Is FT4 useful for fertility?

FT4 (free thyroxine) is a thyroid hormone that can play a role in fertility assessment, but it is not recommended as a standalone marker. Thyroid dysfunction (hypo- or hyperthyroidism) is associated with menstrual irregularities, ovulatory disorders, and increased miscarriage risk. In practice, TSH is typically used as a screening parameter; FT4 and fT3 complement the workup when TSH is abnormal or symptoms are present. Evidence for the direct utility of FT4 as a fertility marker is moderate: studies show associations, but its predictive value alone is low. A normal FT4 does not rule out thyroid dysfunction, and other factors (age, hormonal status, lifestyle) are equally relevant. For unexplained infertility, thyroid function should always be evaluated in the context of an endocrinologist. The test is useful but insufficient for a comprehensive fertility workup.