The efficacy and safety of once daily treatment with a novel liquid formulation of L-thyroxine sodium was evaluated in an open clinical study in dogs with naturally-occurring hypothyroidism.

In 35 client-owned dogs, the diagnosis of hypothyroidism was confirmed based on suggestive clinical signs, together with free T4 (after equilibrium dialysis) <5.4 pmol/L, which is less likely to be influenced by the non-thyroidal factors than total thyroxine (tT4), and thyroid stimulating hormone (TSH) >0.68 ng/mL. L-thyroxine (Leventa® 1 mg/ml, Intervet) was administered orally once daily at a starting dose rate of 20 µg/kg. The dose was adjusted, if needed, every 4 weeks based on both clinical and thyroid hormone response. At each visit, blood samples were collected for peak tT4 (4-6 hours post-treatment) and TSH concentrations. Once the clinical signs had improved and the hormonal status reached target therapeutic range (peak tT4 35-95 nmol/L), the maintenance dose was established and the dogs were reassessed 9 and 22 weeks later.

Clinical signs improved or resolved after 4 weeks of treatment in 91% of the dogs, at the starting dose. Metabolic signs (lethargy, exercise intolerance) resolved rapidly, whereas dermatological signs (alopecia, hyperpigmentation) took longer. The maintenance dose was established in 74%, 91% and 100% of the dogs after 4, 8 and 12 weeks of treatment, respectively. Six percent of the dogs required a once daily maintenance dose rate of 10-15 µg/kg, 79% a maintenance dose rate of 20 µg/kg and 15% a maintenance dose rate of 30 µg/kg. There was no recurrence of clinical signs after the maintenance dose was established. However, based on peak tT4 and TSH concentrations, further adjustment of the dose in increments of 5-10 µg/kg was required in four dogs. At 9 and 22 weeks after establishing the maintenance dose, peak tT4 concentrations reached therapeutic range in 65% and 79% of the dogs and TSH was below 0.68 ng/mL in 87% and 97% of the dogs, respectively. Few suspected side effects were reported: mild to moderate skin reactions of unknown significance were reported in three dogs and occasional vomiting in two dogs.

The novel liquid L-thyroxine formulation (Leventa®) administered once daily at, in most cases, a dose rate of 20 μg/kg, produced a rapid and good clinical response and, after dose adjustment, normalization of thyroid hormone concentrations.