Myxedema Coma

Definition & Epidemiology

1. Altered mental status (lethargy, stupor, or coma)
2. Hypothermia
3. A concomitant precipitating event

Precipitating Factors

Pathophysiology

• Hypometabolic state creates large pools of empty T3/T4 binding sites that must be filled before free thyroxine can affect tissue metabolism.
• Hypoventilation results from decreased hypoxic and hypercapnic ventilatory drive, leading to CO2 retention (PaCO2 rarely exceeds 50-55 mmHg).
• Hypothermia reflects severely reduced thermogenesis and basal metabolic rate.
• Hyponatremia results from inappropriate ADH secretion (SIADH-like mechanism), sometimes severe enough to cause seizures.
• Hemodynamic compromise stems from decreased cardiac contractility, bradycardia, reduced cardiac output, and increased systemic vascular resistance.
• Cerebral blood flow and glucose metabolism are reduced, contributing to impaired consciousness.
• The EEG shows diffuse background slowing, triphasic waves, and may show altered voltage. - Plum and Posner's, p. 398

Clinical Features

Recognition Box (Rosen's)

Diagnostic Testing

• TSH - most sensitive test; markedly elevated in primary hypothyroidism; low or normal in central hypothyroidism
• Free T4 - low (FT4 <0.6 ng/dL is a scoring criterion)
• T3 - may be low (conversion of T4 to T3 can be impaired in critical illness)
• Cortisol / ACTH stimulation - to rule out concurrent adrenal insufficiency
• BMP - hyponatremia, hypoglycemia
• CBC - look for infection
• Blood cultures, CXR, UA - to identify precipitant
• ABG - CO2 retention, hypoxia
• ECG - bradycardia, long QT, low-voltage complexes, possible pericardial effusion signs
• Elevated CK - elevated muscle enzyme levels are common in myxedema

Do not delay treatment waiting for labs. Once blood is drawn, empiric treatment should begin. - Plum and Posner's, p. 398

Myxedema Coma Scoring Tool (Rosen's Table 117.4)

• Score 8-10: Myxedema coma most likely - proceed with treatment
• Score 5-7: Myxedema coma likely - treat if no other diagnosis fits
• Score <5: Myxedema coma unlikely - consider other diagnoses

Management

1. ICU Admission

2. Thyroid Hormone Replacement (Cornerstone of Treatment)

• Loading dose: 200-400 mcg IV (use lower doses in elderly patients, those with CAD, arrhythmias, or low body weight)
• Maintenance: 1.6 mcg/kg/day IV (give at 75% of the equivalent oral dose); approximately 50-100 mcg IV daily

• Can be added in very ill patients because impaired T4 → T3 conversion may limit the effectiveness of T4 alone
• Dose: 5-20 mcg IV initially, then 2.5-10 mcg every 8 hours (Katzung); or 10 mcg every 6 hours (Braunwald's) until clinical improvement
• Caution: T3 is more cardiotoxic and harder to monitor; elevated T3 levels are associated with increased mortality
• All preparations must be given IV - gut absorption is poor in myxedema - Katzung, p. 1088

3. Glucocorticoids

• Hydrocortisone 100 mg IV every 6-8 hours (stress dose) until adrenal insufficiency is excluded
• Given because concurrent secondary adrenal insufficiency may exist, and starting T4 alone can precipitate adrenal crisis
• Glucocorticoids should be started before or simultaneously with thyroid hormone - Goldman-Cecil, p. block36

4. Supportive Care

5. Hemodynamic Recovery

Differential Diagnosis

• CO2 narcosis / hypercapnic encephalopathy
• Hyponatremic encephalopathy
• Septic encephalopathy
• Drug toxicity (opioids, sedatives)
• Stroke / CNS infection
• Hypothermia from other causes (hypoglycemia, drug overdose, exposure)
• Addisonian crisis

Prognosis

• Mortality: approximately 25-30% with optimal treatment
• Mortality approaches 100% without treatment
• Poor prognostic signs:
  • Temperature <32°C (90°F)
  • Need for mechanical ventilation
  • Need for vasopressors
  • Need for steroids
  • Severe coma on admission (3 of 4 comatose patients in one series died vs. 1 of 7 with lesser impairment) - Plum and Posner's
• Hemodynamics improve within 24-48 hours with adequate treatment

Key Pearls

1. Treat empirically - do not wait for TFT results if the clinical picture is compelling.
2. IV route mandatory - oral absorption is unreliable in this state.
3. Passive rewarming only - active warming worsens hypotension.
4. Cover the adrenals - hydrocortisone before or with T4 to prevent precipitating adrenal crisis.
5. T3 is a double-edged sword - beneficial for faster action, but cardiotoxic at elevated levels.
6. Temperature may be near-normal if infection is the precipitant - do not exclude the diagnosis on that basis.
7. Hyponatremia is both a precipitant and a complication, and is independently associated with increased mortality.

Recent literature note: A 2025 systematic review (Zhang et al., Thyroid Research, PMID 41053871) and a 2025 narrative review on thyroid emergencies (Kruithoff & Gigliotti, Endocrine Practice, PMID 40553957) are the most recent publications addressing ongoing challenges in myxedema coma management, including optimal T3/T4 dosing and the T4-only vs. combination therapy debate.