On the enterohepatic circulation of triiodothyronine in rats: importance of the intestinal microflora.

Until 70 h after a single iv injection of I0 uCi [1251]trllodothyronine (T3) , normal rats excreted 15.8+2.8 % of the radioactivitywith the feces and 17.5+2.7 % with the--urlne, while in intestinedecontaminated rats fecal and urinary excretion over this periodamounted to 25.1+7.2 % and 23.6+4.0 % of administered radioactivity, respeetlvely--(mean~SD, n=4)~ In fecal extracts of decontaminated rats 11.5+6.8 % of the excreted radioactivity consisted of T 3glueuronide ~T3G) and 10.9+2.8 % of T 3 sulfate (T3S) , whereas noconjugates were detected ~n feces from normal rats. Until 26 hafter ig administration of I0 uCi [1251]T3, integrated radioactivity in blood of decontaminated rats was 1.5 times higher than thatin normal rats. However, after ig administration of I0 uCi[1251]T3G or [1251]T3S , radioactivity in blood of decontaminatedrats was 4.9- and 2.8-fold lower, respectively, than in normalrats. The radloactlvlty in tbe serum of control animals was composed of T 3 and iodide in proportions independent of the tracerinjected, while T 3 conjugates represented <I0 % of serum radloactivity. These results suggest an important role of the intestinalmicroflora in the enterohepatlc circulation of T 3 in rats