[Hypothyroid myopathy: histochemical and ultrastructural features with physiopatological correlations (author's...

The Authors describe an adult case of hypothyroid myopathy which occurred after a Hashimoto's thyroiditis. Ultrastructural examination of the deltoid muscle showed two fundamental changes: 1) large collections of mitochondria generally normal in shape, structure and size, especially in the subsarcolemmal sapce of the muscle fibre; 2) glycogen deposits both beneath sarcolemmal membrane and between myofibrils. Histochemical examination reveals an increased activity of mitochondrial oxydative enzymes, such as NADPH and SDH especially in subsarcolemmal regions of many type I fibres. The histogram is normal. It seems that the multiplication of skeletal muscle mitochondria may be compensatory to the slowing down of metabolic activities. This, however, is ineffective because it involves the mitochondria localized more superficially in the fibres with a loosely coupled oxydative phosphorylation. Glycogen accumulations are probably due to deficiency of the thyroid hormone; in fact stimulation of carbohydrate metabolism by thyroxine is well known. The abnormal functioning of the muscular mitochondria and the defective utilisation of glycogen are the most important factors in the pathogenesis of muscular weakness of hypothyroid myopathy.