Pharmacokinetics of total thyroxine in dogs after administration of an oral solution of levothyroxine sodium

Oral l-thyroxine ( l-T4) supplementation is used to replace thyroid hormone concentrations in dogs with hypothyroidism. The pharmacokinetics of l-T4 following administration of a solution (Leventa®) was investigated in healthy dogs. l-T4 was absorbed fairly rapidly (tmax 3 h). A mean bioavailability of 22% was calculated following a single oral administration of 40 μg l-T4/kg body weight. Repeated oral administration at the same dose for 14 consecutive days did not lead to any accumulation of T4 in serum. After intravenous administration of l-T4, a serum half-life of 11.6 h was calculated. Food intake concomitant with l-T4 oral administration delayed l-T4 absorption and decreased its rate and extent by about 45%. The relative bioavailability of l-T4 following administration of a tablet formulation was about 50% of that of the l-T4 solution. The pharmacokinetic properties of liquid l-T4 after oral administration support the use of a dose rate of 20 μg/kg once daily, as a starting dose for replacement therapy in dogs with hypothyroidism.