Free Thyroxine Index (T7)

Introduction

Accurate evaluation of thyroid function requires adjustment for hormone binding. The Free Thyroxine Index (FTI or T7) corrects total thyroxine measurements to estimate active hormone available for tissue uptake.

What Is the Free Thyroxine Index (T7)?

FTI is a calculated marker representing unbound, active thyroxine (Free T4) in circulation. It provides insight into thyroid status when plasma binding proteins vary.

Calculating the Free Thyroxine Index

FTI=Total T4×(T3 Uptake100)\text{FTI} = \text{Total T4} × \bigl(\tfrac{\text{T3 Uptake}}{100}\bigr)

• Total T4: Measures all circulating thyroxine (bound + free). Link to T4 — Total.
  
• T3 Uptake: Reflects binding site availability on thyroxine-binding globulin. Link to T3 Uptake.
  
  

Normal and Optimal Ranges

• Standard: 1.40–3.80 Index
  
• Optimal: 1.70–4.60 Index
  
  

Values outside these ranges may indicate thyroid dysfunction.

Clinical Implications of Low FTI

A decreased FTI suggests underactive thyroid hormone action, even when total T4 and TSH appear normal. This pattern supports diagnoses of primary hypothyroidism, which can present with fatigue, weight gain, depression, and cold intolerance.

Clinical Implications of High FTI

An elevated FTI indicates excess active hormone and guides evaluation for hyperactive thyroid, often seen in conditions like Graves’ disease. Symptoms include weight loss, tremors, heat intolerance, and anxiety.

Other Conditions Associated with Abnormal FTI

Beyond thyroid disorders, abnormal FTI may reflect altered binding protein levels in liver disease or nephrotic syndrome, or central hypothyroidism due to pituitary pathology.

Interfering Factors

• Falsely Decreased FTI: Non-thyroidal illness, assay interference, low binding proteins.
  
• Falsely Increased FTI: Pregnancy, estrogen therapy, hepatic dysfunction.
  
  

Drug Associations

• Decrease FTI: Glucocorticoids, beta-blockers, anticonvulsants.
  
• Increase FTI: Heparin, estrogen, amiodarone.
  
  

Conclusion

In conclusion, a comprehensive evaluation by a functional medicine nurse practitioner in Florida facilitates identification of cellular-level and molecular imbalances underlying thyroid hormone availability. By integrating evidence-based allopathic therapies with longevity medicine medical care services—where hormonal optimization supports systemic resilience—we offer a preventive framework to rebuild health and vitality. Call (904) 799-2531 or schedule online to request your personalized thyroid assessment.

Further Reading

1. Fyffe JA, Ayoub L, Cohen HN, et al. Clinical and Laboratory Evaluation of Four Methods of Assessing Free Thyroxine Status in Thyroid Clinic Patients. Ann Clin Biochem. 1980 Nov;17(6):334–338. PubMed
   
2. dos Remedios LV, Weber PM, Feldman R, et al. Detecting Unsuspected Thyroid Dysfunction by the Free Thyroxine Index. Arch Intern Med. 1980 Aug;140(8):1045–1049. PubMed
   
3. Westcott GP, Mulla CM, Hennessey JV. Use of the Free Thyroxine Index to Refine the Lower Limit of a Free Thyroxine Immunoassay for Detection of Secondary Hypothyroidism. Endocr Pract. 2021 Jan;27(1):8–14. PubMed
4. Gupta N, Heil W, McAllister L, et al. Measurement of Serum Free Thyroxine Index May Provide Diagnosis of Central Hypothyroidism in Pituitary Disease. Int J Endocrinol. 2015;2015:4686635. PubMed