Clinical meaning
The thyroid gland produces triiodothyronine (T3, active form) and thyroxine (T4, prohormone converted to T3 peripherally) through a series of iodine-dependent steps. The hypothalamic-pituitary-thyroid (HPT) axis regulates production: TRH from the hypothalamus stimulates TSH from the anterior pituitary, which stimulates the thyroid. Thyroid hormones regulate basal metabolic rate by increasing oxygen consumption, heat production, and cellular metabolism in virtually every tissue. Hypothyroidism (most commonly Hashimoto thyroiditis, an autoimmune destruction with anti-TPO and anti-thyroglobulin antibodies) causes decreased metabolic rate: bradycardia, weight gain, cold intolerance, constipation, fatigue, dry skin, and myxedema (non-pitting edema from glycosaminoglycan accumulation). Severe untreated hypothyroidism can progress to myxedema coma—a life-threatening emergency with hypothermia, altered mental status, hypoventilation, and cardiovascular collapse. Hyperthyroidism (most commonly Graves disease, with thyroid-stimulating immunoglobulins [TSI] mimicking TSH) causes increased metabolic rate: tachycardia, weight loss despite increased appetite, heat intolerance, diarrhea, tremor, and anxiety. Thyroid storm is the extreme manifestation: hyperthermia >104°F, delirium, tachycardia >140, heart failure, and high mortality without treatment.