In patients undergoing long-term thyroid hormone therapy with constant doses the results of FT4 concentrations measured by radioimmunoassay (FT4-AMERLEX) were compared with calculated FT4 values and related to total thyroxine, serum TBG levels, respectively T4/TBG, to control the validity of radioimmunologic FT4 measurements for clinical routine diagnostic purposes. Based on mathematical analyses on the interdependence of these values the following principles were confirmed: free thyroxine measurements are dependent on total thyroxine with a power of x to the exponent and on T4/TBG-ratio with an exponential function. The percentage of free thyroxine (FT4/TT4 ratio) is inversely and exponentially related to serum TBG concentrations. Indeed, the RIA-FT4 results are largely independent of the concentration of binding globulin within certain limits. In thyroid patients without extrathyroid disorders no grossly false hypo- or hyperthyroid FT4 values are to be expected by deviating thyroxine globulin concentrations.