Clinical Evaluation of a Novel Liquid Formulation of L‐Thyroxine for Once Daily Treatment of Dogs with Hypothyroidism

Background: A liquid solution of levothyroxine (L‐T4) is available for treatment of canine hypothyroidism. Hypothesis: Once daily oral administration of a liquid L‐T4 solution is effective and safe for controlling hypothyroidism in dogs. Animals: Thirty‐five dogs with naturally occurring hypothyroidism. Methods: Dogs received L‐T4 solution PO once daily at a starting dosage of 20 μg/kg body weight (BW). The dose was adjusted every 4 weeks, based on clinical signs and peak serum total T4 (tT4) concentrations. Target peak serum tT4 and thyroid stimulating hormone (TSH) concentrations, 4–6 hours posttreatment, were 35–95 nmol/L and < 0.68 ng/mL, respectively. Dogs were followed for up to 22 weeks after establishment of the maintenance dose. Results: Clinical signs of hypothyroidism improved or resolved in 91% of dogs after 4 weeks of L‐T4 treatment at 20 μg/kg once daily. The maintenance dose was established in 76, 94, and 100% of dogs after 4, 8, and 12 weeks of treatment, respectively. This was 20 μg L‐T4/kg BW for 79% of the dogs, 30 μg/kg BW for 15%, and 10–15 μg/kg BW in the remaining 6%, once daily. Thereafter, median peak tT4 and TSH concentrations were 51 nmol/L and 0.18 ng/mL, respectively, and remained stable during the 22‐week follow‐up; clinical signs did not recur. Conclusions and Clinical Importance: All of the hypothyroid dogs had rapid clinical and hormonal responses to supplementation with the PO‐administered L‐T4 solution. The starting dosage of 20 μg L‐T4/kg BW once daily was suitable for 79% of dogs.