Free Thyroxine Index (FTI) Is Not the Same as TSH
No, a free thyroxine index (FTI) calculation is not the same as TSH (thyroid-stimulating hormone). These are fundamentally different laboratory measurements that assess different aspects of thyroid function 1.
Understanding TSH vs. FTI
TSH (Thyroid-Stimulating Hormone)
- Definition: A pituitary hormone that regulates thyroid function
- What it measures: The brain's assessment of thyroid hormone levels
- Clinical use: Primary screening test for thyroid dysfunction
- Origin: Produced by the pituitary gland
Free Thyroxine Index (FTI)
- Definition: An indirect calculation that estimates the amount of unbound (free) thyroxine
- What it measures: An estimate of the biologically active thyroid hormone
- Clinical use: Assessment of thyroid hormone availability
- Calculation: Typically calculated from total T4 and T3 uptake tests
Clinical Significance of the Difference
The distinction between TSH and FTI is crucial for proper diagnosis and management of thyroid disorders:
- In primary hypothyroidism: TSH is elevated while FTI/FT4 is low 1
- In central (secondary/tertiary) hypothyroidism: Both TSH and FTI/FT4 may be low 2
- In hyperthyroidism: TSH is suppressed while FTI/FT4 is elevated
Diagnostic Algorithm
- High TSH + Low FT4/FTI = Primary hypothyroidism
- Low/normal TSH + Low FT4/FTI = Central hypothyroidism
- Low TSH + High FT4/FTI = Hyperthyroidism
- Normal TSH + Normal FT4/FTI = Euthyroid state
Modern Clinical Practice
Current guidelines recommend:
- Using both TSH and FT4 for comprehensive thyroid assessment 1
- Direct measurement of free T4 (FT4) has largely replaced FTI in modern practice 3
- FT4 is considered superior to FTI, particularly in patients with abnormal thyroid-binding globulin (TBG) levels 3
Common Pitfalls and Caveats
- TSH alone can be misleading: In central hypothyroidism, TSH may remain within normal range despite low thyroid hormone levels 1
- FTI can be inaccurate: In conditions affecting binding proteins (pregnancy, liver disease, certain medications) 4
- Laboratory interference: Thyroid hormone autoantibodies and heterophilic antibodies can interfere with both TSH and FT4/FTI measurements 5
Clinical Recommendations
- For routine screening: Check TSH first, with reflex to FT4 if abnormal
- For suspected central hypothyroidism: Check both TSH and FT4 simultaneously 2
- For monitoring levothyroxine therapy:
In conclusion, while both tests evaluate thyroid function, they measure completely different aspects of the thyroid axis and should be interpreted together for comprehensive thyroid assessment.