Thyrotoxic myopathy. Pathomorphological observations of human material and experimentally induced thyrotoxicosis in rats.

Proximal myopathy and different types of neuromuscular syndromes are frequently observed in hyperthyroid patients with complicated thyrotoxicosis. Conflicting reports on the changes seen in muscle biopsies range from no abnormalities to muscle necrosis with the fibers being replaced by connective tissue and fat. On the basis of 21 skeletal muscle biopsies from chronic thyrotoxic myopathic patients, coupled with studies in experimentally induced triiodothyronine myopathy in rats, the histological, histochemical, immunofluorescent, and electron-microscopic findings diagnostic for thyrotoxic myopathy are defined. These criteria include: clumping of nuclei, loss of cross-striation, vacuolar degeneration, and fatty infiltration of endo- and perimysium, presence of giant mitochondria, increased number of mitochondria, disorganization of contractile elements, swelling of transverse tubules, presence of microtubular aggregations, and a significant increase of lipofuscin granules. Both muscle fiber types or in some cases selective type II fibers display atrophy with signs of reinnervation. These alterations are polyfocal and polyphasic and, therefore, polychronous and stereotyped rather than pathognomonic. However, they are related to the severity of thyrotoxic myopathy and the duration of disease. The results of muscle biopsies have a predictive value in determining the reversibility of irreversibility of pathologic changes with therapeutic consequences.