Establishment of the reference intervals of TSH, FT3, FT4, TT3, and TT4 in serum of pregnant women and study on changing trend

Abstract

Objective To establish the reference intervals of thyroid stimulating hormone (TSH), free triiodothyronine (FT3), free thyroxine (FT4), total triiodothyronine (TT3), total thyroxine (TT4) of normal pregnant women. Methods A total of 1 579 pregnant women received antenatal care and 328 non-pregnant healthy women were studied according to the screening criteria of the National Academy of Clinical Biochemistry (NACB). The levels of serum TSH, FT3, FT4, TT3, and TT4 were measured, the results were statistically analyzed to establish corresponding reference intervals and the changing trends of TSH, FT3, FT4, TT3, and TT4 levels at different stages were analyzed. Results (1) The reference intervals of TSH, FT3, FT4, TT3, and TT4 in the first trimester of pregnancy (5-13 weeks, 589 cases) were 0.06-4.12 μIU/ml, 3.82-7.34 pmol/L, 11.41-23.56 pmol/L, 0.94-2.27 ng/ml, and 6.21-14.7 μg/dl; the reference intervals of TSH, FT3, FT4, TT3, and TT4 in the second trimester of pregnancy (14-27 weeks, 557 cases) were 0.25-4.21 μIU/ml, 3.40-6.91 pmol/L, 8.88-19.79 pmol/L, 1.06-2.68 ng/ml, and 6.73-17.64 μg/dl; the reference intervals of TSH, FT3, FT4, TT3, and TT4 in the third trimester of pregnancy (28-40 weeks, 433 cases) were 0.32-5.25 μIU/ml, 3.01-6.51 pmol/L, 7.33-18.93 pmol/L, 1.05-2.57 ng/ml, and 6.62-17.25 μg/dl. (2)With the progress of pregnancy, the level of TSH decreased significantly in the first trimester, and then gradually increased to the level of slightly higher than the third trimester. The levels of FT3 and FT4 increased slightly in the first trimester, then gradually decreased. The levels of TT3 and TT4 increased significantly in the first trimester, rose to the highest level in the second trimester of pregnancy, and then began to decline, but the level of pregnant group was higher than that of the non-pregnant group. Conclusions The thyroid hormone levels in pregnant women are significantly different from those in non-pregnant women, there are also differences between different stages of pregnancy. The reference intervals of thyroid hormones in non-pregnant women cannot be simply used for the reference intervals of pregnant women. Our study is conducive to the diagnosis and treatment of thyroid disease in pregnant women. Key words: Pregnancy; Thyroid stimulating hormone; Free triiodothyronine; Free thyroxine; Total triiodothyronine; Total thyroxine; Reference interval; Changing trend