A golden retriever presented with signs of hypothyroidism occurring in conjunction with autoantibodies to both triiodothyronine (T3) and thyroxine (T4). The autoantibodies caused the apparent concentrations of total T3, total T4, and free T4 by analog assay to be high. However, free T4 concentration was nondetectable when measured using a dialysis assay. The dog's clinical condition markedly improved in response to L-thyroxine therapy, and the free T4 concentration by dialysis assay increased into the normal range. Thyroid hormone autoantibodies can confuse the diagnostic evaluation for suspected hypothyroidism. In dogs with autoantibodies to T4, measurement of free T4 by dialysis assay is useful for both diagnostic and therapeutic monitoring purposes.