Active thyroid hormone
13 of 22 providers
Thyroid Function
2.3-4.2 pg/mL
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).
| Range Type | Level | Significance |
|---|---|---|
| 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. |
Selenium (200 mcg/day):THE most important nutrient for T4→T3 conversion. The deiodinase enzymes that convert T4 to T3 are selenium-dependent. Brazil nuts (2-3/day), seafood, organ meats, or supplement.
Zinc (15-30 mg/day):Required for thyroid hormone receptor function and conversion. Oysters, red meat, pumpkin seeds.
Iodine (150-300 mcg/day):Building block of thyroid hormones. Seaweed, iodized salt, seafood. Caution:excess iodine can worsen Hashimoto's in some people.
Iron:Iron deficiency impairs thyroid peroxidase enzyme. Ferritin should be >50 ng/mL (optimal >70).
Chronic stress/elevated cortisol shifts T4 conversion away from active T3 toward inactive reverse T3 (rT3), which blocks T3 receptors. This is why stressed individuals often have low Free T3 despite normal TSH/T4.
Interventions:Meditation, sufficient sleep (7-9 hours), adaptogenic herbs (ashwagandha, rhodiola), reducing psychological stressors, avoiding overtraining.
Severe calorie restriction or very low-carb diets cause your body to downregulate T3 production as an energy-conservation mechanism. This is adaptive during famine but problematic for modern dieters. Ensure adequate protein (1.6-2.2 g/kg) and sufficient calories (don't cut below 1200-1500 kcal for extended periods).
~20% of T4→T3 conversion occurs in the gut. Dysbiosis, SIBO, leaky gut can impair this. Probiotic-rich foods (yogurt, kefir, sauerkraut), prebiotic fiber, and addressing gut infections can help.
If you're on levothyroxine (T4-only) and Free T3 remains low, consider:
T4/T3 combination therapy:Add synthetic T3 (liothyronine, Cytomel) to levothyroxine
Natural desiccated thyroid (NDT):Armour Thyroid, NP Thyroid contain both T4 and T3 in ~4:1 ratio (like human thyroid)
Compounded T4/T3:Custom ratios tailored to your needs
Evidence is mixed, but ~10-20% of patients feel significantly better on combination therapy. Work with a knowledgeable endocrinologist or functional medicine doctor.
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.
Source:Yen PM. Physiological and molecular basis of thyroid hormone action. Physiol Rev. 2001;81(3):1097-1142.
~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.
Source:Bianco AC, Salvatore D, Gereben B, et al. Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases. Endocr Rev. 2002;23(38-89).
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.
Source:Wiersinga WM. Paradigm shifts in thyroid hormone replacement therapies for hypothyroidism. Nat Rev Endocrinol. 2014;10(3):164-174.
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.
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.
| Provider | Includes | Annual Cost | Biomarkers |
|---|---|---|---|
 Superpower | ✓ | $199 | 100+ (150 with ratios) |
 WHOOP Advanced Labs | — | $349 | 65 |
 Labcorp OnDemand | — | $398 | 30+ |
| — | $486 | 40+ | |
| ✓ | $468 | 83 | |
| ✓ | $349 | 100+ | |
 InsideTracker | — | $680 | 54 |
 Function Health | ✓ | $365 | 100+ |
| — | $250 | 65 | |
| ✓ | $495 | 70+ | |
| ✓ | $895 | 100+ | |
| ✓ | $1950 | 150+ | |
| — | $399 | 100+ | |
| — | $Varies | 75+ | |
| — | $190 | 100+ | |
| — | $99 | 50 | |
| ✓ | $125 | 60 | |
| — | $199 | 50 | |
| ✓ | $499 | 120+ | |
| ✓ | $4188 | 80+ | |
| ✓ | $375 | 85 | |
| ✓ | $700 | 129 |
13 providers include this biomarker in their panels
This information is for educational purposes only and is not medical advice. Always consult with a qualified healthcare provider about your specific health needs.
