The role of free triiodothyronine in pathogenesis of infertility in levothyroxine-treated women with thyroid autoimmunity – a preliminary observational study

Introduction: The aim of this study was to analyze the possible role of free triiodothyronine (FT3) in infertility and in levothyroxine-treated (LT4) euthyroid women with Hashimoto thyroiditis (HT).Methods: It is an observational retrospective case control study. Twenty one euthyroid women with HT on LT4 replacement therapy and a medical history of idiopathic infertility were included into the study. To achieve higher FT3 level, the dose of LT4 was increased in every patient. Fifteen fertile women with HT on LT4 replacement therapy served as a control group.Results: At baseline in the study group mean thyroid stimulating hormone (TSH) level was 1.96 μU/ml ± 0.84 μU/ml and mean FT3 was 4.07 pmol/l ± 0.78 pmol/l. The mean TSH level after the increase of LT4 was 0.60 μU/ml ± 0.45 μU/ml (p < 0.0001), and the mean FT3 was 5.12 pmol/l ± 0.77 pmol/l (p = 0.0001). Baseline TSH in the study group was higher than in controls (p < 0.0001) and baseline FT3 in the study group was lower than in controls (p = 0.0003).Conclusions: Relatively low levels of FT3 in women with HT on LT4 replacement therapy may contribute to higher infertility rates.