The free thyroxine index (FTI) evaluates thyroid function and identifies hypo- or hyperthyroidism. A decreased FTI reflects a decrease in the levels of free thyroxine (T4) in circulation and the possibility of hypothyroidism, while an increased FTI may indicate hyperthyroidism.
Standard Range: 1.40 – 3.80 Index
The ODX Range: 1.70 – 4.60 Index
Low free thyroxine index can be associated with hypothyroidism (Pantalone 2015).
High free thyroxine index can be associated with hyperthyroidism.
The free thyroxine index (FTI), or T7, is considered the first convenient method to evaluate the amount of free T4 in circulation. It is based on total T4, which primarily represents occupied binding sites, and T3 uptake, which primarily reflects unoccupied binding sites. The FTI is designed to account for alterations in the proteins capable of binding thyroid hormones. An increase in binding proteins, including albumin and thyroid-binding globulin, may increase serum T4 even though the amount of free unbound hormone is normal (Midgley 2001).
Although more direct measurements of free T4 have primarily replaced the FTI, the FTI may still help diagnose central hypothyroidism caused by pituitary dysfunction with TSH within the conventional range and total T4 at the lower end of normal. In such cases, the FTI may be more beneficial than measuring free T4 (Pantalone 2015). However, in some cases, the FTI may be within the conventional range despite a free T4 that is borderline low or below optimal (Westcott 2021).
Midgley, J E. “Direct and indirect free thyroxine assay methods: theory and practice.” Clinical chemistry vol. 47,8 (2001): 1353-63.
Pantalone, Kevin M et al. “Measurement of Serum Free Thyroxine Index May Provide Additional Case Detection Compared to Free Thyroxine in the Diagnosis of Central Hypothyroidism.” Case reports in endocrinology vol. 2015 (2015): 965191. doi:10.1155/2015/965191
Westcott, Gregory P et al. “USE OF THE FREE THYROXINE INDEX TO REFINE THE LOWER LIMIT OF A FREE THYROXINE IMMUNOASSAY FOR DETECTION OF SECONDARY HYPOTHYROIDISM.” Endocrine practice : official journal of the American College of Endocrinology and the American Association of Clinical Endocrinologists vol. 27,1 (2021): 8-14. doi:10.4158/EP-2020-0210