| Three studies were reported showing effects of hyperthyroidism on different aspects of body composition/functioning. Brennan et al., Rochester, Minnesota, USA, found that in subjects with hyperthyroidism low levels exist of myosin heavy chain (MHC) I in combination with altered ratios of MCH IIa and IIx isoforms. MHC I are an important component of type I muscle fibers that subserve activities requiring sustained effort or endurance, while MCH II isoforms are in particular present in type II fibers, subserving fast short-term switch. The here described changes are an explanation for complaints of muscle weakness and fatigability in hyperthyroidism. Dittmar et al., Mainz, Germany, studied body composition, total body potassium, and bone mineral density in 199 patients with hyperthyroidism (140 women) and found, as compared to controls, that in hyperthyroid women bone cell mass and extracellular water was lowered while the ratio extracellular mass/body cell mass was elevated and fat mass was not different. Hyperthyroid men did not differ in body composition from controls. In the female patients bone cellular mass correlated negatively with free T3 and positively with total body potassium and bone mineral density. The authors conclude that the lowered body cellular mass in hyperthyroid women is primarily caused by loss in muscle mass. (Comment: Why no changes in men? Testosteron again?) Biondi et al., Naples, Italy, assessed left ventricular characteristics at rest in 20 hyperthyroid patients (14 females). They found a higher heart rate (37%), wider pulse pressure (55%) but unchanged mean arterial pressure. Stroke and cardiac index were markedly increased by 21% and 64% respectively. Systemic vascular resistance was lower (43%) and ejection fraction higher than in controls (11%) and diastolic volume index was increased (11%). All the here mentioned differences were highly significant. The general finding is that there is an enhanced resting left ventricular performance in hyperthyroidism in humans, predominantly reflecting a significant increase in heart rate and preload rather than changes in afterload and myocardial contractility. |
