The Free Thyroxine Index (FTI), also known as the T7 Index, is a calculated estimate of free thyroxine levels used to evaluate thyroid function. The FTI adjusts the total T4 measurement by accounting for variations in thyroid-binding proteins using the T3 Uptake (T3U) value, making it a useful tool when direct free T4 testing is unavailable or when binding protein abnormalities may affect standard thyroid test interpretation. To use this calculator, enter the Total T4 and T3 Uptake values from a standard thyroid panel blood test. Our calculator will compute the Free Thyroxine Index, providing a reliable estimate of the biologically active free T4 circulating in the blood. The FTI is particularly valuable in clinical situations where thyroid-binding globulin (TBG) levels may be altered, such as during pregnancy, estrogen therapy, liver disease, or nephrotic syndrome, all of which can affect total T4 readings without reflecting a true change in thyroid function. An abnormal Free Thyroxine Index may indicate hyperthyroidism, hypothyroidism, or thyroid-binding protein abnormalities. For healthcare professionals, this tool supports accurate thyroid function assessment, especially when evaluating patients with suspected thyroid disorders, monitoring thyroid hormone replacement therapy, or interpreting thyroid labs in the context of binding protein fluctuations. It also serves as a helpful resource for individuals interested in understanding their thyroid health. By calculating the Free Thyroxine Index, clinicians and patients can gain a clearer picture of active thyroid hormone status, guiding informed treatment decisions and ongoing thyroid health management.
