Free Thyroxine Index (FTI) Calculator

What is the Free Thyroxine Index (FTI)?

The Free Thyroxine Index (FTI), also known as T7, is a calculation that helps estimate the amount of unbound, biologically active thyroxine (free T4) in the blood. It corrects the total T4 measurement for the amount of thyroid hormone-binding proteins present.

In situations where binding proteins like TBG are abnormal (e.g., pregnancy, oral contraceptive use), the total T4 can be misleading. The FTI provides a more accurate picture of the true thyroid status in these cases.

Why is the FTI Clinically Useful?

• Corrects for abnormal levels of thyroid-binding globulin (TBG).
• Provides a better assessment of thyroid function than Total T4 alone when protein abnormalities are suspected.
• It was a cornerstone of thyroid testing before direct free T4 immunoassays became widely available and is still useful today.

Interpreting the Result (Reference Guide)

Clinical Context is Paramount

The FTI result should always be interpreted alongside a TSH (Thyroid-Stimulating Hormone) level and the patient's clinical presentation for an accurate diagnosis.

• A low FTI with a high TSH suggests primary hypothyroidism.
• A high FTI with a low TSH suggests primary hyperthyroidism.
• Normal FTI with abnormal TSH may indicate subclinical disease or other issues.
• Always consult an endocrinologist for a definitive diagnosis and treatment plan.