Peripheral metabolism of thyroid hormones and iodide in healthy and hyperthyroid cats

The metabolic clearance rate, volume of distribution, and fractional clearance rate of thyroxine (T4), triiodothyronine (T3), and iodide were calculated for 6 healthy and 7 hyperthyroid cats, using single-compartmental and noncompartmental methods of analysis. The mean T4 volume of distribution of the hyperthyroid cats was 32.0% less than that of the healthy cats; it was the only variable that was significantly different (P less than 0.05) between these 2 groups of cats. The mean fractional clearance rate of T4 in the hyperthyroid cats was 53.2% greater than that in the healthy cats, but this difference was not significant. Hyperthyroid cats had no significant differences in T3 or iodide kinetic variables when compared with those in healthy cats. Single-compartmental analysis significantly overestimated T4 kinetic variables of healthy cats and T4, T3, and iodide kinetic variables of hyperthyroid cats when compared with the noncompartmental method of analysis.