Changes in serum thyroxine and thyroid-stimulating hormone concentrations in epileptic dogs receiving phenobarbital for one year

A multicentric prospective study was conducted to monitor the effect of phenobarbital on serum total thyroxine (T-4) and thyroid-stimulating hormone (TSH) concentrations in epileptic dogs. Serum T-4 concentrations were determined for 22 epileptic dogs prior to initiation of phenobarbital therapy (time 0), and 3 weeks, 6 months, and 12 months after the start of phenobarbital. Median T-4 concentration was significantly lower at 3 weeks and 6 months compared to time 0. Thirty-two percent of dogs had T-4 concentrations below the reference range at 6 and 12 months. Nineteen of the 22 dogs had serum TSH concentrations determined at all sampling times. A significant upward trend in median TSH concentration was found. No associations were found between T-4 concentration, dose of phenobarbital, or serum phenobarbital concentration. No signs of overt hypothyroidism were evident in dogs with low T-4, with one exception. TSH stimulation tests were performed on six of seven dogs with low T-4 concentrations at 12 months, and all but one had normal responses. In conclusion, phenobarbital therapy decreased serum T4 concentration but did not appear to cause clinical signs of hypothyroidism. Serum TSH concentrations and TSH stimulation tests suggest that the hypothalamic-pituitary-thyroid axis is functioning appropriately.