Free T3
The active thyroid hormone — directly regulates metabolic rate, energy production, and body temperature.
Optimal Range
3.0-4.0 pg/mL (optimal)
Risk-Stratified Targets
| Population / Context | Target |
|---|---|
| Optimal | 3.0–4.0 pg/mL |
| Low-normal (may be suboptimal)Investigate T4-to-T3 conversion if symptomatic | 2.3–3.0 pg/mL |
| Low (hypothyroid symptoms likely) | < 2.3 pg/mL |
| Elevated (possible hyperthyroidism) | > 4.4 pg/mL |
Why It Matters
Free T3 is the metabolically active thyroid hormone. Low Free T3 with normal TSH may indicate poor T4-to-T3 conversion, which can cause fatigue, weight gain, and cognitive fog despite 'normal' thyroid labs.
Understanding Free T3
Free T3 (triiodothyronine) is the biologically active thyroid hormone — the form that actually enters cells and turns on metabolic processes. While T4 is produced in much larger quantities by the thyroid gland, it is essentially a prohormone that must be converted to T3 by deiodinase enzymes in peripheral tissues (liver, kidneys, muscles) to exert its metabolic effects.
The conversion of T4 to T3 is a critical and often underappreciated step. Many factors can impair this conversion: chronic stress (elevated cortisol), caloric restriction, selenium or zinc deficiency, inflammation, liver disease, and certain medications (beta-blockers, amiodarone). When conversion is impaired, patients may have normal TSH and Free T4 but low Free T3 — resulting in clinical symptoms of hypothyroidism (fatigue, weight gain, cold intolerance, brain fog) despite 'normal' standard thyroid labs.
This is why longevity and functional medicine practitioners routinely include Free T3 in thyroid assessment rather than relying on TSH alone. A Free T3 in the lower third of the reference range, even if technically 'normal,' often correlates with suboptimal metabolic function and may benefit from investigation and intervention.