Pathophysiology
Clinical meaning
Myxedema coma is the most severe, life-threatening manifestation of decompensated hypothyroidism. Severe thyroid hormone deficiency reduces cellular metabolism throughout the body: decreased Na+/K+-ATPase activity, reduced oxygen consumption, impaired thermogenesis, and slowed protein synthesis. Cardiovascular effects include bradycardia, decreased cardiac output, reduced stroke volume, and pericardial effusion (accumulation of glycosaminoglycans and fluid). Respiratory failure occurs from central hypoventilation (blunted CO2 response), respiratory muscle weakness, and pleural effusions. CNS effects include decreased cerebral blood flow, altered mental status progressing to coma, and seizures. Hypothermia results from impaired thermogenesis. Hyponatremia from impaired free water excretion (reduced GFR, increased ADH) is common and may be severe. The term 'myxedema coma' is somewhat misleading โ most patients present with altered mental status rather than frank coma, and the 'myxedema' refers to the characteristic non-pitting edema from glycosaminoglycan accumulation in the dermis.