myxoedema coma

clinical identity

The end-stage of untreated hypothyroidism resulting in multisystem organ failure. Defined by Altered Mental Status (AMS) + Hypothermia + Precipitating Event. Mortality is 20–50%.

pathophysiology

• Mechanism: Intracellular T3 depletion $\to$ $\downarrow$ thermogenesis, $\downarrow$ cardiac chronotropy/inotropy, $\downarrow$ respiratory drive.
• Precipitants:
  • Sepsis/Infection (Most common).
  • Cold exposure.
  • Drugs (Sedatives, Narcotics, Amiodarone, Lithium).
  • Acute events (MI, Stroke, GI Bleed).
  • Non-adherence to Levothyroxine.

diagnosis

Diagnosis is CLINICAL. Do not wait for TSH/fT4 to treat.

• Hallmark: Hypothermia (<35.5°C, often lower) + Coma/Obtundation.
• CV: Bradycardia, Hypotension (shock), Pericardial effusion.
• Resp: Hypoventilation (hypercapnia), Macroglossia (airway obstruction).
• Skin: Dry, doughy skin, non-pitting oedema, generalized puffiness.
• Labs:
  • $\uparrow$ TSH, Undetectable fT4.
  • Hyponatremia (SIADH-like physiology + $\downarrow$ GFR).
  • Hypoglycemia.
  • $\uparrow$ CK.

sepsis vs myxoedema

A patient with Myxoedema Coma may not mount a fever even with sepsis due to metabolic slowing. Hypothermia + Normal WBC count may mask a severe underlying infection. Pan-culture and treat aggressively.

management protocol

Order of operations is important.

1. adrenal coverage (immediate)

• Drug: Hydrocortisone 100 mg IV q8h.
• Reason: Autoimmune hypothyroidism often co-exists with Adrenal Insufficiency (Schmidt’s Syndrome). Giving thyroid hormone accelerates cortisol metabolism $\to$ precipitates Adrenal Crisis if not covered.
• Test: Draw random cortisol before first dose (ACTH stim later).

2. thyroid hormone replacement

• Levothyroxine (T4):
  • Load: 200–400 µg IV $\times$ 1 (Use lower end, 200µg, if elderly/cardiac disease to avoid arrhythmia/MI).
  • Maintenance: 1.6 µg/kg/day IV (initially) then switch to PO when extubated/stable.
  • Note: IV dose is ~75% of PO dose.
• Liothyronine (T3):
  • Role: Peripheral conversion of T4 $\to$ T3 is impaired in sick states (low deiodinase activity). T3 crosses blood-brain barrier faster for neurologic recovery.
  • Load: 5–20 µg IV $\times$ 1.
  • Maintenance: 2.5–10 µg IV q8h.
  • Caution: Discontinue if ischaemia/arrhythmia develops.

3. supportive care

• Rewarming: PASSIVE only (blankets, warm room).
  • Contraindication: Active external rewarming (e.g., Bear Huggers) causes peripheral vasodilation $\to$ vascular collapse/shock.
• Ventilation: Low threshold to intubate (macroglossia, respiratory acidosis).
• Fluids/Pressors: Patients are volume depleted (third spacing) but sensitive to overload. May require alpha-agonists if hypotensive despite fluids.
• Hyponatraemia: Usually corrects with fluid restriction + thyroid hormone. Use Hypertonic Saline (3%) only if seizures/severe symptoms.

related pages: Hypothyroidism, Adrenal Insufficiency, Hyponatremia