Free T3 / Free T4

Free T3 (fT3) and free T4 (fT4) are the unbound, active fractions of two thyroid hormones, triiodothyronine and thyroxine. T4 is the main hormone your thyroid secretes. Your tissues then strip an iodine off it (deiodination) to make the more potent T3, which acts on receptors in the cell nucleus to control metabolism, heat production, and heart function. Measuring the free fractions sidesteps interference from changes in binding proteins. It helps tell apart primary thyroid disease, central hypothyroidism, and non-thyroidal illness, when you read it together with TSH.

Last reviewed: June 22, 2026

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