Value of Free Thyroxine Index

Abstract

To the Editor: We wish to point out an error in the tabulation of normal reference laboratory values recently published in the Case Records of the Massachusetts General Hospital.¹ The normal range for the free thyroxine index (fT₄I) is given as 1 to 4 ng per 100 ml, and the method of Sarin and Anderson² is cited. However, Sarin and Anderson calculated the fT₄I as the product of the serum thyroxine (T₄) and the triiodothyronine (T₃) resin-uptake ratio, the latter being the ratio of the observed T₃ resin uptake to a . . .