Free T3 (Triiodothyronine)
Definition
Free T3 is the active, unbound form of triiodothyronine, the most metabolically active thyroid hormone. It regulates metabolism, energy production, body temperature, and cellular function. Free T3 levels reflect how much active thyroid hormone is available to cells throughout the body.
Why It Matters
Free T3 is the thyroid hormone that actually drives metabolism and energy. Low free T3 can cause fatigue, weight gain, cold intolerance, and brain fog—even when TSH is normal. Measuring free T3 helps identify thyroid hormone conversion issues and guides more targeted thyroid treatment.
Related Conditions
Related Modalities
Frequently Asked Questions
Low free T3 can result from poor T4-to-T3 conversion due to nutrient deficiencies (selenium, zinc, iron), chronic stress, inflammation, gut dysbiosis, calorie restriction, liver dysfunction, or certain medications. Addressing these factors helps optimize T3 levels.
Optimize free T3 by ensuring adequate selenium, zinc, and iron intake; managing stress and cortisol levels; supporting gut health; consuming adequate calories and protein; reducing inflammation; and using thyroid-supportive herbs like ashwagandha or guggul when appropriate.
Dr. Sanika Kshirsagar, ND
Doctorate of Naturopathic Medicine (ND)
Bastyr University, Kenmore, WA
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