Hypothyroidism

Abstract

Hypothyroidism or thyroid insufficiefncy is the most common endocrinopathy, affecting 2% of women and 0.2% of men (1-3). In children, its population prevalence is about 0.15%, with a 2.8:1 female tomale ratio (4). Optimal levels of thyroid hormone are required for normal neurodevelopment and growth. By maintaining an appropriate index of suspicion, the clinician can often recognize hypothyroidism in its early stages. The production and secretion of thyroid hormone is regulated by the hypothalamic-pituitary-thyroid axis. The hypothalamus secretes the tripeptide thyrotropinreleasing hormone (TRH), which stimulates the thyrotrophs of the pituitary to secrete thyrotropin. Thyrotropin, also called thyroid-stimulating hormone (TSH), stimulates thyroid growth and the synthesis and secretion of thyroid hormone, primarily as thyroxine and to a lesser extent triiodothyronine. Thyroxine (T4) is converted into the biologically more active 3,5,30-triiodothyronine (T3) by outer ring deiodinases (ORDs) expressed in the peripheral tissues (5,6). Both thyroid hormones, thyroxine and triiodothyronine, exert negative feedback on the hypothalamus and pituitary (Fig. 1).