Levothyroxine may not adequately prepare hypothyroid women for controlled ovarian hyperstimulation

Abstract Purpose: Compare differences in serum and follicular fluid thyroid hormone levels in euthyroid versus levothyroxine-replaced women undergoing controlled ovarian hyperstimulation. Methods: Prospective cohort study involving 46 euthyroid (anti-TPO antibody negative) and 16 levothyroxine-replaced women with baseline TSH < 2.5 mIU/L attending a university-based reproductive medical centre. TSH, free T3 and T4 measurements (serum and follicular fluid) were measured at various time points across controlled ovarian stimulation and numbers of mature oocytes retrieved, fertilisation and pregnancy rates were compared between groups. Results: Median TSH levels were similar at baseline, 1.76 vs 1.24 mIU/L (p=0.053), but free T3 levels were lower (4.5 vs 4.8 pmol/L, p=0.029) in levothyroxine-replaced compared with euthyroid (anti-TPO antibody) women. Serum TSH levels in levothyroxine-replaced women increased across controlled ovarian hyperstimulation (p=0.006) and remained significantly higher than baseline (p=0.030) at pregnancy testing. Follicular fluid free T3 levels were lower in levothyroxine-replaced (median 4.3 vs 4.6 pmol/L, p=0.032) and fertilisation rates fewer (52% vs 71%, p=0.043). Numbers of mature oocytes retrieved, early pregnancy loss and live birth rates did not differ. Conclusion: Levothyroxine women, with baseline TSH levels < 2.5 mIU/L, have lower serum and follicular fluid free T3 levels and poorer fertilisation rates than euthyroid women undergoing a cycle of controlled ovarian stimulation.