Deprecating the diagnostic significance of free thyroxine levels in human serum.

Prime diagnostic importance has been postulated for the measurement of free thyroxine in human serum. During the past decade various innovative and routinely applicable techniques have been developed for its assay, and claims for their superior performance in clinical practice have subsequently been made. However, in our opinion, based on current literature concerning thyroxine metabolism together with basic information about the physicochemical characteristics that govern its intravascular distribution, it seems appropriate to indicate caution against overestimating the diagnostic sensitivity attributed to free thyroxine measurement irrespective of assay methodology and its conceptual elegance.