[Control of muscular bioenergetics by the thyroid hormones]

Thyroid hormones control the expression of genes coding for myosin isoforms, the Na-K ATPase pumps and the Ca-ATPase canals of the sarcoplasmic reticulum. This explains the increase of contractility and relaxation of skeletal muscles observed in hyperthyroidism, as opposed to hypothyroidism. Control of key-enzymes of the main energetic pathways accounts for inhibition of oxidative metabolisms in hypothyroidism and excessive glycolysis recruitment in hyperthyroidism. In both cases muscle performance is reduced, with accumulation of lactic acid at exercise. This is due to defective pyruvate oxidation and proton expulsion in hypothyroidism, and to acceleration of glycolysis in hyperthyroidism. These abnormalities partly explain why subjects with dysthyroidism are intolerant to exertion.