Myxedema and Myxedema Coma

Basics

Overview

Myxedema coma is a rare, life-threatening, manifestation of severe hypothyroidism and is considered an endocrine emergency.
The development of myxedema coma requires a precipitating event that overwhelms normal homeostatic mechanisms. No single event has been identified in animals.
The greatest challenge with myxedema coma is recognizing the syndrome. Once it is recognized, immediate and intensive supportive care is necessary. Successful treatment has been reported; however, mortality rates can be high.

Signalment

Dogs, the majority of which have been Doberman pinschers
Age range: 5–7 years
No sex predilection
Myxedema coma is not reported in cats

Signs

Historical Findings

The common findings in patients with myxedema coma are changes in mental status, altered thermoregulation, and non-pitting skin edema.
Mentation changes, due to brain edema, can range from altered alertness to coma. Mental depression is the most common assessment of mental status. Coma is not consistently reported.
Other signs consistent with hypothyroidism may be reported.
Patients may have been previously diagnosed with hypothyroidism.

Physical Examination Findings

Hypothermia without shivering is a consistent finding with myxedema coma. Thyroxine amplifies catecholamine function, helping to stimulate muscular activity associated with shivering. Reduced T₄ level blunts the ability to shiver.
Cold extremities due to peripheral vasoconstriction and central shunting of blood secondary to hypothermia.
Non-pitting edema of the skin is due to cutaneous deposition of glycosaminoglycans in the interstitial space.
Decreases in lung and heart sounds may be noted due to pleural effusion (present in up to 50% of cases).

Causes & Risk Factors

Myxedema coma results from chronic untreated severe primary hypothyroidism.
Both forms of primary hypothyroidism (lymphocytic thyroiditis and idiopathic thyroid atrophy) have reportedly been associated with the development of myxedema coma.
A secondary precipitating event is usually associated with the onset of a myxedema crisis. Precipitating events can include, but are not limited to, infections, respiratory disease, heart failure, and hypovolemia.
Sporadic reports exist suggesting that exposure to cold temperatures may act as a precipitating event, though this has not been consistently reported.

Diagnosis ⬆ ⬇

Diagnosis

Differential Diagnosis

Weakness may be associated with cardiovascular disease, neurologic disease, and other endocrinopathies.
Dilated cardiomyopathy if patient is a Doberman pinscher.
Hypothermia may be associated with shock and other cardiovascular and endocrine diseases. Hypothermia associated with factors other than myxedema coma will usually be accompanied by shivering.

CBC/Biochemistry/Urinalysis

Mild nonregenerative anemia is most commonly noted
Hypercholesterolemia
Hypertriglyceridemia
Hypoglycemia
Hyponatremia
Hypoxemia
Hypercarbia
Urinalysis is normal

Other Laboratory Tests

Thyroid function tests indicate severe hypothyroidism with low total T₄, low free T₄, and elevated TSH levels.
Buccal mucosal bleeding time is prolonged (hypothermia reduces platelet function).
Fluid analysis of pleural effusion-modified transudate.

Imaging

Thoracic radiographs-pleural effusion in up to 50% of cases. Pulmonary edema localized in the perihilar region is rarely noted.

Pathologic Findings

Skin biopsies may demonstrate dermal thickening, myxedema, and vacuolation of arrector pili muscles.

Treatment ⬆ ⬇

Treatment

Myxedema coma is a medical emergency.
Once a presumptive diagnosis of myxedema coma is made, immediate inpatient treatment is necessary.
Due to the critical and life-threatening nature of myxedema coma, treatment must be initiated before thyroid function test results confirm a clinical suspicion of myxedema.
Immediate provision of a patent airway and resuscitation of hypotension are necessary.
Mechanical ventilation may be necessary.
IV fluid therapy is administered to support blood pressure and to address decreased sodium levels. Patients with myxedema coma have a decreased ability to clear free water.
Administer intravenous fluids: 0.9% sodium chloride (20 mL/kg-initial bolus). Reassess and continue intravenous fluids (2.5–7 mL/kg/h). Rate selected based on blood pressure, heart rhythm, heart rate, and respiratory rate.
The hallmark of therapy is intravenous administration of synthetic thyroid hormone.
Rapid rewarming must be avoided as this may cause peripheral vasodilatation, hypotension, and potential cardiovascular collapse. Correction of hypothermia should be passive and occur over a number of hours.

Medications ⬆ ⬇

Medications

Drug(s)

Definitive immediate treatment: levothyroxine, 5 µg/kg (0.005 mg/kg) IV q12h.
A more conservative replacement dose should be used when there is concern about cardiac function, especially the heart's ability to deal with a sudden and rapid increase in metabolic rate. Decrease levothyroxine dose by 50–75% in these cases.
Once the patient's condition has stabilized and the patient is able to swallow, oral levothyroxine therapy should be initiated at 0.02 mg/kg PO q12h.

Contraindications/Possible Interactions

Lower IV levothyroxine dose should be considered in patients with cardiac disease.
Rapid rewarming must be avoided.

Follow-Up ⬆ ⬇

Follow-Up

Prognosis is grave.
If patient survives initial treatment, treatment and monitoring as recommended for hypothyroidism must be initiated.
Evaluation for potential underlying precipitating event.

Miscellaneous ⬆

Miscellaneous

See Also

Hypothyroidism

Abbreviations

T₄ = thyroxine, tetraiodothyronine
TSH = thyroid stimulating hormone

Suggested Reading

Finora K, Greco DS. Hypothyroidism and myxedema coma in veterinary medicine-physiology, diagnosis and treatment. Compend Contin Educ Pract Vet 2007, 29:19–32.

Henik RA, Dixon RM. Intravenous administration of levothyroxine for treatment of suspected myxedema coma complicated by severe hypothermia in a dog. J Am Vet Med Assoc 2000, 216:713–717.

Author Deborah S. Greco

Consulting Editor Deborah S. Greco

Acknowledgment The author and editors acknowledge the prior contribution of Kevin Finora.

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Basics ⬇

Diagnosis ⬆ ⬇

Treatment ⬆ ⬇

Medications ⬆ ⬇

Follow-Up ⬆ ⬇

Miscellaneous ⬆