++
Iodine deficiency remains a common cause of hypothyroidism worldwide. In areas of iodine sufficiency, autoimmune disease (Hashimoto’s thyroiditis) and iatrogenic causes (treatment of hyperthyroidism) are most common (Table 383-1).
++
+++
CONGENITAL HYPOTHYROIDISM
++
Hypothyroidism occurs in about 1 in 2000–4000 newborns, and neonatal screening is performed in most industrialized countries. It may be transient, especially if the mother has thyroid-stimulating hormone (TSH) receptor (TSH-R)–blocking antibodies or has received antithyroid drugs, but permanent hypothyroidism occurs in the majority. The causes of neonatal hypothyroidism include thyroid gland dysgenesis in 65%, inborn errors of thyroid hormone synthesis in 30%, and TSH-R antibody mediated in 5% of affected newborns. The developmental abnormalities are twice as common in girls. Mutations that cause congenital hypothyroidism are being increasingly identified, but most remain idiopathic. These can be broadly categorized as mutations causing (1) central hypothyroidism because of abnormal hypothalamic-pituitary development or the loss of specific components of the thyrotropin-releasing hormone (TRH)/TSH hormonal pathways; (2) abnormal thyroid gland development or dysgenesis; or (3) abnormal thyroid hormone synthesis and processing, or dyshormonogenesis (Table 383-2). Transplacental passage of maternal thyroid hormone occurs before the fetal thyroid gland begins to function and provides partial hormone support to a fetus with congenital hypothyroidism.
++