What to watch with FT3 and fertility?

FT3 (free triiodothyronine) is the active thyroid hormone and plays a key role in metabolism, which directly impacts fertility. Both low and high FT3 levels can disrupt ovulation, egg quality, and implantation. In women with unexplained infertility or recurrent miscarriage, thyroid function including FT3, FT4, and TSH is commonly assessed. However, FT3 alone is insufficient – interpretation requires TSH, FT4, and possibly antibodies (TPO, Tg). Genetic variants in deiodinases (e.g., DIO1, DIO2) can affect T4-to-T3 conversion, leading to low FT3 despite normal TSH. Nutrients like selenium and zinc are also critical for T3 synthesis. Caveat: FT3 can be low in non-thyroidal illness (euthyroid sick syndrome) without true hypothyroidism. Supplements like biotin can interfere with FT3 assays. For fertility, an optimal (not just 'normal') FT3 level is desirable, but this should always be managed by a physician.