Free T3: What It Means for Your Health
Free T3 (free triiodothyronine) is the most metabolically potent thyroid hormone, 3-5 times more active than T4. Most T3 is produced by converting T4 to T3 in peripheral tissues (liver, kidneys, muscles). Free T3 is particularly useful for diagnosing T3 thyrotoxicosis (a form of hyperthyroidism where T3 is elevated but T4 is normal) and for evaluating patients on thyroid medication.
Reference Range
Reference ranges may vary between laboratories, age groups, and sexes. The values shown are general adult reference ranges. Always compare your results to the specific range provided by your testing laboratory.
What Is Free T3?
Free T3 (free triiodothyronine) is the most metabolically potent thyroid hormone, 3-5 times more active than T4. Most T3 is produced by converting T4 to T3 in peripheral tissues (liver, kidneys, muscles). Free T3 is particularly useful for diagnosing T3 thyrotoxicosis (a form of hyperthyroidism where T3 is elevated but T4 is normal) and for evaluating patients on thyroid medication.
Commonly included in:
What Does High Free T3 Mean?
Elevated Free T3 is commonly seen in Graves' disease and T3 thyrotoxicosis. It may be elevated before Free T4 rises in early hyperthyroidism. Symptoms include rapid heart rate, weight loss, nervousness, tremor, and excessive sweating.
What Does Low Free T3 Mean?
Low Free T3 can occur in severe hypothyroidism, nonthyroidal illness (sick euthyroid syndrome), malnutrition, or during caloric restriction. The body may reduce T4-to-T3 conversion as a protective mechanism during illness or starvation to conserve energy.
Related Biomarkers in Thyroid Function
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This information is for educational purposes only and is not intended as medical advice, diagnosis, or treatment. Reference ranges may vary between laboratories. Always consult a qualified healthcare professional for interpretation of your blood test results and before making any health decisions.