Endocrine Abstracts

Background: During pregnancy, the physiology of the thyroid gland undergoes important changes. As a result, change levels of thyroid hormones. In the latest clinical guidelines, experts have recommended to perform population-based studies to determine reference intervals for thyroid hormones.

Aim: To evaluate the reference intervals of free thyroxine in different trimesters of pregnancy in women living in regions with mild iodine deficiency.

Materials and methods: We have conducted the observational multicenter cross-sectional study included 2008 healthy pregnant women at different trimesters of pregnancy, from three regions of the Russian Federation (Moscow, Ivanovo and Smolensk). We assessed the level of free thyroxine, antibodies to thyroid peroxidase, antibodies to serum thyroglobulin (Architect, Abbot, reference range 9-20 pmol/l), the level of iodine concentration in the morning portion of urine (cerium arsenic method) and we have conducted a questionnaire (date birth and gestational age). Women with elevated titers of anti-TPO and/or anti-TG antibodies were excluded from the study (245 women).

Results: We have determined the median iodine concentration: Moscow 111 µg/l, Ivanovo 125 µg/l, Smolensk 133 µg/l, which confirms the presence of mild iodine deficiency in the regions areas (median iodine concentration less than 150 µg/l in pregnant women corresponds to mild iodine deficiency). Reference intervals for free thyroxine are presented using the median, 2.5 and 97.5 percentiles with 95% confidence interval (Me [2.5;97.5] CI+-95%). We obtained the following results: 1st trimester (n=386): Me 11.7 [9.03;16.1] CI+1.96; -1.74; 2nd trimester (n=478): Me 10.7 [8.35; 13.8] CI+1.43; -1.28; 3rd trimester (n=899): Me 9.9 [7.73; 13.0] CI+1.41; -1.30.

Conclusions: Based on the results of our study, we determined the reference intervals of free thyroxine in healthy pregnant women without anti-TPO/TG antibodies living in mild iodine deficiency regions. We revealed a decrease in the level of free thyroxine by the third trimester, which can be physiological isolated hypothyroxinemia.