What it measures.
Free T3 is the biologically active thyroid hormone with 15x greater receptor affinity than T4. Only 20% comes directly from the thyroid; 80% is converted from T4 in peripheral tissues. Low Free T3 directly impacts energy, metabolism, and overall vitality.
The concentration of unbound triiodothyronine (T3) in blood. T3 contains three iodine atoms and has a short half-life of about 1 day, reflecting real-time thyroid hormone activity.
Why it matters.
Free T3 is what actually enters cells and activates thyroid receptors. Someone with normal TSH and T4 but low T3 may still experience hypothyroid symptoms. It's particularly important when assessing conversion issues.
Physiology.
The thyroid produces only 20% of T3 directly. Most T3 is produced by deiodinase enzymes converting T4 in the liver, kidneys, and other tissues. Stress, inflammation, and nutrient deficiencies can shift conversion toward inactive reverse T3 instead.
Testing & preparation.
How to prepare
- No fasting required
- Can be drawn anytime
- Stop biotin supplements 72 hours before
- Note if taking T3-containing medications
When to test
When hypothyroid symptoms persist despite normal TSH/T4, assessing T4-to-T3 conversion, or monitoring T3-containing therapy.
How often
Every 6-8 weeks during dose changes; as needed based on symptoms.
Interpretation.
High free t3
Common causes:
- Hyperthyroidism
- T3-containing medication (Cytomel, desiccated thyroid)
- T3 toxicosis
- Early Graves' disease
Implications:
- Rapid heart rate, palpitations
- Anxiety, irritability, tremor
- Weight loss despite good appetite
- Risk of cardiac complications
Low free t3
Common causes:
- Hypothyroidism
- Poor T4 to T3 conversion
- Chronic stress or illness (euthyroid sick syndrome)
- Nutrient deficiencies (selenium, zinc, iron)
- Inflammation or chronic disease
Implications:
- Fatigue despite normal TSH
- Weight gain, cold intolerance
- Brain fog, depression
- Slow metabolism despite adequate T4
Optimization.
Diet
- Selenium-rich foods for deiodinase enzyme function
- Adequate protein for thyroid hormone synthesis
- Zinc from oysters, meat, pumpkin seeds
- Anti-inflammatory foods to support conversion
Lifestyle
- Stress management (cortisol impairs T4 to T3 conversion)
- Regular exercise (upregulates T3-producing enzymes)
- Quality sleep for optimal hormone balance
- Address gut health (20% of T3 conversion occurs in gut)
Supplements
- Selenium 200mcg for conversion support
- Zinc 15-30mg if deficient
- Ashwagandha may support thyroid function
FAQs.
Why might I have low T3 with normal T4?
Poor T4-to-T3 conversion is common. Causes include chronic stress (elevated cortisol), inflammation, nutrient deficiencies (selenium, zinc, iron), gut dysfunction, and chronic illness. The body shifts conversion toward inactive reverse T3 to conserve energy. Addressing root causes often improves conversion.
Should I take T3 medication?
T3 supplementation remains controversial. Some patients with persistent hypothyroid symptoms despite adequate T4 therapy benefit from combination T4/T3 or desiccated thyroid. However, T3 has a short half-life requiring multiple daily doses or sustained-release forms. Work with an experienced provider to assess if T3 therapy is appropriate.