To investigate respiratory muscle strength in patients with hypothyroidism, global respiratory muscle strength was assessed by measuring mouth pressure during PImax and PEmax efforts. Maximum pressures, VC, FEV1, FVC, T3, T4, and TSH were measured in 43 hypothyroid patients. Measurements were made before and three months after replacement therapy with thyroxine. The results showed that the mean value of PImax and PEmax increased after treatment. Significant change was found in the mean value of VC, FEV1, and FVC after treatment but not in the FEV1/FVC ratio. A highly statistically significant linear relationship was found between PImax and TSH and between PEmax and TSH as well as between PImax and T3 and PEmax and T3. We conclude that hypothyroidism affects respiratory muscle strength and that this weakness is linearly related to thyroid hormone levels. Respiratory muscle weakness is present in both inspiratory and expiratory muscles and is reversible with treatment.