Background: Thyroid physiology undergoes major changes during pregnancy. Recent findings have indicated that thyroidal stimulation by hCG may be impaired in TPOAb positive women, but it remains unknown if this is similar for TgAb positivity. Furthermore, no study to date has been able to study how gestational thyroid stimulation by hCG affects (F)T3 levels.

Methods: This study included 2357 participants of the Swedish Environmental Longitudinal, Mother and Child, Asthma and allergy study, a population-based prospective pregnancy cohort. The associations of hCG with TSH, FT4, TT4, FT3 and TT3 were studied using multivariable linear regression models and reported as standardized coefficients to facilitate comparison between effect estimates. Interactions terms were added to study effect modification by TPO and/or Tg antibody positivity.

Results: Thyroid function was measured at a median (95% range) gestational age of 10 (9–14) weeks and TPO and Tg antibody positivity were present in 11% and 9% of the study population respectively. There was a negative association of hCG with TSH (β=-.29 [SE=.02], p<.01) and a positive association of hCG with FT4 (β=.23 [SE=.02], p<.01). However, there was no association of hCG with FT3 (β=.04 [SE=.02], p=.06) and a weak association with TT3 (β=.04 [SE=.02], p=.04).

There was a positive association of hCG with FT4 in TPOAb negative women (β=.27 [SE=.02]), but not in TPOAb positive women (β=-.01 [SE=.06]; Pinteraction<.01), with the same pattern for FT3 (Pinteraction=.02). There was a negative association of hCG with TSH among TgAb negative women (β=-.32 [SE=.02]) and to lesser extent among TgAb positive women (β=-.09 [SE=.07]; Pinteraction=.02). There was no effect modification by TgAb positivity for the association of hCG with FT4 (Pinteraction=.76) and FT3 (Pinteraction=.20).

Conclusion: While thyroidal stimulation during pregnancy changes TSH and FT4 concentrations, it has a little to no effect on TT3 and FT3 concentrations. TPOAb positivity, but not TgAb positivity, is associated with an impaired thyroidal response to hCG.