Myxoedema Coma
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Direct answer
Myxoedema coma is decompensated severe hypothyroidism, with depressed consciousness, hypothermia, bradycardia and hypoventilation, usually triggered by infection, cold exposure, sedatives or surgery in a patient with long-standing thyroid failure. It is a medical emergency needing airway support, hydrocortisone given before intravenous levothyroxine, and cautious correction of hyponatraemia.
What you must remember
- Precipitants: infection, most often pneumonia or urosepsis, cold exposure, trauma, stroke, myocardial infarction, sedatives, opioids, amiodarone and stopping levothyroxine.
- Clinical clues: hypothermia often below 35 °C, bradycardia, hypotension, hypoventilation with CO2 retention, hyponatraemia, hypoglycaemia, ileus and delayed reflex relaxation.
- Biochemistry: low free T4 with raised TSH in primary disease, plus hypercapnia on arterial gas and often raised creatine kinase.
- Hydrocortisone first: given before levothyroxine, because thyroid hormone speeds cortisol clearance and can unmask adrenal insufficiency.
- Levothyroxine: intravenous loading then daily intravenous maintenance at a lower dose, with cardiac monitoring because arrhythmia and ischaemia risk rises in older patients.
- Warming: passive rewarming with blankets; active external rewarming risks vasodilatation and collapse.
- Supportive care: ventilation for hypercapnia, correction of hypoglycaemia, fluid restriction for dilutional hyponatraemia and antibiotics for the precipitating infection.
Worked example: an elderly woman found drowsy in winter
A 71-year-old woman who stopped her levothyroxine two years earlier is brought in drowsy in winter. Her temperature is 33.8 °C, pulse 46 and blood pressure 90/60 mmHg, and she is hypoventilating.
- Airway and breathing: These come first, with ventilation if carbon dioxide retention is significant.
- Bloods before hormones: Take TSH, free T4, arterial gas, glucose, sodium, creatine kinase and cortisol before any hormone, because a cortisol sample is uninterpretable after hydrocortisone.
- Hydrocortisone first: Give it intravenously, since thyroid hormone can precipitate adrenal crisis if adrenal insufficiency coexists.
- Levothyroxine with monitoring: Start intravenous levothyroxine with an ECG and cardiac monitoring, as the elderly heart tolerates the extra drive poorly.
- Passive warming: Use blankets and a warm room rather than hot packs.
- Infection and sedation: Take cultures and start antibiotics for any likely infection, since sepsis is the commonest trigger. Avoid sedatives, because respiratory depression is already present.
- Review: Judge progress over one to three days by temperature, heart rate and mental state, and restart oral replacement once she can swallow safely.
Where the examiner sets the traps
Four sequencing and safety traps recur.
- Trap: waiting for thyroid results before treating. Correct: myxoedema coma is a clinical diagnosis, so treatment must start on suspicion.
- Trap: giving levothyroxine before hydrocortisone. Correct: hydrocortisone precedes levothyroxine, and the cortisol sample is taken before the steroid is given.
- Trap: warming actively to correct the hypothermia. Correct: active rewarming drives peripheral vasodilatation and can cause collapse, so warming is passive.
- Trap: sedating an agitated hypothermic patient. Correct: sedatives are dangerous here because they depress ventilation further.
The stem may also describe an elderly patient with dementia-like features and a past thyroidectomy, and the diagnosis is missed unless myxoedema is considered.
Frequently asked questions
Why is hydrocortisone given before levothyroxine in myxoedema coma?
Hydrocortisone is given before levothyroxine because thyroid hormone increases cortisol clearance and may precipitate adrenal crisis in a patient with unrecognised adrenal insufficiency. Myxoedema coma therefore starts with intravenous hydrocortisone, with a cortisol sample taken beforehand where possible.
What precipitates myxoedema coma most commonly?
Infection, particularly pneumonia and urinary sepsis, is the commonest precipitant of myxoedema coma, followed by cold exposure, trauma, stroke and sedative drugs. Stopped or missed levothyroxine is also a classic precipitant in patients with known hypothyroidism.
Why should active rewarming be avoided in myxoedema coma?
Active rewarming in myxoedema coma causes peripheral vasodilatation that can lead to hypotension and cardiovascular collapse. Passive rewarming with blankets and a warm environment is preferred, because the hypothermic circulation cannot tolerate sudden peripheral heat.
What laboratory findings are typical in myxoedema coma?
Low free T4 is typical, with raised TSH in primary hypothyroidism, and arterial blood gas often shows CO2 retention. Hyponatraemia and hypoglycaemia are common, and the diagnosis is made clinically, with treatment started before results return.
Which drugs should be avoided in myxoedema coma?
Sedatives, opioids and anaesthetic agents should be avoided in myxoedema coma because they depress ventilation and worsen hypothermia and hypoglycaemia. Respiratory depressants are particularly dangerous in patients who are already hypoventilating.