Provides a more accurate assessment of actual thyroid hormone availability than total T4 alone because it accounts for changes in binding proteins that can dramatically affect total T4 without changing the active hormone. This calculation helps identify true thyroid dysfunction while avoiding misdiagnosis in situations where binding proteins change like pregnancy, birth control use, or serious illness. Free T4 is the metabolically active form that enters cells to regulate metabolism, energy production, heart rate, body temperature, and countless other functions. The index isn't as accurate as directly measuring free T4 (which most labs now do), but it remains useful when direct free T4 testing isn't available or when trying to understand discrepancies in thyroid testing.