Free Thyroxine Index

**Semantic type:** Laboratory Procedure

**Definition:** A measurement of the thyroid status (function, disease diagnosis, or therapy effectiveness) in a biological specimen. This is calculated by a mathematical formula that takes into account the total thyroxine and unbound thyroxine binding globulins.

**Synonyms:** - T4FRIDX - Thyroxine, Free Index - Thyroxine, Free Index