To Örebro University

Context: Establishing reference ranges as well as identifying and quantifying the determinants of thyroid function during pregnancy is important for proper clinical interpretation and optimizing research efforts. However, such data are sparse, specifically for (F)T3 measurements and most studies do not take into account thyroid antibodies or hCG.

Objective: To determine reference ranges and to identify/quantify determinants of TSH, FT4, FT3, TT4 and TT3.

Design, Setting and Participants: This study included 2,314 participants of the Swedish Environmental Longitudinal, Mother and child, Asthma and allergy study, a population-based prospective pregnancy cohort of mother-child pairs. Reference ranges were calculated by 2.5-97.5th percentiles after excluding TPOAb and/or TgAb positive women.

Intervention: None.

Main Outcome Measures: TSH, FT4, FT3, TT4 and TT3 in prenatal serum.

Results: After exclusion of TPOAb positive women, reference range were: TSH: 0.11-3.48 mU/L, FT4: 11.6-19.4 pmol/L, FT3: 3.72-5.92 pg/mL, TT4: 82.4-166.2 pmol/L and TT3: 1.28-2.92 nmol/L. Additional exclusion of TgAb positive women did not change the reference ranges substantially. Exposure to tobacco smoke, as assessed by questionnaires and serum cotinine, was associated with lower TSH and higher FT3 and TT3. BMI and gestational age were the main determinants of TSH (only for BMI), FT4, FT3, TT4 and TT3.

Conclusions: We show that the exclusion of TgAb positive women on top of excluding TPOAb positive women hardly affects clinical reference ranges. We identified various relevant clinical determinants of TSH, FT4, FT3, TT4 and TT3 which could reflect endocrine disrupting effects and/or effects on thyroid hormone transport or deiodination.