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]
Overview
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.
- Even in severe hypothyroidism a balance of metabolic homeostasis is achieved through adaptive neurovascular mechanisms. However in conditions such as respiratory or urinary tract infections, cardiac, acute myocardial infarction or stroke interfere with this adaptive mechanisms by decreasing the blood volume and ventilation triggering myxedema coma.
| Hypothyroidism | |||||||||||||||||||||||||||||||||||
| Precipitating Factor | |||||||||||||||||||||||||||||||||||
| ↓T4 | |||||||||||||||||||||||||||||||||||
| ↓ Intracellular T3 | |||||||||||||||||||||||||||||||||||
| Hypothalamus | Respiratory | Cardiovascluar | Renal | ||||||||||||||||||||||||||||||||
| ↓ thermogenesis | Hypercapnia Hypoxia | ↓ inotropic bradycardia | ↓ Volume status | ||||||||||||||||||||||||||||||||
| Hypothermia | Hypoventilation | ↓ Cardiac output ↓ blood volume | ↓ GFR ↑ ADH | ||||||||||||||||||||||||||||||||
| Alteration of mental state | Cerebral anoxia | ↓arterial pressure/shock | Hyponatremia Edema | ||||||||||||||||||||||||||||||||
| 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