Free T3
Also known as Free Triiodothyronine
The bioactive thyroid hormone — small changes have outsized clinical effects.
Definition
T3 (triiodothyronine) is the biologically active thyroid hormone, produced both directly by the thyroid and via peripheral conversion from T4. Free T3 is the bioavailable fraction. Tissue-level thyroid function depends most directly on this number, which is why peptide-readiness labs include it even when TSH is normal.
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TSHThyroid-Stimulating Hormone
The pituitary hormone that drives thyroid output — the primary screening test for thyroid function.
BiomarkerFree T4Free Thyroxine
The unbound, bioavailable fraction of thyroxine — the main circulating thyroid hormone.
Clinical ConceptThyroid AxisHypothalamic-Pituitary-Thyroid Axis
The feedback loop between hypothalamus (TRH), pituitary (TSH) and thyroid (T4, T3) — governs metabolic rate across tissues.
$149 panel.
A clinician reads it in 24 hours.
Order the Peptide Readiness Panel, draw at a Quest or LabCorp near you, and your clinician sends back a matched protocol with a confidence score.