Free T3
Free T3 (triiodothyronine) is the most metabolically active thyroid hormone, approximately 3-5 times more potent than T4. About 80% of T3 is produced by peripheral conversion of T4 to T3 (via deiodinase enzymes), not directly by the thyroid. Only 0.3% of total T3 circulates in free form.
Why it matters: FT3 is the hormone that actually drives metabolic rate, energy, body temperature, heart rate, and cognitive function. Low FT3 with normal FT4 indicates impaired T4-to-T3 conversion (common in chronic illness, caloric restriction, and selenium deficiency). In hyperthyroidism, FT3 may be elevated before FT4 (T3 thyrotoxicosis).
May increase with:
May decrease with:
What is Free T3?
Free T3 (triiodothyronine) is the most metabolically active thyroid hormone, approximately 3-5 times more potent than T4. About 80% of T3 is produced by peripheral conversion of T4 to T3 (via deiodinase enzymes), not directly by the thyroid. Only 0.3% of total T3 circulates in free form.
What might a high or low Free T3 mean?
FT3 is the hormone that actually drives metabolic rate, energy, body temperature, heart rate, and cognitive function. Low FT3 with normal FT4 indicates impaired T4-to-T3 conversion (common in chronic illness, caloric restriction, and selenium deficiency). In hyperthyroidism, FT3 may be elevated before FT4 (T3 thyrotoxicosis).
What is the typical reference range for Free T3?
The general-population reference range shown here is 3.1 – 6.8 pmol/L. Reference ranges describe the general population and are not a personal target — discuss your results with your physician.
What can affect Free T3?
It may be higher with: Graves' disease (excess thyroid production), T3 thyrotoxicosis, Excess T3-containing medications, Biotin (assay interference). It may be lower with: Non-thyroidal illness (sick euthyroid — impaired T4→T3 conversion), Caloric restriction and fasting, Selenium deficiency (deiodinase requires selenium), Chronic stress (cortisol inhibits conversion), Aging, Medications (amiodarone, beta-blockers, corticosteroids), Zinc deficiency.