Free T3 (Triiodothyronine)
Active thyroid hormone
13 of 22 providers
Thyroid Function
2.3-4.2 pg/mL
What is Free T3 (Triiodothyronine)?
Free T3 (triiodothyronine) is the unbound, biologically active form of thyroid hormone circulating in your blood. Unlike Free T4, which acts as a "prohormone,"T3 is 3-4 times more potent and is the actual hormone that binds to receptors in your cells to drive metabolism, energy production, body temperature, heart rate, and countless other functions. Only about 0.3% of total T3 is "free"(unbound to proteins)—the rest is bound and inactive.
Here's the critical insight:roughly 80% of circulating T3 doesn't come directly from your thyroid gland—it comes from the conversion of T4 to T3 in peripheral tissues (liver, kidneys, muscles, gut). Your thyroid produces mostly T4, which then gets converted to T3 where it's needed. This conversion process depends on enzymes called deiodinases, which require key nutrients like selenium, zinc, and iron to function properly.
This is why you can have normal or even optimal TSH and Free T4, yet still have hypothyroid symptoms if your Free T3 is low. If your body isn't efficiently converting T4 to T3, you won't have enough active hormone to support optimal metabolism and energy—even if your thyroid gland itself is working fine. Factors that impair T4→T3 conversion include chronic stress, calorie restriction, selenium deficiency, chronic illness, certain medications (beta blockers, corticosteroids), and elevated reverse T3 (rT3).
Why Free T3 is Essential Despite Normal TSH/T4
- Chronic stress:Elevated cortisol shifts T4 conversion toward inactive reverse T3 (rT3) instead of active T3
- Nutrient deficiencies:Selenium, zinc, iron deficiency impairs deiodinase enzyme function
- Chronic illness/inflammation:Cytokines inhibit T4→T3 conversion as part of "euthyroid sick syndrome"
- Calorie restriction/low-carb diets:Body downregulates T3 to conserve energy
- Medications:Beta blockers, corticosteroids, amiodarone can block conversion
- Gut dysfunction:20% of T4→T3 conversion happens in the gut;dysbiosis impairs this
Optimal vs Standard Ranges
Optimal (Longevity)3.2-4.2 pg/mL▼
- Mid-upper normal range;best energy, metabolism, cognitive function
- Most people feel best here
Suboptimal (Low-Normal)2.3-3.1 pg/mL▼
- Low-normal;may have hypothyroid symptoms despite normal TSH/T4
- Consider T4→T3 conversion optimization or T3 supplementation
Low (Hypothyroid)<2.3 pg/mL▼
- Frank hypothyroidism at tissue level;requires treatment even if TSH/T4 normal
- May need T3-containing medication
High (Hyperthyroid)>4.2 pg/mL▼
- Hyperthyroidism;accelerated metabolism, heart rate, anxiety
- Requires evaluation for Graves'disease or thyroiditis
Scientific Evidence
T3 Potency and Cellular Function
T3 is 3-4x more potent than T4 at binding thyroid hormone receptors. Almost all genomic effects of thyroid hormone are mediated by T3 binding to nuclear receptors, which regulate transcription of genes controlling metabolism, development, and cellular differentiation.
Peripheral T4→T3 Conversion
~80% of circulating T3 comes from peripheral deiodination of T4 in liver, kidney, and muscle. Only ~20% is directly secreted by the thyroid. Deiodinase type 1 (D1) and type 2 (D2) convert T4→T3;both are selenium-dependent enzymes.
Free T3 and Hypothyroid Symptoms
Studies show that some patients with persistent hypothyroid symptoms on levothyroxine have normal TSH/T4 but low-normal Free T3, suggesting impaired T4→T3 conversion. A subset of these patients (~10-20%) report significant symptomatic improvement when T3 is added to their regimen.
Selenium and T3 Conversion
Selenium supplementation (200 mcg/day) in selenium-deficient individuals improves T4→T3 conversion by supporting deiodinase enzyme activity. Selenium deficiency impairs both T3 production and clearance of reverse T3.
Source:Duntas LH. Selenium and the thyroid:a close-knit connection. J Clin Endocrinol Metab. 2010;95(12):5180-5188.
Stress Cortisol and Reverse T3
Chronic stress and elevated cortisol inhibit type 1 deiodinase (D1) and increase type 3 deiodinase (D3), shifting T4 metabolism toward inactive reverse T3 instead of active T3. This creates a state of "functional hypothyroidism"despite normal TSH.
Source:Chatzitomaris A, et al. Thyroid allostasis-adaptive responses of thyrotropic feedback control to conditions of strain, stress, and developmental programming. Front Endocrinol. 2017;8:163.
Which Providers Test Free T3 (Triiodothyronine)?
Full Provider Comparison
| Provider | Includes | Annual Cost | Biomarkers |
|---|---|---|---|
 Superpower | ✓ | $199 | 100+ (150 with ratios) |
 WHOOP Advanced Labs | — | $349 | 65 |
 Labcorp OnDemand | — | $398 | 30+ |
| — | $486 | 40+ | |
| ✓ | $444 | 288 | |
| ✓ | $349 | 100+ | |
| — | $761 | 54 | |
 Function Health | ✓ | $365 | 160+ |
| — | $250 | 65 | |
| ✓ | $495 | 70+ | |
| ✓ | $895 | 100+ | |
| ✓ | $1950 | 150+ | |
| ✓ | $375 | 80+ | |
| — | $Varies | 75+ | |
| — | $190 | 100+ | |
| — | $99 | 50 | |
| ✓ | $124 | 60 | |
| — | $199 | 50 | |
| ✓ | $499 | 120+ | |
| ✓ | $4188 | 70-80+ | |
| ✓ | $375 | 85 | |
| ✓ | $700 | 128 |
Frequently Asked Questions
What is Free T3 (Triiodothyronine)?
Active thyroid hormone
What is the optimal range for Free T3 (Triiodothyronine)?
The standard reference range for Free T3 (Triiodothyronine) is 2.3-4.2 pg/mL. Optimal ranges may differ based on individual health goals and expert recommendations.
Which blood test providers include Free T3 (Triiodothyronine)?
13 out of 22 blood testing providers include Free T3 (Triiodothyronine) in their panels. This biomarker is widely available across major providers.
What category does Free T3 (Triiodothyronine) fall under?
Free T3 (Triiodothyronine) is categorized under Thyroid Function. This category includes biomarkers that help assess related aspects of health and wellness.
Medical Disclaimer
This information is for educational purposes only and is not medical advice. Always consult with a qualified healthcare provider about your specific health needs.
Last reviewed:2026-02-20