T3 is the most active thyroid hormone at the tissue level. Most circulating T3 is bound to proteins, with only the small free fraction biologically active. Free T3 measurement avoids the influence of binding protein variations and provides the most accurate picture of available T3.
This test is most useful in evaluating hyperthyroidism — including T3-predominant hyperthyroidism, where T4 may be normal — and in monitoring patients with thyroid disease where T3 levels matter clinically. It is generally interpreted alongside TSH and free T4. Sample type is serum.
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