[Clinical analysis of the specific reference intervals of thyroid index for normal pregnant women]

Objective: To explore the reference intervals of thyrotropin (TSH) and free thyroxine (FT(4)) in normal pregnant women. Methods: Prospective and longitudinal sequential collection of the cases were performed. A total of 155 singleton pregnant women who had regular prenatal examination and delivery in West China Second Hospital of Sichuan University from January 2015 to December 2015 were included as the research group. Blood samples were sequentially collected from the pregnant women at the first (9-13(+6) weeks) , the second (24-27(+6) weeks) , the third (32-36 weeks) trimesters, respectively. Another 155 non-pregnant women were selected as the control group simultaneously, in an effort to establish a range of reference intervals of thyroid index in each period of pregnancy. Meanwhile, neonatal plantar blood were also collected on special filter paper and TSH levels were measured. Results: (1) TSH reference intervals [percentile 2.5-percentile 97.5 (P(2.5)-P(97.5)) ] were 0.08-3.29 mU/L, 0.59-4.22 mU/L and 0.81-4.33 mU/L in three trimesters respectively. FT(4) reference intervals were 11.88-20.06 pmol/L, 9.89-15.80 pmol/L and 9.22-15.77 pmol/L in three trimesters respectively. (2) The median serum TSH in the first trimester (1.35 mU/L) was lower than that in the second trimester (2.15 mU/L) , the third trimester (2.19 mU/L) , and the control group (2.19 mU/L) . The differences were all statistically significant (P<0.05) . There was no significant difference in median serum TSH between the second trimester and the third trimester, the second trimester and the control group, and the third trimester versus the control group. (3) The median serum FT(4) in the first trimester (15.16 pmol/L) was higher than that in the second trimester (12.39 pmol/L) and the third trimester (12.26 pmol/L) . The differences were both statistically significant (P<0.05) . The median FT(4) in the second trimester and the third trimester was lower than that in the control group (15.64 pmol/L) , and the differences were both statistically significant (P<0.05) . (4) In the first trimester, the median serum total triiodothyronine (TT(3)) level (2.32 nmol/L) was higher than that in the control group (1.56 nmol/L) , total thyroxine (TT(4)) level (154.60 nmol/L) was also higher than that in the control group (98.25 nmol/L) , and free triiodothyronine (FT(3)) level (4.70 pmol/L) was lower than that in the control group (4.84 pmol/L) , the differences were all statistically significant (P<0.05) . (5) The TSH levels of all neonatus were normal [ (2.1±1.3) mU/L]. Conclusions: The thyroid hormone levels between pregnant women and non-pregnant women are significantly different. Moreover, the reference intervals are vary with pregnancy period. It is important to establish the specific reference intervals of thyroid hormones in the first, second and third trimester of pregnancy specific to local region or unit.