What Does a Free T3 Test Measure?
Free T3 measures the unbound, active form of triiodothyronine — the most potent thyroid hormone. While your thyroid produces mainly T4, about 80% of T3 is produced by converting T4 in your liver, kidneys, and other tissues. Free T3 is important because some patients have poor T4-to-T3 conversion, showing normal TSH and Free T4 but low Free T3 — which explains persistent symptoms.
How the Test Works
The Free T3 test is performed via a standard blood draw at any Quest Diagnostics location. Your sample is sent to a CLIA-certified lab for analysis. Results are typically available within 1-2 business days in your JustLabs account.
Conditions It Helps Diagnose
Who Should Get Tested
Patients with persistent thyroid symptoms despite normal TSH and Free T4, those on T3 medication (Cytomel, compounded T3), and when evaluating T3 thyrotoxicosis (hyperthyroidism driven by T3).
Order your Free T3 test — $10