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 and free thyroid hormones, thyroid-binding globulin (TGB) at 8, 14, 20, 29, and 36 weeks of gestation, along with urinary iodide excretion, in 10 healthy women from a moderately iodine deficient region (group A), and compared them with 6 women from an iodine sufficient region (group B). Serum total thyroxine (T4) fell significantly in group A, and was significantly lower than in group B at 29 and 36 weeks (p < 0.05). TBG saturation was significantly lower in group A throughout pregnancy, and declined in both groups as pregnancy progressed. Free thyroxine (T4) and triiodothyronine (T3) concentrations fell in both groups, and FT4 values were significantly lower in group A than group B in the third trimester (p < 0.05). Urinary iodine excretion was lower in group A women with respect to group B and did not vary significantly in either group as gestation progressed. The serum T3/T4 molar ratio increased through pregnancy only in group B. Thyrotropin concentrations rose in both groups through pregnancy, and were higher in group A at term (p < 0.01). The incidence of isolated hypothyroxinemia or biochemical hypothyroidism doubled (30% to 70%) between midgestation and term in group A, suggesting that moderate iodine deficiency may result in maternal thyroid failure during the later stages of pregnancy.