Thyroid dysfunction during late gestation is associated with excessive iodine intake in pregnant women.

Context: Adequate iodine intake during pregnancy is essential for both the synthesis of maternal thyroid hormones and the maintenance of normal fetal brain development. Scant evidence is available on the effects of excessive iodine intake during pregnancy.

Objective: The study assesses the relationship between iodine nutritional status and thyroid function of pregnant women with excessive iodine intake during late gestation.

Design And Participants: A cross-sectional study of 384 pregnant women was carried out in Tianjin and Haixing from April to October in 2010.

Main Outcome Measures: Morning urine samples and blood samples were obtained from all subjects. Serum levels of free T(3), free T(4), and sensitive TSH and urinary iodine concentration were measured.

Results: The median urinary iodine concentration of pregnant women with excessive iodine intake was significantly higher than those with adequate iodine intake (P < 0.001). The prevalence of thyroid disease, especially subclinical hypothyroidism, in pregnant women with excessive iodine intake was significantly higher than in those with adequate iodine intake (P < 0.05). Subclinical hypothyroidism was the most frequent pattern of thyroid disease for pregnant women and those with positive or negative thyroid autoantibodies. Living with high water iodine content and having urinary iodine concentration higher than 250 μg/liter are associated risk factors for subclinical hypothyroidism in pregnant women (OR(1) = 41.822, OR(2) = 6.202; P < 0.05, where OR(1) is the odds ratio for living with high water iodine content and hypothyroidism and OR(2) is the odds ratio for urinary iodine concentration >250 μg/liter and hypothyroidism).

Conclusions: Excessive iodine intake during late pregnancy may lead to maternal thyroid dysfunction, particularly subclinical hypothyroidism. The appropriate measurements should be performed to monitor the onset of hypothyroidism in pregnant women with excessive iodine intake.