Reference intervals for thyroid hormones in pregnant Chinese women

Objective: To establish gestation-related reference intervals for thyroid hormones in a Chinese population.

Materials and methods: A prospective study with 343 healthy pregnant women (5-41 weeks) and 63 non-pregnant controls. Thyroid stimulating hormone (TSH), free thyroxine (T₄) and tri-iodothyronine (T₃) (and human chorionic gonadotrophin) were measured by immunoassays. The median, 2·5th and 97·5th percentiles at 4-week intervals werecalculated. Data were also analysedfor significant trends using ANOVA.

Results: Free T₃ decreased during pregnancy, whereas free T₄ initially increased, peaking between 9-13 weeks and then decreased, the decline becoming significant by week 21. TSH mirrored changes in free T₄. Conclusion : The gestation-related reference intervals for thyroid hormones should alleviate the misinterpretation of thyroid function in pregnancy.