Free T3 (triiodothyronine) is the active thyroid hormone. It directly controls your metabolism, energy, heart rate, and how every cell in your body functions.
Free T3 (free triiodothyronine) measures the unbound, biologically active form of T3 — the most potent thyroid hormone. While T4 is the main hormone your thyroid produces, T3 is what actually enters cells and turns on genes that control metabolism.
About 80% of T3 in your body comes from conversion of T4 in peripheral tissues (liver, kidneys, muscles), not directly from the thyroid. According to the American Thyroid Association, Free T3 testing is useful when hyperthyroidism is suspected or when symptoms persist despite normal TSH and T4.
Understanding how T3 and T4 relate helps interpret your results:
| Feature | T4 (Thyroxine) | T3 (Triiodothyronine) |
|---|---|---|
| Role | Storage/prohormone | Active hormone |
| Potency | 1x (baseline) | 3-4x more potent |
| Half-life | ~7 days | ~1 day |
| Source | 100% from thyroid | 20% thyroid, 80% converted from T4 |
Healthy active thyroid hormone level
Reduced active hormone, may cause symptoms
Significant hypothyroidism or T4-to-T3 conversion issue
Excess active hormone, overactive thyroid
Significant hyperthyroidism, evaluation needed
Select your Free T3 value for a detailed explanation of what it means and what to do next.
Low Free T3 (typically <80 pg/dL) can occur with hypothyroidism or impaired T4-to-T3 conversion. The American Thyroid Association notes that T3 is often the last thyroid hormone to fall in hypothyroidism.
Low T4 production means less substrate for T3 conversion. TSH is elevated, T4 is low, T3 follows.
Normal TSH/T4 but low T3. Caused by illness, stress, selenium deficiency, or aging. Called "low T3 syndrome" or "euthyroid sick syndrome."
Beta-blockers, amiodarone, and steroids can impair T4-to-T3 conversion.
Low-calorie diets and fasting reduce T4-to-T3 conversion as a metabolic adaptation.
High Free T3 (typically >200 pg/dL) with suppressed TSH indicates hyperthyroidism. According to the Mayo Clinic, T3 may rise before T4 in some forms of hyperthyroidism.
Autoimmune stimulation of thyroid. Both T4 and T3 elevated, TSH suppressed. Most common cause of hyperthyroidism.
Rare condition where T3 is elevated but T4 is normal. TSH is suppressed. Often seen with toxic nodules.
Inflammation releases stored hormone. T3 may spike during the hyperthyroid phase before transitioning to hypothyroid.
Taking liothyronine (T3) medication can elevate Free T3. Timing of blood draw matters — T3 peaks 2-4 hours after dose.
TSH and Free T4 are usually sufficient for routine thyroid evaluation. Free T3 adds value in specific situations:
T3 toxicosis — T3 rises before T4 in some cases
Normal TSH/T4 but still symptomatic — may have T4-to-T3 conversion problem
Monitor T3 levels to ensure appropriate dosing
Confirms adequate T3 production or conversion in treated patients
Upload your thyroid panel results and monitor TSH, T4, and T3 trends. See the complete picture of your thyroid health.
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