What does a low FT3 result mean?

A low FT3 (free triiodothyronine) result may indicate hypothyroidism, but it is not diagnostic on its own. FT3 is the active thyroid hormone; low levels often occur in primary hypothyroidism alongside elevated TSH and low FT4. However, isolated low FT3 can also be seen in 'euthyroid sick syndrome' (non-thyroidal illness) due to severe infection, fasting, or chronic disease, without true thyroid dysfunction. Certain medications (beta-blockers, corticosteroids) can also lower FT3. Interpretation requires full context: TSH, FT4, possibly reverse T3, and clinical symptoms (fatigue, cold intolerance, weight gain). Consumer lab tests (e.g., fingerstick) are screening tools, not diagnostic. If FT3 is low, consult a doctor for proper workup. Caveat: Do not self-treat with thyroid hormones – risk of overdose and cardiac issues.