This test measures the unbound, biologically active form of triiodothyronine (T3) circulating in the blood.
Diagnostic Value: Free T3 reflects the immediate availability of active thyroid hormone to tissues and is crucial in detecting hyperthyroidism, even when T4 or TSH values are borderline.
Clinical Significance: Abnormal FT3 levels can indicate thyroid dysfunction, metabolic disorders, or issues with hormone conversion from T4 to T3. It is especially useful when symptoms of hyperthyroidism appear despite normal T4 results.
Symptom Correlation: Elevated FT3 may cause anxiety, rapid heartbeat, weight loss, or tremors. Low FT3 can lead to fatigue, weight gain, cold intolerance, or depression.
The Free T3 test helps identify active thyroid hormone imbalance and provides deeper insight into thyroid metabolic activity.