Clinical and pharmacokinetic evaluation of a liquid formulation of L-thyroxine for dogs with primary hypothyroidism

Abstract

The purpose of this study was to evaluate clinical and pharmacokinetic responses to once daily administration of an oral solution of L-thyroxine (20 µg/kg body weight) for treatment of primary hypothyroidism in dogs. The 10 dogs included in the study had been diagnosed with primary hypothyroidism based on a plasma total thyroxine (tT4) concentration < 10 nmol/l, in most cases a plasma TSH concentration > 0.60 µg/l, a scintiscan showing no or inadequate uptake in the thyroid glands, and a very low response (< 5 nmol/l) to TSH stimulation. After 4±1 weeks of treatment the clinical improvements were evaluated and a pharmacokinetic profile of tT4 over 34 hours was made. Several of the clinical manifestations of hypothyroidism had improved. The plasma TSH concentrations had significantly decreased and plasma tT4 concentrations had increased. Seven of 10 dogs had a plasma tT4 concentration that was within the reference range during the first 24 hours. Nine of 10 dogs reached plasma tT4 concentrations in the upper half of or above the reference range. The results suggest that once daily oral administration of 20 mg L-thyroxine/kg is sufficient for most dogs treated for primary hypothyroidism. Still, this should be monitored individually and if needed adjusted for each dog.