A comparison of the interference of fatty acids in the competitive binding radioassay and radioimmunoassay for serum T4.

Several investigations have reported an increase, in stored serum, in apparent thyroxine (T4) concentration measured by competitive binding radioassay (CPBR) with thyroxine-binding globulin (TBG) as the binding reagent [l-7]. Watson and Lees [4] attributed this increase to the production of a “mimic T4” during storage. Recently, we [6] and Rootwelt [7], working independently, discovered that this “mimic T4” was a result of the conversion of esterified fatty acids to nonesterified fatty acids. The former, such as those in triglycerides, are nonreactive with TBG, whereas nonesterified fatty acids do react with TBG. Since T4 values have been reported [5] stable when measured by radioimmunoassay (RIA), we decided to evaluate the interferent reactivity of fatty acids with T4 specific antisera. Failure of fatty acids to interfere extensively with T4-specific antisera would help explain the stability of serum T4 values by RIA and their instability by CPBR.