Trimester-specific reference data of thyroid hormones for normal pregnancy

：Objective To set up thetrimester-specific reference ranges of thyroid hormones for normal pregnant women toprovide reference criteria for diagnosis, treatment and monitoring or screening of thyroiddisease during pregnancy and related research. Methods A cross-sectional survey wasconducted in pregnant and non-pregnant women in iodine sufficient areas. A total of 505normal pregnant women and 153 normal non-pregnant women (as control) were selected forestablishing trimester-specific reference ranges of thyroid hormones after rigorousscreening through the survey questionnaire and laboratory tests. Thyroid hormones weremeasured by Bayer automated chemiluminescence immunoassay, and the reference range of eachhormone was calculated as median (the 50th percentile value) and two-sided limits (the2.5th and 97.5th percentile values). Results All women investigated were in iodinesufficient status within optimal urine iodine level. The serum TSH level during the 1sttrimester was obviously declined compared with that in the non-pregnant individuals (P< 0.01), and started to rise during the 2nd trimester, but was still not restored tonon-pregnant level until the 3rd trimester. Serum FT4 and FT3 levels gradually decreasedfrom the 2nd trimester to the 3rd (P < 0.01), and the TT4 and TT3 levels were markedlyelevated since early pregnancy (P < 0.01) and reached peak levels at the 2nd trimesterapproximately making up to 1.5 times of those in the non-pregnant individuals. ConclusionThe thyroid hormone levels during pregnancy differ completely from those of thenon-pregnant individuals, and also differ during different gestation periods. Therefore,to establish trimester-specific reference data of thyroid hormones during normal pregnancymay be important for clinical practice.