Thyroid Physiology and Thyroid Function Testing

Abstract Thyroid dysfunction is common and occurs two to three times more often in women than men. 1 , 2 , 3 In iodine-sufficient regions, subclinical and overt hypothyroidism occurs in up to 7% whereas subclinical and overt hyperthyroidism occurs in 0.2% to 1.3% of the general population. 4 Both hypothyroidism and hyperthyroidism can have variable clinical presentations; thus biochemical testing and, when indicated, radiologic imaging should be pursued to confirm suspected disease. This chapter reviews the fundamentals of thyroid hormone synthesis, metabolism, and action, which are then applied to the evaluation and treatment of hypothyroidism and hyperthyroidism, including in pregnancy and nonthyroidal illness.