Further studies of the metabolism of thyroxine and 3,5,3′-triiodothyronine in the guinea-pig

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Abstract
T4 labelled with 125I and T3 labelled with 131I haVe been administered simultaneously to guinea-pigs and their metabolism studied by whole body counting and measurement of radioactivity in urine, feces, and blood. The half-life of T3 in the body is about 19 hr. significantly less than that of T4 (41 hr). After T3 administration, an unidentified iodinated compound appears in the blood which complicates the estimation of the half-life of this hormone by measurement of the radioactivity in peripheral blood. A previous report, based on measurements of blood radioactivity, that the half-lives of the 2 hormones are similar in the guinea-pig was not confirmed.