[Hypothyroid myopathy. A clinical and histologic prospective study of 19 patients]

Nineteen patients with primary hypothyroidism were prospectively studied before starting replacement therapy with thyroid hormones. Most patients had mild to moderate muscle symptoms: generalized muscle weakness, myalgia and cramps, and increased serum levels of muscle enzymes. In two patients these features were the motive for consulting the physician. The patients with muscular features had significantly higher plasma levels of lactic dehydrogenase and aldolase than asymptomatic patients. The major histological muscle findings were atrophy of type II fibers, abnormalities in the oxydative pattern of muscle cells, and glycogen deposition. In the biochemical investigations an increased muscle glycogen with reduced activity of the glycolysis and glucogenolysis enzymes were found. The patients with more marked histological abnormalities had lower levels of thyroid hormones, but the differences did not reach significant values. Five patients in euthyroid status after 12 months of therapy underwent a new biopsy. Although the remaining clinical and laboratory parameters had returned to normal, some of the histological muscle abnormalities persisted. This finding leads to the suspicion that the lack of thyroid hormones may induce irreversible structural damage in skeletal muscle.