Free Triiodothyronine (T3)
Disclaimer: Information on this page is for educational purposes. Consult a physician to interpret your test results. Lissa Health helps track biomarker trends but does not replace medical advice.
What This Test Measures
Free T3 is the most active thyroid hormone. T4 converts to T3 in peripheral tissues.
T3 toxicosis is hyperthyroidism with elevated T3 and normal T4 (early Graves disease, toxic nodular goiter).
Low T3 syndrome in critical illness is an adaptive response and does not always require treatment.
Normal Ranges
| Group | Reference Range |
|---|---|
| Men | 2.0–4.4 pg/mL (2.6–5.7 pmol/L) |
| Women | 2.0–4.4 pg/mL |
| Children | 2.3–5.0 pg/mL |
Reference ranges may vary by laboratory and assay method.
Causes of High Levels
- Hyperthyroidism
- T3 toxicosis
- Thyroid hormone overdose
- Thyroiditis
Causes of Low Levels
- Hypothyroidism
- Non-thyroidal illness (low T3 syndrome)
- Selenium deficiency
Test Preparation
- Morning draw
- Avoid biotin supplements
- Before levothyroxine
Related Biomarkers
| Biomarker | Relationship |
|---|---|
| Free Thyroxine (T4) | Thyroid axis |
| Thyroid Stimulating Hormone (TSH) | Regulation |
| Cortisol | Stress axis interaction |
FAQ
How often should I take this test?
In hyperthyroidism — at diagnosis and during therapy monitoring.
What should I do if my result is abnormal?
Isolated high T3 — repeat with TSH/T4; confirm hyperthyroidism before treatment.
Last updated: June 2026