Hypothyroidism exerts a significant effect on several body systems, including the functioning of muscle and nerve. Described here is the case of a severely hypothyroid patient with weakness and numbness of the legs who showed marked clinical improvement with thyroid hormone treatment. Concurrently obtained phosphorus 31-nuclear magnetic resonance (NRM) spectra and nerve conduction studies showed changes in muscle phosphorus metabolism with increases in conduction velocities. Phosphorus 31-NMR spectroscopy is useful for evaluating metabolism in muscle in hypothyroid patients.