Effects of butyl-4-hydroxy-3, 5-diiodobenzoate on the metabolism of I131-thyroxine and triiodothyronine.

Comparatively little is known about the interference in actions and metabolism of hormones by their appropriate analogues, though thyroxine has been studied in this regard more than have other hormones. One analogue which has been demonstrated by Maclagan et ~2. (1) to prevent the calorigenic effect of thyroxine is butyl+Ghydroxy-3,5-diiodobenxoate (BHDB). The mechanism of antagonism has not been elucidated, but the effects of this antagonist on thyroxine distribution, degradation, and excretion seemed worthy of study. The effects of BHDB on triiodothyronine metabolism also would be important to know, since, even though the actions of triiodothyronine appear to be very similar to those of thyroxine in kind (2), there are significant quantitative differences in their activities and contrasts in their metabolism (3-6). In general, triiodothyronine is more rapidly accumulated by tissues in vivo, it exerts a more potent and rapid calorigenic effect, it is degraded more rapidly, and its effect on oxygen consumption is more transient. Certain observations have been reported previously on the effect of BHDB on degradation of the 1131-labeled hormones. In rats, Wilkinson et al. found that the degradation of both thyroxine and triiodothyronine, as measured by the rate of urinary excretion of radioactivity, was inhibited by BHDB (7, 8). These findings were contrary to those reported by Roche, Deltour, and Michel for 113’-thyroxine and BHDB (9), but, as later demonstrated by Wilkinson and Feetham (lo), the findings of Roche’s group could be explained by the fact that administration of BHDB was not continued throughout the experiment. The studies reported herein confirm the observations of Wilkinson et al. (7, 8) on the excretion pattern of radioactivity following the administration of 1131-thyroxine and P-triiodothyronine t,o rats and also demon-