The usefulness of 'effective thyroxine ratio' in correcting for T4-binding abnormalities in serum

Abstract

The effective thyroxine ratio (ETR) simultaneously reflects the total thyroxine (T4) in the serum and the binding capacity of T4-binding globulin (TBG). It should correlate closely with the important moiety determining the physiological status of the individual, namely the free T4 concentration and this was confirmed recently in this laboratory. However, the ETR is a very much simpler test than the free T4 concentration. The present study was undertaken to determine the relative effectiveness of free T4 concentration, free T4 index and ETR in defining the euthyroid state in patients with abnormalities in T4 binding. Obser¬vations were made in three groups of euthyroid subjects: (1) those with normal TBG or TBPA levels, (2) those with low TBPA or TBG levels as in hypoalbuminaemia or chronically sick patients, and (3) those with high TBG or TBPA levels usually due to oestrogen medication or to pregnancy. Summary of Results Reference ranges (95% limits) Observations in reference ranges (%) Normal TBP (29) * Low TBP (28) High TBP (39) Total T4. I (μg%) 4.6-6.9 39 41 T3 uptake 78-110 29 51 Free T4 Index 4.1-6.8 64 87 Free T4 Cone. (ng%) 2.9-6.7 71 95 ETR 0.90-1.04 68 87 * Figures in parentheses indicate number of subjects in group The free T4 index, free T4 concentration and ETR are all more accurate than the total T4 or the T3 uptake in defining the euthyroid state. The accuracy of the ETR is between those of the free T4 indes and the free T4 concentration. ETR is recommended as a simple and single thyroid function test. Res-o-Mat ETR, T3 and T4 kits were supplied by courtesy of Malljnckrodt Chemical Works.