Qualitative and quantitative aspects of thyroxine metabolism in sheep.

Qualitative and quantitative aspects of thyroxine degradation and excretion in adult, intact and biliary-fistulated sheep in a near-basal state are reported. Thirty-eight per cent of the T4 metabolized was deiodinated, about 56% of this iodide being taken up by the thyroid, 36% being excreted in urine and 8% in feces; 62% of the T4 secreted was excreted unchanged or as a thyroxine conjugate. Of this, 39 % was in bile, 22 % in urine and 39 % was excreted in feces from a nonbiliary source. Although T4 secretion per unit metabolic body mass (weight0–7) is much higher in sheep than in man and some other domestic animals, T4 deiodination/unit metabolic body mass is very similar to other species. This is consistent with investigations on laboratory animals which have shown that T4 excreted in feces has not influenced metabolism. Therefore, it is suggested that, in investigations in which thyroid function is correlated with some metabolic response, the large and potentially variable fraction excreted unchanged o...