Free thyroxine index

This method is an indirect measurement of FT4. The calculation formula is FT4I = total T4 × measured 125I-T3 uptake ratio T4 or PBI measurement is affected by TBG; 125I-T3 uptake test is also affected by TBG. For example, during pregnancy or after taking the pill, TT4 or PBI is increased due to an increase in blood TBG concentration, which is misdiagnosed as hyperthyroidism; an increase in TBG reduces the 125I-T3 uptake rate accordingly, which can be misdiagnosed as hypothyroidism . FT4I can avoid the effect of increasing or decreasing TBG.

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