Comparison between determination of free thyroxine index and effective thyroxine ratio in human serum

Four thyroid function indices were determined in the sera of 181 patients: total thyroxine (T-4), the triiodothyronine resin uptake (T-3 B.C.-index), the free thyroxine (T-4) index, and the effective thyroxine ratio (E.T.R.).A statistically significant correlation between the free T-4 index and E.T.R. could be established though this correlation seemed to be worse than could be expected from the literature. Two sets of correlation coefficients were shown: one obtained with the ordinarily used Pearson or product-moment correlation and one with the Spearman rank correlation. Because the assumptions and requirements for the proper use of the product-moment correlation statistics, such as a normal distribution of both variables, are seldom met, expecially not if a selected number of controls and patients is used for the correlation, it is advisable to use some kind of rank correlation statistics instead.The advantages and disadvantages of the E.T.R. and free T-4 index determination are described.