Thyroid Dysfunction

Thyroid hormones influence metabolic rate, thermogenesis, cardiovascular function, gastrointestinal motility, neuromuscular activity and lipid metabolism. The hypothalamus releases thyrotropin-releasing hormone, the pituitary releases thyroid-stimulating hormone (TSH), and the thyroid produces primarily thyroxine (T4), which is converted in tissues to the more active triiodothyronine (T3). Circulating T4 and T3 normally suppress further TSH and thyrotropin-releasing hormone release through negative feedback. TSH is the pituitary’s signal to the thyroid, not a direct measurement of thyroid hormone action. In primary hypothyroidism, the thyroid gland cannot produce enough hormone; falling free T4 removes negative feedback, so TSH rises. In central hypothyroidism, pituitary or hypothalamic disease prevents an appropriate TSH response, so free T4 is low while TSH is low or inappropriately normal. A “normal” TSH therefore does not reassure when pituitary or hypothalamic disease is possible. Chronic autoimmune thyroiditis, commonly called Hashimoto thyroiditis, gradually damages thyroid follicles and commonly causes primary hypothyroidism. Graves disease is caused by antibodies that stimulate the TSH receptor, driving increased hormone synthesis. Both conditions may produce a diffuse goitre, but their physiology is opposite: Hashimoto is gland failure, whereas...