[Establishment of reference ranges for thyroid-related indicators in normal pregnant women]

To establish the gestational age-specific reference ranges for thyroid-related indicators of pregnant women in our hospital, especially for serum thyroid stimulating hormone (TSH) in the first trimester.

A total of 548 pregnant women in the first trimester were collected in Peking University First Hospital from June 2013 to April 2014. Among them, 254 pregnant women with single fetus who conformed to the National Academy of Clinical Biochemistry criteria, without adverse pregnancy outcomes and obstetric complications, were enrolled in the current study. To establish their own self-sequential longitudinal reference intervals, serum TSH, total thyroxine (TT4) and free thyroxine (FT4) levels were measured using Siemens detection kits during three different trimesters. Urine iodine (UI) was measured using digestion instrument in some of the pregnant women.

The gestational age-specific reference ranges for thyroid-related indicators of pregnant women in our hospital were as follows: in the first trimester: TSH 0.23-4.08 mU/L, TT4 92.59-186.25 nmol/L, FT4 13.36-20.81 pmol/L; in the second trimester: TSH 0.78-4.25 mU/L, TT4 103.20-180.95 nmol/L, FT4 11.57-16.62 pmol/L; in the third trimester: TSH 0.65-4.52 mU/L, TT4 78.20-174.70 nmol/L, FT4 10.01-20.57 pmol/L. The median level of TSH during 7 to 12 weeks of gestational age was lower than that of 4 to 6 weeks, but with no significant statistical difference (P=0.063). The medians of UI during three trimesters of pregnancy were 211.60 μg/L, 195.55 μg/L and 198.65 μg/L, respectively, which were all classified as adequate iodine status.

The gestational age-specific reference ranges for thyroid-related indicators are different from the kits' reference ranges, which are also different among the three trimesters. It may be more reasonable to establish different reference ranges for thyroid-related indicators at 4 to 6 weeks and 7 to 12 weeks in the first trimester separately.