Thyroxine Metabolism in the Dog

Publisher Website
Google Scholar
Abstract
T4 metabolism in the dog was evaluated by quantitative and qualitative analysis of serum, urine, fecal and bile radioactivity following the intravenous administration of radioactive T4. In addition, T4 binding by serum proteins was also studied. Protein-bound serum radioactivity decreased in a multi-exponential fashion with an average half-time of disappearance of 0.43 and 5. 6 days for the slower 2nd and 3rd components. Urinary and fecal radioactivity accumulated at similar rates to plasma disappearance with 40% of the dose in the feces and 41% in urine by 48 hr. Fecal radioactivity was primarily T4 and urinary radioactivity iodide. A variety of T4 metabolities were found in the bile, and biliary "clearance" averaged 0. 58 cc/min., considerably less than rat and greater than man when expressed in terms of body weight. Four T4 binding proteins were identified in dog serum with binding characteristics distinctly different from man. These data suggest that T4 metabolism in the dog differs quantitatively and qualitatively from that in the rat and in man.