{"id":"free-t3","name":"Free T3 (Free Triiodothyronine)","shortName":"Free T3","aliases":["Free T3","FT3","T3, Free","Triiodothyronine, Free","Free Triiodothyronine","T3, Free, Direct","Triiodothyronine (T3) Free","Triiodothyronine (T3), Free, Serum","T3 (Triiodothyronine), Free, Serum","T3, Free, Serum","Free T3 (FT3)","T3 libre"],"abbreviations":["FT3","fT3"],"loinc":[{"code":"3051-0","primary":true},{"code":"14928-6","primary":false},{"code":"29239-1","primary":false}],"specimen":"serum-or-plasma","units":[{"ucum":"pg/mL","display":"pg/mL"},{"ucum":"pmol/L","display":"pmol/L"},{"ucum":"ng/dL","display":"ng/dL"},{"ucum":"ng/L","display":"ng/L"}],"referenceRanges":[{"population":"20+","low":2,"high":4.4,"unit":"pg/mL","unitDisplay":"pg/mL","note":"Labcorp (over 19 years) and Mayo Clinic Laboratories (19 years and over) print the same adult interval; both use a Roche cobas electrochemiluminescence immunoassay. Other labs and methods (including equilibrium dialysis) can differ. The interval is not meant for pregnancy. Use the range printed on your own report.","sources":[{"title":"Triiodothyronine (T3), Free (test 010389)","url":"https://www.labcorp.com/tests/010389/triiodothyronine-t3-free","publisher":"Labcorp"},{"title":"T3 (Triiodothyronine), Free, Serum (test T3FR)","url":"https://www.testcatalog.org/show/T3FR","publisher":"Mayo Clinic Laboratories (Extended Catalog, testcatalog.org)"}]}],"review":{"status":"ai-verified","label":"AI-verified against cited sources. Awaiting expert review","lastReviewed":"2026-10-03"},"summary":"The unattached, active form of the thyroid hormone T3, which drives how fast your cells use energy. It is usually checked after TSH and free T4.","url":"https://bloodtesttracker.com/markers/free-t3","notice":"Information, not medical advice. Ranges differ between labs, so the range on your own report is the one your result is judged against."}