Congenital hypothyroidism (CH) represents one of the most common preventable causes of mental retardation. The fetal hypothalamic-pituitary-thyroid axis begins to function by midgestation and is mature in the term infant at delivery. If fetal hypothyroidism develops, untoward effects may be demonstrated in certain organ systems, including the central nervous system and skeleton. However, most infants with CH appear normal at birth. Recent data suggest that the hypothyroid fetus is protected to a certain extent by placental transfer of maternal thyroid hormone; serum thyroxine (T4) levels in the cord blood of athyroid fetuses approximate one third of maternal levels.1 In addition, studies in animal models of hypothyroidism demonstrate increased levels of brain iodothyronine deiodinase, the enzyme which converts T4 to triiodothyronine (T3). In the hypothyroid fetus, this increased enzyme acting on T4 of maternal origin is sufficient to produce near normal fetal brain T3 concentrations.2 Thus, it appears that early detection and treatment of congenital hypothyroidism should have the potential to completely reverse the effects of fetal hypothyroidism in all but the most severe cases, for example, athyreotic infants born to mothers with thyroid problems resulting in inadequate placental transfer of maternal thyroid hormone.

Since the development of pilot screening programs for CH in Quebec and Pittsburgh in 1974,3 newborn screening for CH has become routine in essentially all developed countries of the world and is under development in Eastern Europe, South America, Asia, and Africa. In North America it is estimated that more than 5 million newborns are screened, with approximately 1400 infants with congenital hypothyroidism detected annually.

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