Free Thyroxine Index (FTI): Meaning and Clinical Significance in Thyroid Function
The Free Thyroxine Index (FTI) is an indirect calculation used to estimate the amount of free (unbound) thyroxine in the blood, providing a reliable assessment of thyroid function even when thyroid hormone-binding proteins are abnormal. 1
Definition and Calculation
- FTI is calculated by multiplying the total thyroxine (T4) concentration by a factor derived from a T-uptake test, which assesses the availability of binding sites on thyroid hormone-binding proteins 2
- It serves as an alternative to direct free T4 measurement, particularly useful when binding protein abnormalities are present 3
- The normal reference range for FTI is typically between 121 and 360, though specific ranges may vary by laboratory 4
Clinical Utility
- FTI helps differentiate true thyroid dysfunction from conditions that only affect binding protein levels 5
- It provides a more accurate assessment of thyroid status than total T4 alone, especially in:
- FTI has high sensitivity (1.0) and specificity (0.996) for detecting thyroid dysfunction 4
Comparison with Other Thyroid Tests
- FTI performs similarly to direct free T4 measurement in most clinical scenarios, with comparable sensitivity and specificity 3
- In patients with normal binding protein levels, FTI and direct free T4 measurements provide equivalent diagnostic information 6
- FTI may be less accurate than direct free T4 measurement in certain complex binding protein abnormalities 3
- When used alongside TSH, FTI provides comprehensive information about thyroid function 1
Clinical Applications
- FTI is recommended alongside TSH for evaluating suspected thyroid dysfunction in pregnant women 1
- It helps monitor treatment effectiveness in patients with hyperthyroidism, with the goal of maintaining FTI in the high-normal range when using thioamide therapy 1
- Regular monitoring of FTI (every 2-4 weeks) is recommended during treatment adjustments for thyroid disorders 1
- FTI is particularly valuable when binding protein abnormalities might confound total T4 interpretation 5
Limitations and Considerations
- Some automated methods for calculating FTI may be affected by known inhibitors of thyroid hormone binding 2
- Different laboratory methods for T-uptake measurement can yield varying results, with some correlating better with TBG concentrations than others 2
- In patients with non-thyroidal illness, FTI results should be interpreted with caution 2
- Direct free T4 measurement may be superior to FTI in patients with high TBG levels 3
Clinical Pearls
- An abnormal FTI has a predictive value of 67% for detecting thyroid dysfunction when the prevalence is 0.81% 4
- FTI is considered cost-effective for case finding in thyroid dysfunction 4
- When interpreting FTI, consider factors that might affect binding proteins, such as pregnancy, medications, and liver disease 5
- In cases where thyroid status remains unclear after FTI assessment, additional testing (such as TSH stimulation or TRH tests) may be warranted 6