Free Thyroxine Index (T7)
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.
AI-drafted. Awaiting verificationAlso called FTI, FT4ILOINC 32215-6
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What it is
The free thyroxine index (FTI, also called T7) is not measured directly. A lab calculates it from two blood tests, total T4 and T3 uptake. Most T4 in your blood rides on carrier proteins, and the amount of carrier protein changes in pregnancy and with some medicines. Total T4 alone can then look high or low even when the active, unattached part is normal. The index adjusts for that. Many labs now measure free T4 directly instead, so FTI is less common than it used to be.
Why it matters
FTI can give a clearer picture of thyroid hormone than total T4 alone when carrier proteins are higher or lower than usual. It is read together with TSH. Because it is an older, indirect estimate, your clinician may also want a direct free T4 test.
If it’s low
An index below your lab's range means less active T4 may be available than usual. This is often linked to an underactive thyroid, usually with a raised TSH. A problem with the pituitary gland, which sends the TSH signal, can also lower it, and then TSH may not be raised.
Common causes
- An underactive thyroid, such as Hashimoto's thyroiditis
- Thyroid surgery or radioactive iodine treatment
- Not enough thyroid medicine, or missed doses
- Serious illness or a hospital stay
- Rarely, a pituitary problem that lowers the TSH signal (central hypothyroidism)
What can help
- Ask whether your TSH and free T4 fit with this result
- If you take thyroid medicine, ask whether how and when you take it could play a role
- Ask whether to repeat the test
If it’s high
An index above your lab's range means more active T4 may be available than usual. This is often linked to an overactive thyroid, usually with a low TSH. Taking extra thyroid hormone, or a recent dose, can also raise it.
Common causes
- An overactive thyroid, such as Graves' disease or a nodule that makes extra hormone
- Thyroiditis, an inflammation that releases stored hormone for a while
- Taking more thyroid hormone than needed, including some supplements
- Biotin supplements, which can change some thyroid test results
- Rarely, inherited or unusual binding proteins that distort the test
What can help
- Ask whether your TSH and free T4 fit with this result
- Tell your clinician about every supplement and medicine you take, including biotin and any sold for thyroid support
- Ask whether to repeat the test or measure free T4 directly
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
For adults: 1.4–3.8 index. Source: Cureus, Case Reports in Endocrinology.
- Adults (this one applies)1.4–3.8 index
Sources: Cureus, Case Reports in Endocrinology
What can affect your result
- Pregnancy and estrogen-containing medicines, which raise carrier proteins
- Androgens and glucocorticoids, which lower carrier proteins
- Serious illness
- How the lab reports T3 uptake and calculates the index, since scales differ
- Biotin supplements
- Test method and lab
- Medicines that raise thyroid-binding proteins (estrogen, clofibrate, opiates such as methadone, 5-fluorouracil, perphenazine) or lower them (androgens, glucocorticoids) shift total T4. The index is meant to offset this, but the correction is not perfect.
- Medicines that pull T4 off its carrier proteins (salicylates, phenytoin, carbamazepine, furosemide, NSAIDs) lower total T4, and heparin can do the same in the test tube.
- High-dose biotin can interfere with some thyroid immunoassays; ask your clinician or lab whether to pause it before the draw.
- Pregnancy and serious illness change binding proteins and thyroid hormone levels.
Follows total T4, so it takes a few weeks to settle after a change in thyroid medicine.
When to talk to a clinician
Talk with a clinician about any result outside your lab's range, especially if you have symptoms of an overactive or underactive thyroid. No single universal cutoff marks an urgent result. If you have chest pain, a very fast or irregular heartbeat, fainting or confusion, call 911 or your local emergency number.
Questions you could ask
- Is a free thyroxine index useful for me, or is a direct free T4 test enough?
- Could pregnancy or a hormone medicine I take affect this result?
- Does this result fit with my TSH and free T4?
- Could a supplement or the timing of a medicine dose explain it?
- Should we repeat it or measure free T4 directly?
- Does this result fit with my TSH and free T4?
Questions people ask
Why can my total T4 be high while my free thyroxine index is in range?
The index adjusts total T4 for how much carrier protein you have. In pregnancy, or with estrogen-containing medicines, carrier proteins rise, so total T4 can be high while the active part is in range. The index (and free T4) help sort that out. Your clinician can tell you which applies to you.
Why does my lab report free T4 and not FTI?
Many labs now measure free T4 directly with an automated test. The free thyroxine index is an older approach that still appears on some thyroid panels.
Can I calculate it myself?
If your report gives total T4 in µg/dL and T3 uptake in percent, multiply them and divide by 100. For example, 9.7 × 30 ÷ 100 is about 2.9. Labs report T3 uptake in different ways, so the lab's own number is the one to trust.
Show the science and sources
For clinicians and researchers
FTI estimates free T4 as total T4 (µg/dL) multiplied by T3 uptake (%) and divided by 100. One published laboratory table fits this arithmetic: total T4 9.7 µg/dL with T3 uptake 30% gave an index of 2.9. When binding proteins rise, total T4 rises and traditional T3 uptake falls, so the product stays steady; the reverse happens when binding proteins fall. The ATA-commissioned 2023 review notes that uptake tests were renamed thyroid hormone binding ratios and are scaled to a reference assigned 1.00 or 40% depending on method, so numbers are not interchangeable between labs. One hospital using a Roche Elecsys analyzer divides T4 by a T-uptake ratio (reference 0.7 to 1.2) and prints an FTI interval of 6 to 11 µg/dL, a different scale from the 1.4 to 3.8 index above. Medicines that raise or lower binding proteins, or displace T4 from them, move total T4 without changing free T4, which is the situation the index is meant to correct. Indirect free-hormone estimates, including direct free T4 immunoassays, are still not fully independent of binding-protein changes and need method-specific reference intervals. A small report of four people with central hypothyroidism described a low FTI alongside free T4 and TSH in the usual range.
LOINC 32215-6 (primary), 3022-1. Specimen: calculated.
Sources
- Thyroid Stimulating Hormone and Thyroid Hormones (Triiodothyronine and Thyroxine): An American Thyroid Association-Commissioned Review of Current Clinical and Laboratory Status, Thyroid (American Thyroid Association) (2023)
- Post-COVID Myoclonus-Ataxia Syndrome: A Case Report of Successful Recovery With Intravenous Immunoglobulin (IVIG) Treatment (Table 3: laboratory thyroid panel with reference ranges), Cureus (2025)
- 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), Cureus (2024)
- A Rare Case of Primary Thyroid Lymphoma (Table 1: laboratory results with reference ranges), Cureus (2026)
- Measurement of Serum Free Thyroxine Index May Provide Additional Case Detection Compared to Free Thyroxine in the Diagnosis of Central Hypothyroidism, Case Reports in Endocrinology (2015)
AI-drafted. Awaiting verification. Last reviewed 2026-10-03. How we build and check these pages.