Pharmacokinetics of L-thyroxine after its oral administration in dogs

Twelve mature (5 sexually intact males, 4 castrated males, and 3 females) mixed-breed dogs were surgically thyroidectomized and used in a Latin-square design pharmacokinetic study of orally administered L-thyroxine. The dogs were treated with 44, 22, and 11 micrograms of L-thyroxine/kg as a single morning dose or in divided doses, morning and evening. Serum concentration of thyroxine (T4) was evaluated to determine a number of pharmacokinetic variables for comparison. Mean steady-state concentrations (Css) were determined from the area under the curve. Variables were analyzed for comparisons between dosages by use of ANOVA. Concentration at steady state was highest for dogs of the 44-micrograms/kg of body weight once-daily group and was lowest for dogs of the group given 11 micrograms/kg in 2 daily doses. Single daily administration resulted in higher Css, except at the 22-micrograms/kg/d dosage. Clearance was faster for the 22- and 44-micrograms/kg/d dosages than for the 11-micrograms/kg/d dosage. The half-life (t1/2) and mean residence time (MRT) also were shorter for the 44-micrograms/kg/d dosage, possibly indicating more rapid elimination of the drug at higher doses and dose-dependent kinetics. Perhaps, as the dogs' metabolism increased with higher iodothyronine concentrations, hormone degradation was accelerated. Interval (divided vs single dose) caused some expected changes: maximal concentration was higher and minimal concentration was lower when single administration was used. These undulations resulted in iodothyronine concentrations above the physiologic range for a number of hours, whereas concentration closer to physiologic ranges was achieved by use of divided doses.(ABSTRACT TRUNCATED AT 250 WORDS)