Free T3 (fT3)
Stomach and gut healthFree T3 is the active thyroid hormone that regulates metabolic activity and energy use in cells.
Why this matters
Free T3 directly controls metabolic rate, body temperature, heart rate, and energy production in every cell. Low levels can cause fatigue, weight gain, cold intolerance, depression, and sluggish metabolism, while high levels may lead to anxiety, weight loss, heat intolerance, and rapid heartbeat. Unlike total T3, free T3 isn't affected by changes in binding proteins from pregnancy, medications, or illness, making it a more accurate measure of thyroid function at the tissue level
How this connects to other biomarkers
- Low fT3 with low Free T4 (fT4) and elevated TSH indicates underactive thyroid (primary hypothyroidism).
- Low fT3 with normal/low fT4 and normal/low TSH points to non-thyroidal illness ("euthyroid sick syndrome") seen in chronic disease, malnutrition, or critical illness — cross-reference Reverse T3 to confirm impaired hormone conversion.
- Elevated fT3 with low TSH suggests overactive thyroid (Graves' disease, toxic nodule); normal fT3 with low TSH can indicate subclinical hyperthyroidism or T3-only thyrotoxicosis.
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