The T4/TBG ratio in healthy subjects and in patients with disorders of thyroid function: correlation with FT4 concentrations

Summary: The proportion of free hormones can be estimated in the presence of protein-bound hormones by a variety of direct assay approaches or by a two test strategy to calculate a free hormone »index«. The T4/TBG ratio, determined from measurements of total thyroxine (TT4) and thyroxine-binding globulin (TBG) concentrations, is an appropriate index with improved diagnostic accuracy compared with T4. In this study, we calculated the T4/TBG ratio after determining serum TT4 and TBG by radioimmunoassay (INEP, SCG) in euthyroid persons (n=102), in patients with hypothyrosis (n=33) and hyperthyrosis (n=66), as well as in healthy pregnant women (n=346). The reference range for each group of patients was determined. Values obtained for the T4/TBG ratio in healthy subjects varied from 2.1 to 4.4. They were lower in hypothyrosis (0.5 ’2.2) and elevated in hyperthyrosis (4.7–13.1). The reference intervals of the T4/TBG ratio were also determined for each trimester of pregnancy. A significant correlation was obtained between the values for T4/TBG ratio and FT4 concentration, determined by the two-step radioimmunoassay (CIS-bio international, France), in healthy subjects and patients with thyroid disorders (r=0.93), as well as during pregnancy (0.59). In conclusion, we determined reference intervals for the T4/TBG ratio, confirming its practical usefulness in the assessment of thyroid function.