Evidence check: FT4 in the context of fertility

Thyroid function is crucial for fertility. FT4 (free thyroxine) is a key lab marker. Low FT4 (hypothyroidism) is linked to menstrual irregularities, anovulation, and increased miscarriage risk. Subclinical hypothyroidism (elevated TSH, normal FT4) is also associated with fertility issues. However, FT4 alone is insufficient; assessment must include TSH and often TPO antibodies. Evidence for the thyroid-fertility link is human-strong (multiple studies, guidelines). Evidence for FT4 as a standalone predictor is human-moderate because reference ranges are wide and individual variation requires clinical context. For those trying to conceive, a TSH target < 2.5 mU/L is recommended. A consumer FT4 test can provide initial insight but does not replace medical evaluation. Caution: minor deviations are often benign. The thyroid is just one piece of the complex fertility puzzle.