Assessment of thyroid function during first-trimester pregnancy: what is the rational upper limit of serum TSH during the first trimester in Chinese pregnant women?

CONTEXT: Guidelines of the American Thyroid Association (ATA) proposed that the upper limit of the TSH reference range should be 2.5 mIU/L in first trimester, but the reported ranges in China are significantly higher.

OBJECTIVE: Our objective was to establish a rational reference range of serum TSH for diagnosis of subclinical hypothyroidism in the first trimester of pregnant women in China.

DESIGN: We screened 4800 pregnant women in the first trimester and 2000 women who planned to become pregnant and evaluated 535 pregnant women in follow-up visits during the second and third trimester.

RESULTS: Median concentrations of serum TSH decreased significantly from the seventh week of gestation. The median of TSH from 4 to 6 weeks was significantly higher than from 7 to 12 weeks (2.15 [0.56-5.31] mIU/L vs 1.47 [0.10-4.34] mIU/L, P<.001); however, there was no significant difference compared with nonpregnant women (2.07 [0.69-5.64] mIU/L; P=.784). The median of free T4 was not significantly altered in the first trimester. The prevalence of subclinical hypothyroidism in the 4800 pregnant women was 27.8% on the diagnostic criteria of TSH>2.5 mIU/L and 4.0% using the reference interval derived by our laboratory (0.14-4.87 mIU/L).Additionally, of 118 pregnant women who had serum TSH>2.5 mIU/L in the first trimester, only 30.0% and 20.3% of them at the 20th and 30th week of gestation had TSH>3.0 mIU/L.

CONCLUSIONS: The reference range for nonpregnant women can be used for the assessment of pregnant women at 4 to 6 weeks of gestation. The upper limit of serum TSH in the first trimester was much higher than 2.5 mIU/L in Chinese pregnant women.