Muscle mass and function in thyrotoxic patients before and during medical treatment

Development of muscle weakness and atrophy are well known complications of thyrotoxicosis, although little is known about its clinical course. The present longitudinal study was therefore undertaken to monitor muscle mass and strength before and during treatment of hyperthyroidism.

Five patients (2 male, 3 female; Age 41 +/- 6 years; BMI 22.2 +/- 1.1 kg/m2) with newly diagnosed hyperthyroidism were studied with respect to muscle area, muscle strength, body composition and substrate metabolism at baseline and after 1, 3, 6, 9 and 12 months of treatment.

Midthigh muscle areas were assessed by computed tomography (CT), while bioelectrical impedance analysis (BIA) was used for assessment of body composition. The isometric strength of the biceps brachialis and quadriceps muscles was assessed by means of a dynamometer and the maximal static ins- and ex-piratory mouth pressures were measured with a respiratory pressure module.

Prior to treatment thyrotoxic patients all displayed elevated levels of total and free T3 and T4 together with suppressed TSH. BMI, fat mass and lean body mass increased significantly during the treatment period, while energy expenditure (EE) decreased. Thigh muscle areas increased by 24% (101.5 +/- 11.5 vs. 125.3 +/- 13.1 cm2, P < 0.05) from entry to peak. Peak time was 9 +/- 0.9 months. During treatment a significant (P < 0.01) increase in muscle strength was observed; arm capacity increased by 48%, while leg capacity increased by 51%. Peak time (months) was: Right arm: 8 +/- 3, left arm: 7 +/- 2, right leg: 5 +/- 3, left leg: 9 +/- 2. Respiratory muscle strength, expressed as maximal ins- or ex-piratory mouth pressure, was significantly impaired among patients at entry. A significant increase in inspiratory and expiratory strength was found from entry to peak (P < 0.05), as inspiratory strength increased by 35% and expiratory by 19%. Inspiratory strength peaked after 7 +/- 1 months, expiratory muscle strength after 6 +/- 1 months.

In conclusion we find that in patients with thyrotoxicosis muscle mass is reduced by approximately 20% and muscle strength by approximately 40% and that between 5 and 9 months elapse before normal muscle mass and function are reestablished.