20 patients afflicted with primary hypothyroidism were studied in order to evaluate the association of clinical or sub-clinical myopathy, detected by neurophysiological (electromyography) (EMG) or neuropathological methods (muscular biopsy with enzymatic study). 70% of the patients had muscular weakness (moderate in 30% and severe in 40%) of the scapular and pelvic muscles. 60% of the patients had muscular cramps. There was no myodema nor muscular atrophy or hypertrophy. Seric CPK was high in 70% of the cases. EMG was myopathic in 65%. All cases with weakness registered EMG alterations. The histological findings were import findings were important. The enzymatic techniques showed alterations of the fiber subtypes in 90% of the cases. The type I fibers had sarcolemmal and mitochondrial accumules in 85% and 70% had areas without oxidative activity, similar to "core". In this study, we did not find any correlation between the evolution time of hypothyroidism, hormonal levels, CPK increase, and muscular weakness. The EMG was myopathic in cases with severe weakness, however, in patients with moderate weakness it could also prove abnormal. There was no correlation between the electric myopathic pattern, CPK levels and thyroid hormones.