Levothyroxine may not adequately prepare hypothyroid women for controlled ovarian hyperstimulation.

Background: Thyroid axis dysregulation during controlled ovarian hyperstimulation (COH) is more pronounced in hypothyroid-treated women. Whether or not this leads to compromised thyroid hormone levels within the ovarian follicular fluid is not known.

Aims: To determine whether ovarian follicular thyroid hormone levels are compromised in adequately replaced hypothyroid women undergoing controlled ovarian hyperstimulation (COH), and/or influence cycle/pregnancy outcomes.

Materials and methods: Prospective cohort study involving 46 euthyroid (anti-thyroid peroxidase antibody negative) and 16 levothyroxine-replaced women with baseline thyroid-stimulating hormone (TSH) <2.5 mIU/L attending their first COH cycle. Follicular fluid TSH, free triiodothyronine (T3) and free thyroxine (T4) were recorded at oocyte pick-up. Serum levels were measured at: (i) baseline; (ii) human chorionic gonadotropin trigger day; and (iii) cycle conclusion. The number of mature oocytes retrieved, fertilisation, early pregnancy loss and live birth rates were compared.

Results: Median serum 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 to euthyroid women, with serum TSH levels increasing across ovarian stimulation (P = 0.006) into pregnancy testing (P = 0.030). Follicular fluid free T3 levels were lower in levothyroxine-replaced women (median 4.3 vs 4.6 pmol/L, P = 0.032). Fertilisation rates were lower (52% vs 71%, P = 0.043) in women requiring levothyroxine replacement, but numbers of mature oocytes retrieved, early pregnancy loss and live births did not differ.

Conclusion: Adequately replaced hypothyroid women achieve lower ovarian follicular fluid free T3 levels and poorer fertilisation rates compared to euthyroid women undergoing COH. Optimising T3 levels may be pivotal in improving COH outcomes in hypothyroid women.