In a procedure for determining total seru thyroxine by (a) extracting the thyroxin (T4) from the T4-binding globulin (TBG) in a serum specimen, (b) mixing a tracer amt. of TBG satd. with radioactively labelled T4 with the extracted T4, (c) intimately contacting the mixt. with an ion-exchange resin, (d) incubating with the ion-exchange resin, (e) measuring the initial radioactivity, (f) removing the T4/TBG mixt. from the resin, and (g) measuring the residual radioactivity on the ion-exchange resin, a small amt. of the original serum specimen is added to the T4 extracted in step (a) so as to provide further competitive binding sites and normal serum is added to the standard curve so as to normalise abnormal serum thyroxine values caused by alterations of the TBG due to physiological conditions. The new procedure (normalised serum thyroxine method) eliminates errors due to e.g., pregnancy or oestrogen therapy and gives results which are more consistent than those obtd. by the usual T3 and T4 tests.
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