Maternal thyroid function at 11-13 weeks of gestation.

OBJECTIVE To establish normal ranges of maternal serum thyroid-stimulating hormone (TSH), free thyroxine (FT4) and free triiodothyronine (FT3) at 11-13 weeks of gestation. METHODS Maternal serum concentrations of FT3, FT4, TSH, anti-thyroperoxidase (anti-TPO) and anti-thyroglobulin (anti-Tg) antibodies were measured at 11-13 weeks. Normal ranges were constructed from the data of singleton pregnancies with no anti-thyroid antibodies resulting in live births after 34 weeks of phenotypically normal neonates with birth weight above the 5th percentile. Adjustments were made for maternal characteristics found by multiple regression analysis to affect the levels of TSH, FT3 and FT4. RESULTS 3,592 of the 4,318 pregnancies examined were antibody negative, and in this group serum TSH increased whereas FT3 and FT4 decreased with gestation, and all three were lower in black than in white women. Serum FT3 and FT4 decreased but TSH did not change significantly with maternal age; TSH and FT3 increased whereas FT4 decreased with body mass index; TSH decreased whereas FT3 and FT4 increased with serum free beta-hCG. In the antibody-positive group, compared to the negative group, median TSH was higher and median FT3 and FT4 were lower. CONCLUSION The study established normal ranges for maternal thyroid function at 11-13 weeks.