Measure your Free T3 levels! Free T3 is the most active thyroid hormone and the best indicator of your true metabolic health. It measures only the unbound portion of Triiodothyronine that can enter your cells, controlling energy, mood, and heart rate. Crucial for diagnosing Hyperthyroidism and identifying T4-to-T3 conversion issues—a common cause of persistent fatigue and brain fog despite normal TSH. Get the accurate picture of your cellular metabolism. Check your Free T3!