Serum iodothyronine concentrations in intestinally decontaminated rats treated with a 5′-deiodinase type I inhibitor 6-anilino-2-thiouracil

Enteric bacteria have been postulated to have a role in thyroid economy by promoting the hydrolysis of thyroid hormone conjugates of biliary origin, thus permitting the absorption and recycling of thyroxine (T 4 ) and triiodothyronine (T 3 ). An enterohepatic circulation of T 3 might be more pronounced under conditions in which type I iodothyronine deiodinase activity (5'D-I) is inhibited, because this augments the accumulation of T 3 sulfate conjugates in bile. This potential of increased gut reabsorption of T 3 might explain, at least in part, the failure of serum T 3 values to decrease appreciably when marked reductions in peripheral 5'D-I activity are induced by selenium deficiency or 6-anilino-2-thiouracil (ATU) administration. Thus, studies were performed to determine the effect of intestinal decontamination. in the absence and in the presence of 5'D-I inhibition, on plasma T 4 and T 3 concentrations. Groups of adult male rats received either enteric antibiotics or no antibiotics for 12 days and then, in half of the rats in each group, treatment for 10 days with ATU, a 5'D-I inhibitor that does not affect thyroid hormone synthesis. The activity of intestinal arylsulfatase and arylsulfotransferase, enzymes that catalyze hydrolysis of thyroid hormone conjugates, was reduced markedly by approximately 87% in rats that received antibiotics, regardless of whether or not they also received ATU. The ATU treatment markedly inhibited liver 5'D-I activity in antibiotic-treated as well as in non-antibiotic-treated rats (control = 399 ± 32 U/mg protein (mean ± SEM) ; = 152 ± 17 ; antibiotics = 351 ± 29 ; antibiotics + ATU=130 ± 10 ; p < 0.01) and significantly increased plasma T 4 and T 3 sulfate (T 4 S, T 3 S) concentrations (control : T 4 S = 2.8±0.4 and T 3 S = 6.7 ± 1.3 ng/dl ; ATU : T 4 S = 6.2 ± 1.4 and T 3 S = 10.6 ± 2.1 ng/dl : antibiotics : T 4 S = 1.8 ± 0.2 and T 3 S = 3.6 ± 1.0 ng/dl : antibiotics + ATU : T 4 S = 6.8 ± 0.7 and T 3 S = 9.7 ± 1.8 ng/dl ; p < 0.05). The ATU treatment was associated with a significant increase in plasma T 4 and rT 3 concentrations but did not affect plasma T 3 concentrations, and intestinal decontamination did not alter these ATU-associated effects on circulating thyroid hormones. These results suggest that anaerobic enteric bacteria in the rat do not have an important role in recycling of thyroid hormones, either under normal conditions or in circumstances where 5'D-I activity is markedly reduced, and that increased gut absorption of T 3 from T 3 S cannot explain the near-normal serum T 3 values found when peripheral 5'D-I activity is markedly decreased.