Serum total thyroxine, total triiodothyronine, free thyroxine, and thyrotropin concentrations in epileptic dogs treated with anticonvulsants

Objective-To determine whether administration of phenobarbital, potassium b romide, or both drugs concurrently was associated with abnormalities in bas eline serum total thyroxine (T-4), triiodothyronine (T-3), free T-4, Or thy rotropin (thyroid-stimulating hormone; TSH) concentrations in epileptic dog s. Design-Prospective case series. Animals-78 dogs with seizure disorders that did not have any evidence of a thyroid disorder (55 treated with phenobarbital alone, 15 treated with phen obarbital and bromide, and 8 treated with bromide alone) and 150 clinically normal dogs that were not receiving any medication. Procedure-Serum total T-4, fetal T-3, free T-4, and TSH concentrations, as well as serum concentrations of anticonvulsant drugs, were measured in the 78 dogs with seizure disorders. Reference ranges for hormone concentrations were established on the basis of results from the 150 clinically normal do gs. Results-Total and free T-4 concentrations were significantly lower in dogs receiving phenobarbital (alone or with bromide), compared with concentratio ns ill clinically normal dogs. Administration of bromide alone was not asso ciated with low total or free T-4 concentration. Total T-3 and TSH concentr ations did not differ among groups of dogs. Clinical Implications-Results indicate that serum total and free T-4 concen trations may be low (ie, in the range typical for dogs with hypothyroidism) in dogs treated with phenobarbital. Serum total T-3 and TSH concentrations were not changed significantly in association with phenobarbital administr ation. Bromide treatment was not associated with any significant change in these serum thyroid hormone concentrations.