyroid follicles, where follicular cells utilize the sodium-iodide symporter (NIS) and pendrin to transport iodide for the synthesis of thyroid hormones. It explains the intricate process of organification and coupling catalyzed by thyroid peroxidase (TPO) within the thyroglobulin precursor to form thyroxine (T4) and triiodothyronine (T3). Key concepts covered include the storage of hormones in the colloid, their release via pinocytosis and proteolysis, and their transport in the blood primarily bound to thyroxine-binding globulin (TBG). The summary elucidates the peripheral metabolism of these hormones, describing how deiodinase enzymes (D1, D2, and D3) activate T4 into the potent T3 or inactivate it into reverse T3 (rT3), a balance significantly altered by fasting and illness (euthyroid sick syndrome). Mechanistically, the text describes how T3 binds to nuclear receptors (TR) that heterodimerize with retinoid X receptors (RXR) to modulate gene transcription at thyroid hormone response elements (TREs). Physiological discussions focus on the hormone's thermogenic action on oxidative phosphorylation and uncoupling proteins, its synergy with growth hormone for skeletal development, and its indispensable role in fetal brain maturation to prevent mental retardation. Finally, the chapter contrasts the pathophysiology of hyperthyroidism, such as Graves disease (mediated by TSH receptor antibodies), with hypothyroidism caused by Hashimoto thyroiditis or iodine deficiency, highlighting clinical presentations like goiter, myxedema, and weight fluctuations.