Increased risk of maternal thyroid failure with pregnancy progression in an iodine deficient area with major iodine deficiency disorders

In an effort to assess the impact of moderate iodine deficiency on maternal thyroid function during pregnancy, we measured serum thyrotropin, total an d Gee thyroid hormones, thyroid-binding globulin (TGB) at 8, 14, 20, 29, an d 36 weeks of gestation, along with urinary iodide excretion, in 10 healthy women from a moderately iodine deficient region (group A), and compared th em with 6 women from an iodine sufficient region (group B). Serum total thy roxine (T-4) fell significantly in group A, and was significantly lower tha n in group B at 29 and 36 weeks (p < 0.05). TBG saturation was significantl y lower in group A throughout pregnancy, and declined in both groups as pre gnancy progressed. Free thyroxine (T-4) and triiodothyronine (T-3) concentr ations fell in both groups, and FT4 values were significantly lower in grou p A than group B in the third trimester (p < 0.05). Urinary iodine excretio n was lower in group A women with respect to group B and did not vary signi ficantly in either group as gestation progressed. The serum T-3/T-4 molar r atio increased through pregnancy only in group B. Thyrotropin concentration s rose in both groups through pregnancy, and were higher in group A at term (P < 0.01). The incidence of isolated hypothyroxinemia or biochemical hypo thyroidism doubled (30% to 70%) between midgestation and term in group A, s uggesting that moderate iodine deficiency may result in maternal thyroid fa ilure during the later stages of pregnancy.