A new method for the assessment of thyroxine metabolism in the rat

A new method for assessing thyroxine (T4) metabolism in the rat has been devised and the results that it yields have been compared to those obtained by other methods for assessing peripheral T4 metabolism. The new method uses a simple analysis of the kinetics of exogenous T4 metabolism in rats chronically treated with daily injections of exogenous labeled T4 to determine T4 clearance rates via both fecal and deiodinative pathways. Fecal clearance rates of endogenously labeled hormone measured in rats brought into approximate 125I/127I specific activity equilibrium by administration of iodide of know specific activity in the drinking water were nearly the same as those determined with this new method. Furthermore, total T4 clearance rate determined with the new method agreed well with that measured by the standard single injected technique utilizing radioactive T4. These results indicate that quantitation of the clearance rate of endogenous T4 via not only the fecal, but also the deiodinative, pathway for T4 disposal can be achieved with this new method. In rats made hyperthyroid with exogenous T4, clearance rates of T4 via both deiodinative and fecal pathways were markedly increased. This result supports the concept that hyperthyroidism increases the activity of the tissue mechanisms for metabolizing T4.