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Free T3 measures the unbound, active form of the T3 hormone that's available for the body to use. It's considered a more accurate measure of thyroid function than total T3, since it isn't affected by protein levels in the blood.

T4 is the main hormone produced by the thyroid gland and serves as a precursor to T3. Measuring T4 helps assess overall thyroid function and is commonly used to diagnose and monitor hypothyroidism and hyperthyroidism.