Establishment of reference range for specific thyroid function during pregnancy and analysis of influencing factors of hypothyroxinemia

Objective To establish a reference range for specific thyroid function during pregnancy and to explore the influencing factors of hypothyroxinemia during pregnancy. Methods A retrospective analysis of 2 996 cases of thyroid function in the pregnant women who were with single pregnancy and without thyroid diseases and family history of those diseases. Results (1) Establish a unified reference range for specific thyroid function during pregnancy; the early, middle, and late trimesters thyrotropin (TSH) ranges were 0.02-6.39, 0.16-6.23, 0.64-6.59 mU/L, respectively, while free thyroxine (FT4) ranges were 11.32-23.00, 9.39-18.92, 8.54-16.73 pmol/L respectively. The specific reference ranges of Han and Uygur pregnant women were established separately. There was no difference in the detection rates of various thyroid diseases when using their respective reference ranges and the unified reference range of the hospital (P>0.05). (2) The detection rate of various thyroid diseases (except subclinical hyperthyroidism) of our subjects with China guideline reference range was significantly higher than the reference range with the hospital (P<0.05). (3) The detection rates of hypothyroxinemia in all pregnant women with FT4 cut points of P2.5 and P5 were 4.3% and 7.4%, respectively, of which the Han population was 4.3% and 7.1%, respectively, and the Uygur population was 4.3% and 7.9%, respectively. (4) Comparing the mean age, gestational age, median urine iodine, and thyroid antibody positive rate between the hypothyroxinemia group and the control group, only the mean age and gestational age were different (P<0.05); Logistic binary regression analysis showed that age was the risk factor for hypothyroxinemia during pregnancy (OR=1.035, 95%CI 1.006-1.066, P<0.05). Conclusions The Han and Uygur pregnant women in this area both can use the thyroid reference range of our hospital during pregnancy. The establishment of thyroid reference range may avoid over-diagnosis of thyroid disease during pregnancy. Age is a possible influencing factor of hypothyroxinemia during pregnancy. Key words: Pregnancy; Thyroid hormone; Han; Uygur; Hypothyroxinemia