This is a prospective cohort study on neurologically asymptomatic patients with primary hypothyroidism. It was conducted to evaluate the frequency and pattern of neurophysiological changes in this group of patients. Twenty-three subjects were included over a period of 2(1/2) years. Neurophysiological evaluation included nerve conduction studies (NCS) of median, ulnar, and peroneal motor nerves as well as median palmar and ulnar and sural sensory responses. Electromyography of deltoid, first dorsal interosseous, vastus lateralis, and tibialis anterior muscles was performed with concentric needle electrodes in which duration, amplitude, and stability of motor unit action potentials, recruitment, and interference pattern were evaluated. NCS showed that 52% of the patients had some abnormality, predominantly of the motor demyelinating pattern, as evidenced by prolonged F-wave and distal latencies with normal amplitudes in most affected nerves. Thirty percent of patients had median mononeuropathy consistent with carpal tunnel syndrome. Nondisfigurative myopathic changes in the form of myopathic motor unit action potentials without spontaneous activity were seen in 74% of the patients, most commonly in deltoid (70%). Frequencies of involvement of other muscles were 39% in the vastus lateralis muscle, 26% in tibialis anterior muscle, and 9% in the first dorsal interosseous muscle. We conclude that electromyographic/NCS changes commonly exist in treated, neurologically asymptomatic patients with hypothyroidism and are most frequently myopathic. Median neuropathy is the most common nerve abnormality. Other nerves are involved, with a higher tendency for motor nerve demyelination. We speculate that some neuromuscular changes secondary to hypothyroidism persist after treatment and that motor nerve abnormalities are less likely to be symptomatic than sensory nerve changes in these patients.