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We undertook this study to investigate respiratory muscle strength in relation to thyroid function in 20 thyrotoxic patients and in a group of 20 normal subjects matched for age and sex. Global respiratory muscle strength was assessed by measuring mouth pressure during maximal static inspiratory (PImax) and expiratory (PEmax) efforts. We also measured VC, FVC, and FEV1 as well as thyroid-related hormones (T3, T4, TSH). Measurements were made once in normal subjects and twice in thyrotoxic patients, before and 3 months after medical treatment. Our results showed that both maximal pressures were significantly reduced (p less than 0.0001) before treatment in thyrotoxic patients in relation to the mean values of the normal subjects (p less than 0.0001), and they increased significantly (p less than 0.0003) after treatment. Lung volumes were significantly reduced (p less than 0.0001) before and increased significantly (p less than 0.008) after treatment. The ratio FEV1/FVC did not change. A statistically significant linear relationship was found when PImax of patients with thyrotoxicosis before treatment and of normal subjects were plotted against thyroid hormones (T3, T4) (r = -0.746 and r = -0.745, respectively, p less than 0.001). Similarly, a statistically significant linear relationship was found between PEmax and T3 and T4 (r = -0.837 and r = -0.838, respectively, p less than 0.001). No relationship was found between maximal pressures and TSH. Finally, a significant linear relationship was found between PImax and PEmax (r = 0.872, p less than 0.001). Our results confirm that in thyrotoxicosis respiratory muscle weakness occurs that affects both inspiratory and expiratory muscles.(ABSTRACT TRUNCATED AT 250 WORDS)