On the enterohepatic cycle of triiodothyronine in rats; importance of the intestinal microflora.

Until 70 h after a single iv injection of 10 uCi [125I]triiodothyronine (T3), normal rats excreted 15.8 +/- 2.8% of the radioactivity with the feces and 17.5 +/- 2.7% with the urine, while in intestine-decontaminated rats fecal and urinary excretion over this period amounted to 25.1 +/- 7.2% and 23.6 +/- 4.0% of administered radioactivity, respectively (mean +/- SD, n = 4). In fecal extracts of decontaminated rats 11.5 +/- 6.8% of the excreted radioactivity consisted of T3 glucuronide (T3G) and 10.9 +/- 2.8% of T3 sulfate (T3S), whereas no conjugates were detected in feces from normal rats. Until 26 h after ig administration of 10 uCi [125I]T3, integrated radioactivity in blood of decontaminated rats was 1.5 times higher than that in normal rats. However, after ig administration of 10 uCi [125I]T3G or [125I]T3S, radioactivity in blood of decontaminated rats was 4.9- and 2.8-fold lower, respectively, than in normal rats. The radioactivity in the serum of control animals was composed of T3 and iodide in proportions independent of the tracer injected, while T3 conjugates represented less than 10% of serum radioactivity. These results suggest an important role of the intestinal microflora in the enterohepatic circulation of T3 in rats.