Free T3 (Triiodothyronine)
Free T3 is the active thyroid hormone that drives metabolism. Low free T3 can cause fatigue, cold intolerance, and slow metabolism — even when TSH looks normal.
Have your own lab results?
Upload your test and get a personalized meal plan targeting your specific free t3 (triiodothyronine) level.
What Free T3 (Triiodothyronine) measures
T3 is the most metabolically active thyroid hormone — it directly drives energy, body temperature, heart rate, and tissue activity. The 'free' portion (not bound to proteins) is the bioavailable fraction. Most circulating T3 is converted from T4 inside tissues, so issues with conversion can produce symptoms despite normal TSH.
Normal ranges
Reference ranges may vary slightly by lab. Always use the range provided on your specific test report.
What affects your free t3 (triiodothyronine) level
- Hypothyroidism (most common cause of low free T3)
- Selenium and iodine deficiency (impair T4-to-T3 conversion)
- Chronic illness or inflammation (sick euthyroid syndrome)
- Severe calorie restriction and prolonged dieting
- Gut conditions (impair conversion)
- Beta-blockers and amiodarone
- High reverse T3 (a converted-but-inactive form)
Foods that may help
Brazil nuts (1–2 per day)
Highest selenium source — needed for T4-to-T3 conversion
Seafood (cod, tuna, shrimp)
Iodine plus selenium together
Eggs
Iodine, selenium, and tyrosine — the thyroid building block
Beef and oysters
Zinc and iron, both required for thyroid hormone production
Leafy greens
Magnesium supports thyroid receptor function
When to see your doctor
Any abnormal thyroid value should be evaluated alongside TSH, free T4, and (if autoimmune is suspected) thyroid antibodies. Don't self-supplement thyroid hormone, iodine, or 'thyroid support' formulas without testing first — over-supplementation can cause real harm.
Related biomarkers
Get your own personalized plan
Upload your health test PDF and our AI will create a personalized interpretation and meal plan targeting your specific biomarkers, in minutes.
Educational content only · Not medical advice