Free T3 & Free T4
For informational purposes only — not medical advice. Always consult a qualified healthcare provider before making changes to your health regimen. Full disclaimer →
- TSH reflects the pituitary's assessment of thyroid function, not the actual hormones at the tissue level. A normal TSH means the pituitary is satisfied with the thyroid hormone signal it's receiving — but the pituitary lives in a privileged vascular environment and its exposure to thyroid hormones may not reflect what peripheral tissues are experiencing. People with symptoms of hypothyroidism (fatigue, cold intolerance, weight gain, cognitive slowing, constipation) despite normal TSH may have suboptimal Free T3 or Free T4 that TSH testing is failing to capture.
- Free T3 is the biologically active hormone — it is what actually binds to receptors and drives metabolism. Free T4 is largely a reservoir and prohormone. The conversion of FT4 to FT3 by type 2 deiodinase in peripheral tissues is the critical step that determines metabolic effect. Factors that impair this conversion — selenium deficiency, chronic inflammation, elevated cortisol, severe caloric restriction, and certain genetic polymorphisms in the DIO2 gene — can produce functional hypothyroidism at the tissue level despite normal TSH and FT4.
- People on levothyroxine (T4-only therapy) are at particular risk for low Free T3. Levothyroxine replaces T4 but relies entirely on peripheral conversion to generate T3. Some people, particularly those with DIO2 gene variants or compromised conversion capacity, cannot efficiently convert the replacement T4 to adequate T3. These individuals may have normalized TSH and FT4 on levothyroxine but have suboptimal FT3 and persistent symptoms. Adding small amounts of liothyronine (T3) to the regimen, or switching to desiccated thyroid extract (which contains both T4 and T3), can improve FT3 and symptom resolution in this subset.
- Free T3 declines with age independent of TSH — a potential contributor to the metabolic slowing associated with aging. Multiple studies have found that Free T3 levels decline progressively from middle age onward, even in people with stable TSH. This age-related decline in the active thyroid hormone may contribute to the reduction in resting metabolic rate, the changes in body composition, and the cognitive slowing that characterize normal aging. Whether optimizing Free T3 in older adults with low-normal levels produces meaningful longevity benefits is an active area of research.
- The FT3:FT4 ratio is a practical marker of T4-to-T3 conversion efficiency. A low ratio (below approximately 0.25 using standard units) suggests conversion impairment — more FT4 is present relative to FT3 than expected if conversion were working efficiently. This pattern, combined with symptoms, is the primary indication to evaluate conversion factors (selenium status, inflammatory markers, cortisol, dietary intake) or consider T3 supplementation under medical supervision.
Why TSH Alone Is Not Enough
TSH is an excellent screening test. It is highly sensitive for detecting overt hypothyroidism and hyperthyroidism, is well-standardized across laboratories, and is appropriate as a first-line test for most people with potential thyroid symptoms.
But TSH has a structural limitation that makes it insufficient as the sole thyroid assessment in anyone doing serious metabolic or longevity monitoring: it measures the pituitary's response to thyroid hormone levels, not the hormones themselves. The pituitary sits in a highly vascularized environment, receiving thyroid hormones through a specialized portal system that may not accurately reflect what peripheral tissues — muscle, liver, brain, heart — are experiencing.
The analogy is instructive: using TSH alone to assess thyroid function is like measuring blood pressure in one location to infer what's happening in an entirely different vascular bed. The correlation is good enough for population-level screening, but misses the cases where the relationship between central and peripheral thyroid hormone exposure is decoupled.
The most common decoupling scenario is impaired T4-to-T3 conversion — the most critical step in thyroid hormone metabolism, and one that TSH does not assess at all. If peripheral deiodinase activity is reduced, Free T3 can be suboptimal while TSH and Free T4 remain entirely normal. 1
The T4-to-T3 Conversion Problem
The conversion of inactive T4 to active T3 is regulated by three deiodinase enzymes, primarily type 2 deiodinase (DIO2) in most tissues and type 3 deiodinase (DIO3), which deactivates T3 to RT3. This conversion is the metabolic gateway through which thyroid hormone exerts its effects.
Factors that reduce DIO2 activity — and therefore reduce FT3 — include:
Selenium deficiency: DIO2 is a selenoprotein — its activity depends entirely on an adequate selenium supply. Selenium deficiency reduces DIO2 activity, impairing T4-to-T3 conversion. This is the most nutritionally actionable conversion impairment and should be assessed (and corrected) before pharmaceutical interventions are considered.
Chronic inflammation: Elevated IL-6, TNF-α, and other pro-inflammatory cytokines downregulate DIO2 expression in peripheral tissues. This is the mechanism behind the 'low T3 syndrome' or 'non-thyroidal illness syndrome' seen in chronic disease — thyroid function tests look normal except for a low FT3, reflecting inflammation-driven conversion impairment.
Chronic stress and elevated cortisol: Cortisol upregulates DIO3 (which converts T3 to inactive RT3) while downregulating DIO2. This shunts the T4-to-T3 conversion pathway toward RT3, reducing active T3 and producing the functional hypothyroidism that is so commonly associated with chronic stress states.
| Marker | Standard Range | Longevity Optimal | Notes |
|---|---|---|---|
| Free T3 | 2.3–4.2 pg/mL | 3.2–4.0 pg/mL | Active hormone; most directly affects metabolism |
| Free T4 | 0.8–1.8 ng/dL | 1.1–1.5 ng/dL | Prohormone reservoir; reflects thyroid output |
| FT3:FT4 ratio | Varies by units | ≥ 0.25 (pg/mL:ng/dL) | Low ratio suggests conversion impairment |
| Range Type | Value (pg/mL (Free T3) · ng/dL (Free T4)) | Notes |
|---|---|---|
| Standard Clinical Range | Free T3: 2.3–4.2 pg/mL · Free T4: 0.8–1.8 ng/dL | Designed to identify disease risk — not longevity optimisation. |
| Longevity-Optimal Target | Free T3: 3.2–4.0 pg/mL · Free T4: 1.1–1.5 ng/dL |
Associated with reduced all-cause mortality and extended healthspan.
Standard reference ranges for Free T3 and Free T4 include people with subclinical hypothyroidism and suboptimal thyroid function. Longevity-focused interpretation targets the upper half to upper third of the standard range for Free T3, where metabolic rate, cardiac function, and tissue responsiveness are optimal. Critically, Free T3 and Free T4 must always be interpreted alongside TSH — a low Free T4 with high TSH confirms primary hypothyroidism; a low Free T3 with normal TSH and normal Free T4 suggests impaired peripheral T4-to-T3 conversion; a low Free T3 and Free T4 with low TSH suggests secondary (central) hypothyroidism or non-thyroidal illness. The ratio of FT3 to FT4 is increasingly used as a marker of T4-to-T3 conversion efficiency — a low ratio suggests conversion impairment.
|
Already have your results? See what your Free T3 & Free T4 and other markers reveal about your longevity in 60 seconds.
Analyze My Labs →Affiliate disclosure: we may earn a commission if you purchase through these links at no extra cost to you.
My TSH is normal but I have all the symptoms of hypothyroidism. Should I test Free T3 and Free T4?
Yes — this is the primary clinical indication for measuring Free T3 and Free T4. A normal TSH with ongoing hypothyroid symptoms (fatigue, cold intolerance, weight gain, cognitive slowing, dry skin, hair loss, constipation, depression) is a common clinical scenario that standard screening misses. Several patterns can produce this: suboptimal Free T3 with normal FT4 (conversion problem); FT3 and FT4 both in the lower portion of the normal range but below individually optimal levels; or elevated thyroid peroxidase antibodies indicating autoimmune thyroid disease (Hashimoto's) that has not yet produced abnormal TSH. Free T3 below 3.0 pg/mL combined with hypothyroid symptoms is clinically meaningful even with a normal TSH. Having this data gives you and your clinician a much more complete picture than TSH alone.
What is Reverse T3 and should I test it alongside Free T3?
Reverse T3 (RT3) is an inactive isomer of T3 produced when T4 is converted down an alternative pathway. In normal physiology, a small proportion of T4 is converted to RT3 rather than active T3. Under conditions of severe physiological stress — critical illness, major caloric restriction, prolonged elevated cortisol, severe trauma — the body preferentially shunts T4 toward RT3 production, essentially applying the brakes on metabolism to conserve energy. In these states, Free T3 falls, RT3 rises, and the FT3:RT3 ratio decreases dramatically. Measuring RT3 is most valuable in the context of acute illness, post-surgery, or severe stress. Its value in routine wellness monitoring is more contested — proponents argue it captures chronic stress-related conversion impairment; critics argue RT3 is cleared so quickly that its serum level in non-acutely-stressed people is not a reliable marker of tissue thyroid hormone action. Measuring it alongside Free T3 and Free T4 in symptomatic patients is reasonable; as a standalone routine test in asymptomatic people, its utility is lower.
What nutrients are required for optimal T4-to-T3 conversion?
T4-to-T3 conversion is performed by selenium-dependent deiodinase enzymes. Selenium deficiency is the most well-documented nutritional impairment of conversion, and selenium repletion in deficient individuals has been shown to improve FT3 levels. Iron deficiency can impair thyroid hormone synthesis (iron is needed for thyroid peroxidase activity) and may affect conversion efficiency. Zinc is involved in thyroid hormone receptor function. Iodine is essential for thyroid hormone synthesis — both deficiency and excess can impair thyroid function. Vitamin D deficiency has been associated with Hashimoto's thyroiditis prevalence. In someone with low FT3 and suspected conversion impairment, checking selenium, ferritin, zinc, and vitamin D alongside the thyroid panel is a logical first step before considering pharmaceutical interventions.
What is the difference between Free T3/T4 and Total T3/T4?
Total T3 and Total T4 measure all T3 and T4 in the blood — both the portion bound to carrier proteins (inactive) and the free portion (active). Because binding protein levels vary significantly between individuals — elevated in pregnancy, oral contraceptive use, liver disease, genetic conditions; reduced in nephrotic syndrome, malnutrition, certain medications — Total T3 and T4 can be misleading. A person with high thyroid-binding globulin (TBG) will have high Total T3 and T4 but potentially normal or low Free T3 and T4. Free hormone assays measure only the biologically active unbound fraction and are not affected by changes in binding protein levels. For this reason, Free T3 and Free T4 are more accurate measures of actual thyroid hormone availability to tissues. Most current guidelines recommend Free over Total for clinical thyroid assessment.