The T3/TBG ratio and the biochemical investigation of thyrotoxicosis

The relationship between serum tri-iodothyronine (T3) and thyroxine-binding globulin (TBG) has been studied in euthyroid subjects. This relationship is similar to that previously described for thyroxine (T4) and TBG. Serum T3, TBG and T4 data are presented for euthyroid subjects of all ages and the T4:T3 and T3:TBG ratios calculated. Similar data has been derived for patients with clinically proven thyrotoxicosis and for patients in which the diagnosis was in doubt. The results suggest the T3:TBG ratio enhances the use of the T3 assay to confirm thyrotoxicosis. The fact that 25% of our patients suffering from thyrotoxicosis had apparently normal serum T4 levels reinforces the contention that a serum TBG assay must be carried out as part of a basic thyroid function screen.