Thyroid function in the third trimester of pregnancy and after delivery in an area of adequate iodine intake

Objective: To establish whether the higher thyroid stimulating hormone (TSH) levels and lower levels of the 2 free thyroid hormones noted toward the end of pregnancy are in relation with iodine supply. Methods: We compared these hormones' levels in the third trimester of pregnancy and 4 months after delivery in 116 consecutive women without thyroid disease and otherwise healthy. The study was conducted in Slovenia, an iodine-sufficient area. The Mann-Whitney test, the Kruskal-Wallis rank test, and Spearman analysis were used for statistical analysis. Results: In the third trimester TSH was significantly higher and both free thyroid hormones were significantly lower than after delivery (P=0.003 and P<0.001), but the free thyroxine to free triiodothyronine ratios in the third trimester and 4 months after delivery did not significantly differ. Urinary iodine concentration (UIC) was significantly higher during pregnancy than after delivery (P=0.044). We found no significant correlations between UIC and TSH or between UIC and both free thyroid hormones during pregnancy or after delivery. Conclusion: The decrease of both free thyroid hormones in the third trimester of pregnancy is most likely due to reasons that are not related to iodine supply. (C) 2010 Published by Elsevier Ireland Ltd. on behalf of International Federation of Gynecology and Obstetrics.