T3 is produced both directly by the thyroid and through conversion from T4 in body tissues. Total T3 includes both bound and free forms, with most circulating T3 bound to proteins. Elevated T3 is a characteristic finding in many cases of hyperthyroidism, sometimes appearing before T4 rises.
This test is used to evaluate suspected hyperthyroidism, especially when TSH is suppressed but T4 is normal (T3 toxicosis), monitor antithyroid therapy, and assess unusual thyroid function patterns. Free T3 is often preferred when binding protein variations might be present. Sample type is serum.
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