Aims
Iodine supplements are recommended for women planning pregnancy, but their impact on thyroid function during controlled ovarian hyperstimulation (COH) and into pregnancy is unknown. The aim of this study was to assess the impact of iodine supplementation on thyroid function during COH.

Methods
One‐hundred and six euthyroid women (thyroid stimulating hormone (TSH) 0.4–2.5 mIU/L) planning their first COH cycle were subdivided according to iodine supplementation (nil, <6 months, ≥6 months) and compared to levothyroxine (LT4)‐treated controls. Serial TSH, free thyroxine, free triiodothyronine and thyroglobulin (Tg) levels were recorded at four time points: (i) baseline, (ii) day 7 ovarian stimulation, (iii) ovulation trigger and (iv) two weeks post oocyte retrieval. Oocyte numbers, fertilisation rates and pregnancy outcome were recorded. Results TSH increased during COH for those women taking iodine supplements for ≥6 months (P = 0.025). One quarter recorded a TSH level >2.5 mIU/L before embryo transfer. A similar increase in TSH was demonstrated by LT4‐dependent controls (P = 0.024) but not the remaining subgroups. Tg levels did not change during COH in any group but decreased significantly post oocyte retrieval if nil iodine (P < 0.0001) or supplemented for ≥6 months (P < 0.005). Iodine supplementation did not influence oocyte count, fertilisation or implantation rates. Women taking iodine for <6 months were four times more likely to achieve a live birth than women taking iodine for longer. Conclusions Women taking iodine supplements for ≥6 months are less able to adapt to the thyroidal demands of COH, with responses comparable to LT4‐dependent patients.