What It Measures
This test measures the amount of triiodothyronine (T3) circulating freely in the blood, not bound to proteins. Free T3 is the active form that enters cells and drives metabolic processes including oxygen consumption, protein synthesis, and energy production.
Free T3 measures the unbound, biologically active form of triiodothyronine, the most potent thyroid hormone. It is critical for regulating metabolism, energy production, body temperature, and cognitive function. Free T3 is considered the best indicator of actual thyroid hormone activity at the cellular level.
Current Value
What High Means
Elevated Free T3 may indicate hyperthyroidism (Graves disease, toxic nodular goiter), excessive thyroid hormone supplementation, thyroiditis (early inflammatory phase), or rarely a TSH-secreting pituitary adenoma. High levels accelerate metabolism, increase heart rate, and can lead to bone loss and cardiac complications if sustained.
Possible Symptoms
Rapid or irregular heartbeat, anxiety, tremors, weight loss despite normal appetite, heat intolerance, excessive sweating, insomnia, diarrhea, muscle weakness, irritability, bulging eyes (Graves disease), hair thinning
What Low Means
Low Free T3 may indicate hypothyroidism, poor T4-to-T3 conversion (common with chronic stress, inflammation, selenium/zinc deficiency, or liver/gut dysfunction), euthyroid sick syndrome (non-thyroidal illness), caloric restriction, or reverse T3 dominance. Low Free T3 is associated with fatigue, weight gain, depression, and impaired cognitive function.
Possible Symptoms
Fatigue, weight gain, cold intolerance, constipation, dry skin, hair loss, brain fog, depression, muscle aches, slow heart rate, puffy face, elevated cholesterol, menstrual irregularities, poor memory
Risk Factors
Hashimoto thyroiditis, Graves disease, hypothyroidism, hyperthyroidism, thyroid nodules, pituitary disorders, chronic stress, nutrient deficiencies (selenium, zinc, iron, iodine), liver disease, chronic illness, eating disorders, aging
Actionable Advice
Supplements
- •Selenium (200 mcg/day — supports T4-to-T3 conversion)
- •Zinc (15-30 mg/day — required for thyroid hormone synthesis)
- •Ashwagandha (supports thyroid function and reduces stress)
- •Iron (if deficient — necessary for thyroid peroxidase activity)
- •Vitamin D3 (supports immune regulation and thyroid health)
- •B-complex vitamins (support cellular energy metabolism)
- •Iodine (150-200 mcg/day — essential precursor, use cautiously with Hashimoto)
- •Magnesium (supports enzymatic conversion processes)
- •Vitamin A (supports thyroid hormone receptor sensitivity)
Diet & Lifestyle
- •Manage chronic stress — cortisol impairs T4-to-T3 conversion
- •Prioritize 7-9 hours of quality sleep nightly
- •Eat adequate protein and calories — severe restriction lowers T3
- •Include selenium-rich foods: Brazil nuts, sardines, eggs
- •Exercise regularly but avoid overtraining which can suppress T3
- •Address gut health — 20% of T4-to-T3 conversion occurs in the gut
- •Minimize exposure to endocrine disruptors (BPA, phthalates, perchlorate)
- •Limit excessive raw cruciferous vegetable consumption if thyroid is compromised
- •Ensure adequate carbohydrate intake — very low-carb diets can reduce T3
Ask AI
Ask questions about your T3 Free results, trends, and what you can do to optimize.
Only one data point — trend chart will appear after multiple tests.
Resources & Studies
All Readings
| Date | Value | Change |
|---|---|---|
| 2026-01-28 | 3.1 pg/mL | — |