Myths vs facts: FT3 and Fruchtbarkeit

FT3 (free triiodothyronine) is often marketed as the 'active' thyroid hormone, but its standalone role in fertility is overemphasized. Fact: TSH remains the primary screening marker because it reflects the hypothalamic-pituitary axis. FT3 fluctuates diurnally, is affected by non-thyroidal illness ('low T3 syndrome'), and depends on peripheral conversion from T4. An isolated low FT3 with normal TSH and FT4 is usually not a sign of treatable thyroid dysfunction. Myth: 'Optimize FT3 to get pregnant' – no randomized trials support this. Evidence indicates that a euthyroid state (normal TSH, FT4, and TPO antibodies if indicated) matters for fertility, not a single FT3 value. Direct-to-consumer tests that promote FT3 as a 'fertility marker' ignore this complexity. Caveat: If you have known thyroid disease or are planning pregnancy, management should be guided by a physician – FT3 alone is not a treatment target.