Free T3 (Free Triiodothyronine)
The active thyroid hormone that directly affects metabolism. More potent than T4 and useful for diagnosing hyperthyroidism.
What is Free T3?
Free T3 (free triiodothyronine) is the active form of thyroid hormone that directly affects your cells. While T4 is the main hormone produced by your thyroid, it must be converted to T3 to exert metabolic effects.
Only about 0.3% of T3 circulates freely (unbound to proteins) — this "free" portion is biologically active.
Normal Ranges
| Free T3 Level | Interpretation |
|---|---|
| 2.3–4.2 pg/mL | Normal (varies by lab) |
Why It Matters
Hyperthyroidism Diagnosis
Free T3 is often elevated early in hyperthyroidism, sometimes before Free T4 rises. T3 toxicosis (high T3, normal T4) is a recognized pattern.
Conversion Assessment
Some people don't convert T4 to T3 efficiently. Checking both helps assess thyroid hormone availability at the cellular level.
Medication Monitoring
For patients on T3 medication (liothyronine) or combination therapy, Free T3 monitoring is essential.
When Free T3 is Checked
Free T3 isn't part of routine screening. It's ordered when:
- Hyperthyroidism is suspected but T4 is normal
- Evaluating conversion issues
- Monitoring T3-containing medications
Low Free T3
Can occur in:
- Severe illness (euthyroid sick syndrome)
- Caloric restriction
- Chronic stress
Related Biomarkers
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