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 (T-4) and tri-iodothyronine (T-3) (and human chorionic gonadotrophin) were measured by immunoassays. The median, 2(.)5th and 97(.)5th percentiles at 4-week intervals were calculated. Data were also analysed for significant trends using ANOVA. Results: Free T-3 decreased during pregnancy, whereas free T-4 initially increased, peaking between 9-13 weeks and then decreased, the decline becoming significant by week 21. TSH mirrored changes in free T-4. Conclusion: The gestation-related reference intervals for thyroid hormones should alleviate the misinterpretation of thyroid function in pregnancy.