What are the diagnostic criteria and treatment for myxedema coma?

Diagnosis of Myxedema Coma

Myxedema coma should be diagnosed using a combination of clinical presentation (altered mental status, hypothermia, bradycardia), laboratory confirmation of hypothyroidism (elevated TSH, low free T4), and identification of a precipitating factor, with treatment initiated immediately without waiting for complete laboratory confirmation given the 25-65% mortality rate. 1, 2

Clinical Diagnostic Criteria

The diagnosis requires simultaneous assessment of six key variables that together form a validated screening tool with approximately 80% sensitivity and specificity 1:

Core Clinical Features

• Altered mental status: Assess using Glasgow Coma Scale; patients present with decreased consciousness ranging from lethargy to frank coma 1
• Hypothermia: Temperature typically below 35°C (95°F), often presenting in winter months 2
• Bradycardia: Heart rate significantly reduced below normal range 1
• Characteristic physical findings: Look specifically for puffy face, thin eyebrows, severe systemic non-pitting edema, hyporeflexia, and tetraparesis 3, 4

Laboratory Confirmation

• Thyroid function tests: Elevated TSH with decreased free T4 and free T3 levels 3, 1
• Critical caveat: Myxedema coma can rarely occur with subclinical hypothyroidism (elevated TSH but normal free T4/T3), so do not exclude the diagnosis based solely on normal thyroid hormone levels if clinical presentation is compelling 5
• Supportive laboratory findings: Hyponatremia, elevated creatinine phosphokinase, metabolic acidosis, hypercarbia, and hypoxemia 3, 5, 2

Precipitating Factors (Required Component)

Identify at least one triggering event 1, 2:

• Systemic illness (pneumonia, sepsis)
• Sedatives or anesthetics
• Cold exposure
• Acute illness or trauma
• Medication non-compliance in known hypothyroid patients

Additional Diagnostic Workup

Cardiovascular Assessment

• Echocardiogram: May reveal pericardial effusion and heart failure 3, 5
• ECG: Documents bradycardia and may show low voltage 3

Imaging Studies

• Chest X-ray: Evaluate for pleural effusions and precipitating infections 3
• CT/MRI brain: Primarily to exclude other causes of altered mental status; typically normal in myxedema coma 3

Rule Out Concurrent Adrenal Insufficiency

• ACTH stimulation test: Must be performed to exclude adrenal insufficiency, as this can coexist and requires concurrent treatment 5
• Critical point: Administer hydrocortisone 100 mg IV every 8 hours empirically until adrenal insufficiency is ruled out, as thyroid hormone replacement can precipitate adrenal crisis 6

Diagnostic Algorithm

1. Immediate clinical assessment (do not delay for laboratory results):

   • Glasgow Coma Scale score
   • Core temperature measurement
   • Heart rate
   • Physical examination for hypothyroid stigmata (puffy face, thin eyebrows, non-pitting edema) 4, 1

2. Stat laboratory tests:

   • TSH, free T4, free T3
   • Serum sodium, glucose
   • Arterial blood gas (assess for hypercarbia, hypoxemia)
   • Creatinine phosphokinase 3, 5

3. Identify precipitating factor through history and targeted workup 1, 2

4. Begin treatment immediately once clinical suspicion is high, even before laboratory confirmation, as mortality depends on time to treatment initiation 3

Common Diagnostic Pitfalls

• Do not wait for laboratory confirmation: The high mortality rate (25-65%) mandates immediate treatment based on clinical suspicion 1, 2
• Do not exclude diagnosis with normal thyroid hormones: Rare cases present with subclinical hypothyroidism; clinical judgment supersedes laboratory values when presentation is classic 5
• Do not overlook precipitating factors: Absence of an identifiable trigger should prompt reconsideration of the diagnosis 1
• Do not forget to rule out adrenal insufficiency: Always administer empiric hydrocortisone until ACTH stimulation testing excludes this, as thyroid hormone can unmask or worsen adrenal crisis 6, 5