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 were calculated. Data were also analysed for 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.