KOHEL, K.A.*; EALES, J.G.: Effects of Chronic Dietary Triiodothyronine and Thyroxine on Acute In vivo Absorption of Radioactive Triiodothyronine and Thyroxine from the Intestine and Uptake into Enterohepatic Tissues in Rainbow Trout

In teleost fish, peripheral tissues regulate thyroidal status. Hence, the intestine, with access to thyroid hormones (TH), may contribute to TH availability through enterohepatic circulation (EHC) from intestine to blood to liver to bile back to intestine. To evaluate the role of the intestine in regulating thyroidal status in rainbow trout, we studied effects of chronic dietary triiodothyronine (T3) and thyroxine (T4) on acute in vivo EHC of radioactive TH (*T3 and *T4). After feeding rainbow trout food supplemented with TH (T3 TRT and T4 TRT) for 7 days, *T3 or *T4, with low or high non-radioactive carrier (200 fmole or 100 pmole), was injected into an intestinal loop comprised of middle and distal regions. After an hour, blood, liver, bile, intestinal loop, and loop contents were collected to measure: 1) absorption of radioactivity from intestine; 2) uptake of absorbed radioactivity (*TH vs *I- and *TH conjugates) into EHC tissues; and 3) radioactivity potentially returning to intestine. T4 had the greatest potential for EHC with the highest absorption from and potential return to the intestinal loop. T3 TRT (low and high carrier) and T4 TRT (high carrier) decreased this potential. T4 TRT promoted *I- cycling or *TH conjugate excretion. Unlike T4, T3 absorption from the loop was saturable. T3 TRT and T4 TRT (low carrier) increased potential for EHC, and T3 TRT and T4 TRT (high carrier) decreased EHC and increased *I- cycling or *TH conjugate excretion. Therefore, T4 and T3 can potentially contribute to thyroidal status through EHC, with regulation under TH fluctuations. Excess hormone may be regulated by metabolism to I- and cycling or conjugation and excretion.