Myths vs facts: FT3 and fertility

FT3 (free triiodothyronine) is the active thyroid hormone and plays a role in energy metabolism, which can influence fertility. A common myth is that an isolated low FT3 directly impairs fertility. In reality, FT3 is rarely low in isolation; usually there is overt hypothyroidism (elevated TSH, low fT4), which can affect fertility through menstrual irregularities or egg quality. Another myth: FT3 supplements can boost fertility. This is dangerous – overdosing causes thyrotoxicosis, which worsens fertility. Fact: Normal TSH (0.5–2.5 mU/L) and fT4 within reference range are more important for fertility than an isolated FT3 value. Evidence is based on observational studies; randomized trials are lacking. Thyroid values should always be interpreted in context, not in isolation. For fertility issues, medical workup (including TSH, fT4, TPO antibodies) is recommended. DNA tests for thyroid genes have no clinical relevance for fertility.