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.