Hypothyroid myopathy in two dogs.

Biopsy specimens of skeletal muscle and peripheral nerve from two dogs with primary hypothyroidism but without clinical neuromuscular disease were studied with histological, histochemical and morphometric techniques. The most prominent change in skeletal muscle was variation in fiber size associated with a pronounced reduction in mean diameter of type II fibers. Type II fiber loss was apparent in the specimen from one dog. No histological or morphometric differences were noted in nerves from either dog compared with an age-matched control. The myopathic findings suggest a preferential metabolic defect in type II muscle fibers.