What to watch with FT4 and fatigue?

When you have a FT4 (free thyroxine) value and experience fatigue, do not interpret FT4 in isolation. FT4 is a thyroid hormone, but assessing thyroid function requires a full profile: TSH, FT3, possibly antibodies (TPO-Ab, Tg-Ab), and clinical symptoms. An isolated high FT4 can occur in subclinical hyperthyroidism or with levothyroxine over-replacement. Low FT4 with high TSH suggests hypothyroidism, which can cause fatigue. However, even with normal thyroid values, other causes like iron deficiency, vitamin D deficiency, sleep disorders, or stress may underlie fatigue. A DTC genetic test for thyroid function (e.g., TSHR, DIO2) has limited predictive power and does not replace blood work. Always seek medical evaluation before taking supplements or medications. Caveat: fatigue is nonspecific – comprehensive diagnostics are essential.