{"id":"free-thyroxine-index","name":"Free Thyroxine Index (T7)","shortName":"Free thyroxine index","aliases":["Free Thyroxine Index","FTI","T7 Index","Free T4 Index","Thyroxine Free Index (T7)","FT4I","T7 (Free Thyroxine Index)","Free T4 Index (T7)","Free Thyroxine Index (FTI)","Thyroxine Index, Free","FTI (calculated)"],"abbreviations":["FTI","FT4I"],"loinc":[{"code":"32215-6","primary":true},{"code":"3022-1","primary":false}],"specimen":"calculated","units":[{"ucum":"{index}","display":"index"}],"referenceRanges":[{"population":"Adults","low":1.4,"high":3.8,"unit":"{index}","unitDisplay":"index","note":"The classic calculation is total T4 in µg/dL × T3 uptake % ÷ 100. This interval is printed in two published case-report tables that use that scale (the laboratory is not named), and no laboratory directory page could be opened to confirm it. A third case-report table prints 1.0 to 4.5, so the interval varies by laboratory. Analyzers that report T-uptake as a ratio near 1.0 divide T4 by that ratio and print the index on a different scale (6 to 11 µg/dL in one hospital), which cannot be compared with this one. Use the interval printed on your own report.","sources":[{"title":"Post-COVID Myoclonus-Ataxia Syndrome: A Case Report of Successful Recovery With Intravenous Immunoglobulin (IVIG) Treatment (Table 3: laboratory thyroid panel with reference ranges)","url":"https://pmc.ncbi.nlm.nih.gov/articles/PMC12702629/","publisher":"Cureus","year":2025},{"title":"New-Onset Paroxysmal Atrial Fibrillation in a Healthy Male With Post-COVID-19 Painless Thyroiditis: A Case Report (Tables 2 and 3: thyroid tests with reference ranges)","url":"https://pmc.ncbi.nlm.nih.gov/articles/PMC11376423/","publisher":"Cureus","year":2024},{"title":"A Rare Case of Primary Thyroid Lymphoma (Table 1: laboratory results with reference ranges)","url":"https://pmc.ncbi.nlm.nih.gov/articles/PMC13274580/","publisher":"Cureus","year":2026},{"title":"Measurement of Serum Free Thyroxine Index May Provide Additional Case Detection Compared to Free Thyroxine in the Diagnosis of Central Hypothyroidism","url":"https://pmc.ncbi.nlm.nih.gov/articles/PMC4686635/","publisher":"Case Reports in Endocrinology","year":2015}]}],"review":{"status":"ai-draft","label":"AI-drafted. Awaiting verification","lastReviewed":"2026-10-03"},"summary":"A calculated estimate of how much active thyroxine (T4) is available, worked out from total T4 and T3 uptake. It is an older alternative to measuring free T4.","url":"https://bloodtesttracker.com/markers/free-thyroxine-index","notice":"Information, not medical advice. Ranges differ between labs, so the range on your own report is the one your result is judged against."}