Urinary Iodine Excretion During Normal Pregnancy in Healthy Women Living in the Southwest of France: Correlation with Maternal Thyroid Parameters

A prospective study was undertaken to evaluate urinary iodine excretion and changes of maternal thyroid function during pregnancy in healthy women living in the southwest of France. The cohort included a total of 347 pregnant women (mean age 28.0 ± 0.5 years). Iodine concentration in a random urine sample and thyroid tests (free thyroxine [FT₄], free triiodothyronine [FT₃], thyrotropin (TSH), thyroxine-binding globulin [TBG], and thyroglobulin [Tg]) were measured at initial presentation (before 12 weeks of gestation), and during the ninth month of pregnancy. A thyroid ultrasound was performed 1 to 5 days after delivery in 246 mothers. Mean urinary iodine levels were low during the first trimester (6.9 ± 0.4 μg/dL), as well as during the ninth month of pregnancy (8.6 ± 0.6 μg/dL). During pregnancy, FT₄ and T₃ concentrations decreased {p <.001), and TSH and Tg concentrations increased (p <.001). Thyroid hypertrophy (thyroid volume greater than 18 mL) was present in 15.4% of women whose first trimester urinary iodine concentration was less than 5 μg/dL, but was present in only 3.5% of women whose urinary iodine concentration was greater than 10 μg/dL. A goiter (thyroid volume greater than 22 mL) was present in 11% of the mothers. In conclusion, this prospective study shows that urinary iodine excretion is low in pregnant women living in the southwest of France. This low iodine intake is associated with reduced circulating thyroid hormone levels and growth of the thyroid gland. These data point to the need of an increased iodine supply in these pregnant women to reduce the potential consequences of low iodine intake on maternal thyroid economy.