Free Thyroxine Index (FTI)
The Free Thyroxine Index (FTI), also known as the T7 index in some nomenclatures, is a calculated estimate of biologically available thyroxine derived from multiplying the total T4 concentration by the T3 resin uptake ratio. It was developed as a practical surrogate for free T4 measurement during an era when direct immunoassay of free T4 was not widely available, expensive, or technically unreliable. The T3 resin uptake test measures the percentage of radiolabelled T3 that binds to an inert resin, which is inversely proportional to the amount of unoccupied thyroxine-binding globulin (TBG) sites in the sample. When TBG is elevated (as in pregnancy or oestrogen therapy), it 'mops up' more of the added T3, reducing resin uptake; when TBG is reduced (as in nephrotic syndrome or liver disease), resin uptake rises. Multiplying total T4 by this ratio corrects for changes in TBG, yielding an index that tracks free T4 more closely than total T4 alone. The normal FTI range is approximately 1.4 to 3.8 in most older assay systems, though specific reference ranges vary by laboratory. Although largely replaced by modern direct free T4 immunoassays in most clinical laboratories, FTI remains relevant in laboratories with limited resources, in research reviewing older literature, and when understanding the rationale for TBG-corrected estimates. FTI interpretation remains concordant with TSH-based thyroid status assessment: elevated FTI suggests hyperthyroidism; depressed FTI suggests hypothyroidism.
FTI = Total T4 (mcg/dL) x T3 Uptake Ratio (fraction); Normal FTI: 1.4 - 3.8 Where each variable represents a specific measurable quantity in the engineering and electrical domain. Substitute known values and solve for the unknown. For multi-step calculations, evaluate inner expressions first, then combine results using the standard order of operations.
- 1Measure total T4 in mcg/dL (normal range approximately 4.5-12.5 mcg/dL in most adults).
- 2Perform the T3 resin uptake test, which returns a percentage (e.g., 30%). Express this as a ratio relative to a normal mean uptake (e.g., 30%/30% = 1.0 ratio); some systems simply use the raw percentage divided by 100.
- 3Multiply total T4 by the T3 uptake ratio: FTI = Total T4 (mcg/dL) x (T3 Uptake % / 30%).
- 4The result is the FTI. A value of 1.4-3.8 is considered normal in classical systems.
- 5Interpret elevated FTI in the context of a suppressed TSH as supporting hyperthyroidism.
- 6Interpret depressed FTI with elevated TSH as supporting hypothyroidism.
- 7Recognise that FTI corrects for changes in TBG binding capacity but does not account for changes in TBG affinity or other binding proteins, so it may still be misleading in severe hepatic or renal disease.
Euthyroid — consistent with normal thyroid function
Total T4 in the mid-normal range with a normal T3 uptake yields a normal FTI. This is consistent with a normal TSH and clinical euthyroid state.
High total T4 corrected to normal by low T3 uptake
Pregnancy raises oestrogen, which increases hepatic TBG production. Total T4 rises because more is bound to TBG, but free T4 is unchanged. The FTI correctly identifies the euthyroid state by adjusting for the high TBG.
Both total T4 and T3 uptake elevated — genuine hyperthyroidism
In hyperthyroidism, excess thyroid hormone saturates TBG, leaving fewer unoccupied binding sites. Resin uptake rises (less unoccupied TBG to compete). Total T4 is elevated AND T3 uptake is elevated — the FTI is markedly high, confirming hyperthyroidism.
Low total T4 corrected to normal — not hypothyroid
Nephrotic syndrome causes urinary TBG loss, lowering total T4. Without FTI correction, this would appear to be hypothyroidism. The high T3 uptake (reflecting low TBG) corrects the total T4 to reveal a normal FTI and euthyroid state.
Electrical engineers in power distribution companies use Free Thyroxine Index to size conductors, calculate voltage drop across long cable runs, and verify that circuit breaker ratings provide adequate protection against fault currents in residential, commercial, and industrial installations.
Electronics design engineers apply Free Thyroxine Index during printed circuit board layout to determine trace widths for required current capacity, calculate impedance matching for high-speed signal traces, and verify thermal dissipation in surface-mount components under worst-case operating conditions.
Maintenance technicians in manufacturing plants use Free Thyroxine Index to troubleshoot motor control circuits, verify transformer tap settings, and calculate expected current draws when commissioning variable frequency drives and programmable logic controller systems.
Renewable energy system designers rely on Free Thyroxine Index to size solar panel arrays, calculate battery bank capacity for off-grid installations, and determine inverter ratings that match the expected peak and continuous load demands of residential and commercial photovoltaic systems.
Open circuit or infinite resistance
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in free thyroxine index calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Short circuit condition
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in free thyroxine index calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Reactive component dominance
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in free thyroxine index calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Amiodarone therapy
Amiodarone inhibits T4-to-T3 conversion, raises total T4 and FTI in euthyroid patients, and can cause either hypo or hyperthyroidism. FTI is not a reliable monitor in amiodarone-treated patients; TSH and FT4 measured by equilibrium dialysis or ultrafiltration are preferred.
| Condition | Total T4 | T3 Uptake | FTI | True Thyroid Status |
|---|---|---|---|---|
| Normal | Normal | Normal | Normal (1.4-3.8) | Euthyroid |
| Pregnancy / high oestrogen | High | Low | Normal | Euthyroid |
| Nephrotic syndrome | Low | High | Normal | Euthyroid |
| Overt hyperthyroidism | High | High | High (>3.8) | Hyperthyroid |
| Overt hypothyroidism | Low | Low | Low (<1.4) | Hypothyroid |
| FDH (familial dysalbuminaemia) | High | Normal/high | High (false) | Euthyroid — artefact |
Has FTI been replaced by free T4 immunoassay?
In most modern clinical laboratories, yes. Direct free T4 immunoassays (analogue or equilibrium dialysis) have largely superseded the FTI because they are more accurate, do not require the T3 uptake step, and are less affected by TBG abnormalities. However, FTI remains in use in laboratories with older equipment and remains important for understanding historical clinical literature.
What is the T3 resin uptake test measuring?
The T3 resin uptake (T3RU) test measures how much of a radiolabelled T3 tracer binds to an inert resin rather than to TBG in the patient's serum. When TBG binding sites are scarce (low TBG or TBG saturated by excess thyroid hormone), more tracer binds to the resin, giving a high uptake. When TBG has many empty sites (high TBG), more tracer binds to TBG and less to the resin, giving a low uptake.
What conditions raise TBG and how does this affect FTI?
The most influential inputs in Free Thyroxine Index are the primary quantities that appear in the core formula — typically the rate, the principal amount or base quantity, and the time period or frequency factor. Changing any of these by even a small percentage can shift the output significantly due to multiplication or compounding effects. Secondary inputs such as adjustment factors, rounding conventions, or optional parameters usually have a smaller but still meaningful impact. Sensitivity analysis — varying one input while holding others constant — is the best way to identify which factor matters most in your specific scenario.
What conditions lower TBG and how does this affect FTI?
The most influential inputs in Free Thyroxine Index are the primary quantities that appear in the core formula — typically the rate, the principal amount or base quantity, and the time period or frequency factor. Changing any of these by even a small percentage can shift the output significantly due to multiplication or compounding effects. Secondary inputs such as adjustment factors, rounding conventions, or optional parameters usually have a smaller but still meaningful impact. Sensitivity analysis — varying one input while holding others constant — is the best way to identify which factor matters most in your specific scenario.
Is the FTI formula the same in all laboratories?
No. Different laboratories may express T3 uptake differently (as a percentage, as a ratio to a normal mean, or as a fraction), and reference ranges for FTI vary accordingly. It is essential to use the laboratory's own FTI reference range (typically 1.4-3.8 in classical percentage-ratio systems) rather than assuming a universal normal range.
When would FTI still be preferred over modern free T4?
FTI may still be preferred when: direct free T4 immunoassays are unreliable (e.g., in patients with severe illness affecting assay results, or when familial dysalbuminaemia hyperthyroxinaemia is suspected), in resource-limited settings where equilibrium dialysis free T4 is unavailable, or when auditing older medical records that report only total T4 and T3 uptake.
Can the FTI be used in children?
In the context of Free Thyroxine Index, this depends on the specific inputs, assumptions, and goals of the user. The underlying formula provides a deterministic relationship between inputs and output, but real-world application requires interpreting the result within the broader context of engineering and electrical practice. Professionals typically cross-reference calculator output with industry benchmarks, historical data, and regulatory requirements. For the most reliable results, ensure inputs are sourced from verified data, understand which assumptions the formula makes, and consider running multiple scenarios to bracket the range of likely outcomes.
What is familial dysalbuminaemia hyperthyroxinaemia?
Familial dysalbuminaemia hyperthyroxinaemia (FDH) is a genetic condition where a mutant albumin binds T4 with abnormally high affinity, raising total T4 and often causing falsely elevated FTI and direct free T4 (analogue method), while the patient is clinically and biochemically euthyroid. Equilibrium dialysis free T4 and TSH are normal in FDH. It is the prototypical example of a TBG-independent cause of misleading FTI.
Pro Tip
Remember the directional rule: in a genuine thyroid disorder, total T4 and T3 uptake move in the same direction (both up in hyperthyroidism, both down in hypothyroidism). When TBG is abnormal, they move in opposite directions. This directional check alone can tell you whether a total T4 abnormality is real or a TBG artefact.
Did you know?
The FTI was sometimes called the 'T7' because it was thought of as combining the T4 and T3 uptake tests — the combined index being a logical step beyond the individual fourth and third tests. The naming reflected the sequential numbering of thyroid assays as they were developed in clinical laboratories.
References
- ›Larsen PR. Thyroid-pituitary interaction: feedback regulation of thyrotropin secretion by thyroid hormones. NEJM 1982
- ›Stockigt JR. Free thyroid hormone measurement: a critical appraisal. Endocrinol Metab Clin North Am 2001
- ›DeGroot LJ. Non-thyroidal illness syndrome is a manifestation of hypothalamic-pituitary dysfunction. Endocrinol Metab Clin North Am 1999
- ›American Thyroid Association — Clinical Thyroidology for the Public