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Hormonal
Free T3
The active thyroid hormone — directly regulates metabolic rate, energy production, and body temperature.
Optimal Range
3.0-4.0 pg/mL (optimal)
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.