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Information ⬇

Author(s): James Crane and Paul Carroll

At the extreme end of the hypothyroid spectrum lies the rare endocrine emergency of myxoedema coma, which has a prevalence of less than 1 per million per year, and is largely a disease of the elderly. The physical signs of hypothermia from whatever cause closely resemble those of myxoedema coma; however, if there is other evidence of hypothyroidism (Table 92.1), thyroid hormone and hydrocortisone (in case there is coexisting autoimmune adrenal insufficiency) should be given. Even with treatment, mortality is high.

Priorities ⬆ ⬇

Outline

Is This Myxoedema Coma?!!navigator!!

  • Hypothermia and reduced conscious level are the cardinal features (although most patients are not actually comatose, i.e. Glasgow Coma Scale score <8).
  • Bradycardia, bradypnoea and hypoxaemia are common.
  • Hyponatraemia, hypercapnia, hypercalcaemia, hypoglycaemia and elevated creatinine kinase are often present.

What Has Caused Myxoedema Coma?!!navigator!!

  • Myxoedema coma is usually precipitated by an event causing an increased metabolic demand which outstrips the adaptive mechanisms compensating for chronic hypothyroidism, such as infection or trauma.
  • Other triggers include cold weather, sedative agents, general anaesthesia, acute coronary syndrome and stroke.

Immediate Management!!navigator!!

  • ABCDE assessment
    • Airway – may be compromised by oedema of the upper respiratory tract structures. Airway adjuncts or intubation may be required.
    • Breathing – ventilatory failure is usual and should be confirmed with arterial blood gas analysis. Assisted ventilation is often necessary for the first 24–48h.
    • Circulation – cautious fluid resuscitation, bearing in mind the likely impairment of cardiac contractility, can be employed. Glucocorticoids should be administered (50–100 mg 6-hourly IV) as severe hypothyroidism may impair ACTH response to stress. The possibility of undiagnosed autoimmune adrenal insufficiency as a comorbidity must be recognized.
  • Correct hypoglycaemia using intravenous glucose
  • Identify the precipitant and initiate treatment. Investigation needed urgently is given in Table 92.2.
    • Infection may be occult and sepsis is unlikely to be accompanied by an elevated temperature. If in doubt, administer broad spectrum antibiotics.
    • The ECG will be abnormal and usually shows bradycardia, small voltage QRS complexes and flattened or inverted T-waves. Varying degrees of heart block may be present. Measure the QTc interval, which may be prolonged, bringing a risk of polymorphic ventricular tachycardia (torsades de pointes; see Chapter 41). Assess for evidence of myocardial ischaemia or infarction.
    • Cerebellar signs may be the result of severe hypothyroidism, but assess for evidence of an acute stroke.
    • Assess for evidence of an upper gastrointestinal bleed.
    • Obtain a collateral history – there will usually be a history of hypothyroidism or thyroid ablation.
    • Review the drug history for new medications, which may have precipitated the acute presentation.

Alert the critical care team and transfer to an appropriate ICU or HDU bed when stable.

Further Management ⬆ ⬇

Outline

Thyroid Hormone Replacement!!navigator!!

  • Restoration of thyroid hormone activity is essential. There is no high-grade evidence to suggest how this is best achieved. Replacement may be enteral or parenteral; with T4, T3 or both.
  • Restoring normal target tissue thyroid hormone activity as soon as possible to reverse life-threatening disturbance of body systems must be weighed against the possibility of inducing fatal tachyarrhythmias with rapid correction. The enteral route should lead to a less abrupt increase in circulating thyroid hormone levels and allows for the use of T4, which, by virtue of requiring peripheral conversion to T3 for maximal activity, gives a smoother tissue response. However, absorption may be impaired by oedema, slow transit or ileus. The intravenous route limits one to using T3 since intravenous preparations of T4 are not commonly available. The associated rapid increase in thyroid hormone receptor signalling may induce adverse cardiac events.
  • The choice of treatment should be made on a patient-specific basis. The options are:
    • NG T4 alone
    • NG T4 plus T3

    • IV T3: typically 20μgm/24h

      T3 and T4 replacement should be carefully titrated against free thyroid hormone levels, which may be measured daily. Over-replacement risks tachyarrhythmia in what is likely to be a myopathic heart.

Supportive Care!!navigator!!

  • Hypothermia should not be treated with external rewarming since this will induce peripheral vasodilatation, negating the compensatory diversion of blood flow to the vital organs.
  • Glucocorticoids should be continued until coexisting adrenal insufficiency (Chapter 90) has been excluded by a short Synacthen test.
  • Ongoing supportive management of organ failure (e.g. mechanical ventilation or vasopressors) while awaiting response to thyroid hormone replacement is a key determinant of outcome. The time to recovery may be variable depending on the duration of severe hypothyroidism.

Further Reading ⬆

Chiong YV, Bammerlin E, Mariash CN (2015) Development of an objective tool for the diagnosis of myxedema coma. Translational Research 166, 233–243. http://doi.org/10.1016/j.trsl.2015.01.003