Effects of phenobarbital treatment on serum thyroxine and thyroid-stimulating hormone concentrations in epileptic dogs

Objective-To determine whether phenobarbital treatment of epileptic dogs al ters serum thyroxine (T-4) and thyroid-stimulating hormone (TSH) concentrat ions, Design-Cross-sectional study. Animals-78 epileptic dogs receiving phenobarbital (group 1) and 48 untreate d epileptic dogs (group 2). Procedure-Serum biochemical analyses, including T-4 and TSH concentrations, were performed for ail dogs. Additional in vitro analyses were performed o n serum from healthy dogs to determine whether phenobarbital in serum inter feres with T-4 assays or alters free T-4 (fT(4)) concentrations. Results-Mean serum T-4 concentration was significantly lower, and mean seru m TSH concentration significantly higher, in dogs in group 1, compared with those in group 2. Thirty-one (40%) dogs in group 1 had serum T-4 concentra tions less than the reference range, compared with 4 (8%) dogs in group 2, All dogs in group 2 with low serum T-4 concentrations had recently had seiz ure activity. Five (7%) dogs in group 1, but none of the dogs in group 2, h ad serum TSH concentrations greater than the reference range. Associations were not detected between serum T-4 concentration and TSH concentration, ag e, phenobarbital dosage, duration of treatment, serum phenobarbital concent ration, or degree of seizure control. Signs of overt hypothyroidism were no t evident in dogs with low T-4 concentrations. Addition of phenobarbital in vitro to serum did not affect determination of T-4 concentration and only minimally affected fT(4) concentration. Conclusions and Clinical Relevance-Clinicians should be aware of the potent ial for phenobarbital treatment to decrease serum T-4 and increase TSH conc entrations and should use caution when interpreting results of thyroid test s in dogs receiving phenobarbital.