[Hypothyroid myopathy. Physiopathological approach]

Abstract

Patients with thyroid deficiency often complain of muscular weakness, exercise intolerance, cramps and excessive fatiguability. Hypothyroidism induces a metabolic myopathy, with a fall of the energetic production, and especially of the mitochondrial metabolism. This is due to a global inhibition of the main oxidative pathways (substrate incorporation, substrate oxidation) and of the respiratory chain. A diminished energetic consumption is partially related to a transition in the myosin isoforms, which express a slower ATPase, and to an impairment of the trans-sarcolemic transports. Exercise intolerance could be due to an abnormal recruitment of several metabolic pathways, such as glycolysis, related to the mitochondrial metabolism impairment, and including an abnormal accumulation of protons and monovalent phosphate ions, which are involved in the alteration of the actin-myosin interaction, and also by an abnormal Ca++ metabolism. The decreased number of NA+/K+ ATPase dependent pumps could imply an abnormal intracellular Na+ level and explain the frequent disorders of the membrane excitability.