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The purpose of this study was to evaluate changes in muscular strength and endurance, work capacity, and subjective fatigue following surgical treatment of primary hyperparathyroidism (pHPT), and to assess whether changes in muscular function were due to changes in activation of the muscles. A prospective consecutive study design was used, and patients surgically treated for nontoxic goiter served as controls. Nineteen female patients with mild to moderate pHPT and 20 controls were included. Maximal isometric handgrip and quadriceps strength, quadriceps endurance (intermittent stimulation), and quadriceps activation (superimposed twitch technique) were used for evaluation of muscular function. All patients were operated on successfully. Knee extension strength increased by 17 ± 17% (mean ± SD; p= 0.0004) in the patients, whereas no change was observed in the controls. The relative strength increase correlated positively to patient age at operation ( r= 0.52, p= 0.02). Handgrip strength, quadriceps endurance, and general work capacity did not change in any group after operation. Subjective fatigue was preoperatively higher in patients than in controls ( p= 0.01), and decreased postoperatively to the level of controls. In conclusion, women with pHPT increase knee extension force after parathyroidectomy as a result of increased force generation capacity of the muscle. If change in physical performance is a determinant for change in subjective fatigue in pHPT after operation, then change in strength of the quadriceps muscle seems to be of primary importance, whereas handgrip strength, muscular endurance, and work capacity do not seem to be important. The cause of the increasing strength benefit with increasing age at operation as found in this study needs further investigation. [ABSTRACT FROM AUTHOR]