Pharmacokinetics of Total Thyroxine after Repeated Oral Administration of Levothyroxine Solution and its Clinical Efficacy in Hypothyroid Dogs

Background\n\nOral levothyroxine (l-T4) supplementation is commonly used to treat hypothyroid dogs.\n\n\nObjectives\n\nInvestigate the plasma profile and pharmacokinetics of total thyroxine (tT4) after PO administration of a l-T4 solution and its clinical efficacy in hypothyroid dogs.\n\n\nAnimals\n\nTen dogs with naturally occurring hypothyroidism.\n\n\nMethods\n\nAfter hypothyroidism diagnosis and supplementation with l-T4 solution PO q24h at 20 μg/kg BW for minimum 4 weeks, the plasma profile and pharmacokinetics of tT4 were determined over 34 hours and the clinical condition of the dogs was evaluated.\n\n\nResults\n\nBefore dosing for pharmacokinetic evaluation, mean tT4 concentration was 23 ± 9 nmol/L. l-T4 was absorbed rapidly (tmax, 5 hours), reaching a mean maximal tT4 concentration of 56 ± 11 nmol/L. The apparent terminal half-life was 11.8 hours. Clinical signs of hypothyroidism improved or resolved in all dogs after 4 weeks of treatment. The dosage of 20 μg/kg PO q24h was judged appropriate in 5 dogs, and 4 dogs required slight increases (9–16%). Twice daily treatment, with a 30% increase in dosage, was necessary for 1 dog.\n\n\nConclusions and Clinical Importance\n\nThe pharmacokinetics of l-T4 in hypothyroid dogs was similar to that reported in healthy euthyroid dogs. Clinical and hormonal responses to l-T4 solution were rapid in all dogs. The starting dosage of 20 μg/kg PO q24h was suitable for maintenance supplementation in 50% of the dogs, minor dosage modification was required in 4 other dogs, and treatment q12h was required in 1 dog.