Myxedema coma pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] ; Associate Editor(s)-in-Chief: Aditya Ganti M.B.B.S. [2]

Pathophysiology

• Myxedema coma occurs as a result of long-standing, undiagnosed, or undertreated hypothyroidism.
• Myxedema coma is usually precipitated by a systemic illness.

Triggers

• Myxedema coma can result from any of the causes of hypothyroidism, most commonly chronic autoimmune thyroiditis.
• Myxedema coma can also occur in patients who had thyroidectomy or underwent radioactive iodine therapy for hyperthyroidism.
• Rare causes may include secondary hypothyroidism and medications such as lithium and amiodarone.

Pathogenesis

• Thyroid hormone plays an important role in cell metabolism.
• Long-standing hypothyroidism is associated with reduced metabolic rate and decreased oxygen consumption, which affects all body systems. [5]
• Reduced metabolism results in hypothermia.
• Reduced metabolism and decreased oxygen also results in decreased drug metabolism leading to overdosing of medications particularly sedatives, hypnotics, and anesthetic agents; this can precipitate myxedema coma.

The following table summarizes the various effects of reduced thyroid hormone on different organ systems

Organ System Effect due to Decreased Thyroid Hormone Manifestation Cardiovascular Cardiac contractility is impaired Leading to reduced stroke volume

Low cardiac output

Bradycardia

Sometimes hypotension

Reduced stroke volume in severe cases may also be due to pericardial effusions

caused by the accumulation of fluid rich in mucopolysaccharides within the pericardial sac

Hypotension Narrowed pulse pressure

Fluid accumulation in tissue

Pericardial effusions.

Neurologic Altered brain function due to Reduced oxygen delivery and consumption

Decreased glucose utilization

Reduced cerebral blood flow.

Altered consciousness Pulmonary Central depression of ventilatory drive Decreased responsiveness to hypoxia and hypercapnia

Hypoventilation Renal Reduced glomerular filtration rate because of

Low cardiac output

Peripheral vasoconstriction

Rhabdomyolysis

Electrolyte abnormalities

Low volume stimulates

Antidiuretic hormone impairs water excretion leading to hyponatremia

Gastrointestinal mucopolysaccharide infiltration and edema malabsorption

gastric atony

impaired peristalsis,

paralytic ileus

megacolon.

GI bleeding Ascites

Constipation

Hematologic Coagulopathy due to decrease in production of factors V, VII, VIII, IX, and X

Hemorrhage and vitamin B12 deficiency

Bleeding Anemia