{"id":"free-t4","name":"Free T4 (Free Thyroxine)","shortName":"Free T4","aliases":["Free T4","FT4","T4, Free","Thyroxine, Free","Free Thyroxine","T4 Free, Direct","T4, Free, Direct","T4, Free, Direct Dialysis","Free T4 (Direct)","Free T4 by Equilibrium Dialysis","Thyroxine (T4), Free, Direct","Thyroxine (T4), Free, Serum","Free Thyroxine (FT4)","Free Thyroxine Level","T4 libre"],"abbreviations":["FT4","fT4"],"loinc":[{"code":"3024-7","primary":true},{"code":"14920-3","primary":false},{"code":"6892-4","primary":false},{"code":"70217-5","primary":false}],"specimen":"serum-or-plasma","units":[{"ucum":"ng/dL","display":"ng/dL"},{"ucum":"pmol/L","display":"pmol/L"},{"ucum":"ng/L","display":"ng/L"}],"referenceRanges":[{"population":"20+","low":0.82,"high":1.77,"unit":"ng/dL","unitDisplay":"ng/dL","note":"Labcorp interval for adults over 19 (Roche cobas immunoassay). ARUP, on the same kind of immunoassay, prints 0.9 to 1.7 ng/dL from age 20. Ranges depend on the method: Quest prints 0.9 to 2.2 ng/dL for adults on its equilibrium dialysis test with mass spectrometry. In a very large US study, sex, age, time of day and time of year did not change the free T4 interval. The interval is not meant for pregnancy. Use the range printed on your own report.","sources":[{"title":"Thyroxine (T4), Free, Direct (test 001974)","url":"https://www.labcorp.com/tests/001974/t4-free-direct-s","publisher":"Labcorp"},{"title":"Thyroxine, Free (Free T4) (test 0070138)","url":"https://ltd.aruplab.com/Tests/Pub/0070138","publisher":"ARUP Laboratories"},{"title":"T4, Free, Direct Dialysis (test FFT4F)","url":"https://www.testcatalog.org/show/FFT4F","publisher":"Quest Diagnostics Nichols Institute (listed in the Mayo Clinic Laboratories Extended Catalog, testcatalog.org)"},{"title":"Circadian and Circannual Rhythms in Thyroid Hormones: Determining the TSH and Free T4 Reference Intervals Based Upon Time of Day, Age, and Sex","url":"https://pubmed.ncbi.nlm.nih.gov/26061389/","publisher":"Thyroid","year":2015}]}],"review":{"status":"ai-verified","label":"AI-verified against cited sources. Awaiting expert review","lastReviewed":"2026-10-03"},"summary":"The unattached, ready-to-use part of thyroxine (T4), the main hormone your thyroid makes. It shows how much thyroid hormone is available to your body.","url":"https://bloodtesttracker.com/markers/free-t4","notice":"Information, not medical advice. Ranges differ between labs, so the range on your own report is the one your result is judged against."}