What It Measures
This test measures the amount of triiodothyronine (T3) circulating in the blood that is not bound to carrier proteins. Free T3 is the active thyroid hormone that enters cells and drives metabolic processes including oxygen consumption, protein synthesis, and energy metabolism.
Free T3 is the unbound, biologically active form of triiodothyronine, the most potent thyroid hormone. It regulates metabolism, energy production, body temperature, heart rate, and brain function. Because only the free (unbound) fraction is active at the cellular level, Free T3 is a more accurate marker of thyroid function than total T3.
Current Value
What High Means
Elevated Free T3 levels may indicate hyperthyroidism (overactive thyroid), Graves disease, thyroiditis (inflammation causing hormone release), excessive thyroid medication dosing, or T3-secreting thyroid nodules. High Free T3 accelerates metabolism, increases cardiac output, and can lead to bone loss, muscle wasting, and cardiac arrhythmias if sustained.
Possible Symptoms
Rapid or irregular heartbeat, anxiety and nervousness, tremors, heat intolerance and excessive sweating, unintentional weight loss, insomnia, diarrhea, muscle weakness, eye bulging (in Graves disease), irritability.
What Low Means
Low Free T3 levels may indicate hypothyroidism, poor T4-to-T3 conversion (often due to nutrient deficiencies in selenium, zinc, or iron), euthyroid sick syndrome (non-thyroidal illness syndrome), chronic stress or elevated cortisol, caloric restriction, or reverse T3 dominance. Low Free T3 slows metabolism and is associated with fatigue, weight gain, depression, and cognitive impairment.
Possible Symptoms
Fatigue and low energy, weight gain despite normal diet, cold intolerance, dry skin and hair loss, constipation, brain fog and poor concentration, depression, muscle aches and joint stiffness, slow heart rate, menstrual irregularities in women.
Risk Factors
Hashimoto thyroiditis, Graves disease, iodine deficiency or excess, selenium deficiency, chronic stress and elevated cortisol, caloric restriction or eating disorders, liver disease (impaired T4-to-T3 conversion), chronic kidney disease, aging, certain medications (amiodarone, lithium, beta-blockers, corticosteroids).
Actionable Advice
Supplements
- •Selenium (200 mcg)
- •Zinc (15-30 mg)
- •Iodine (150-300 mcg from kelp or potassium iodide)
- •Iron (if deficient)
- •Vitamin D3
- •Ashwagandha (supports T4-to-T3 conversion)
- •B vitamins (especially B12)
- •Magnesium
- •Tyrosine
Diet & Lifestyle
- •Manage chronic stress through meditation, breathwork, or yoga — cortisol inhibits T4-to-T3 conversion
- •Avoid extreme caloric restriction or prolonged fasting which downregulates T3 production
- •Exercise regularly but avoid overtraining which can lower Free T3
- •Prioritize 7-9 hours of quality sleep
- •Eat selenium-rich foods (Brazil nuts, sardines, eggs)
- •Consume adequate protein to support thyroid hormone synthesis
- •Minimize exposure to endocrine disruptors (BPA, phthalates, perchlorate)
- •Limit goitrogenic foods (raw cruciferous vegetables) if thyroid-compromised, or cook them to reduce goitrogen content
- •Address gut health — 20% of T4-to-T3 conversion occurs in the gut
Ask AI
Ask questions about your Free T3 results, trends, and what you can do to optimize.
Only one data point — trend chart will appear after multiple tests.
Related Biomarkers
Resources & Studies
All Readings
| Date | Value | Change |
|---|---|---|
| 2025-08-25 | 3.1 pg/mL | — |