Outline the clinical manifestations, appropriate investigations and treatment of hypothyroidism in Intensive Care.
• Hypothyroidism is very common in the ageing population, many unrecognised. Many clinical manifestations are specifically related to either generalised metabolic slowing (fatigue, delayed relation of deep tendon reflexes, bradycardia, depressed nervous system, and hypothermia) or accumulation of matrix glycosaminoglycans (coarse hair and skin, enlarged tongue, non-pitting oedema [myxoedema]). Other manifestations include pericardial effusion, hypertension, hypercholesterolaemia, respiratory muscle weakness, impaired gut motility, and normochromic normocytic anaemia. In some situations (usually
obvious), hypothyroidism occurs as a result of treatment for hyperthyroidism or after thyroid surgery.
• Investigations should confirm diagnosis and detect complications (eg. hyponatraemia and lipid abnormalities). Confirmatory tests reveal high serum TSH and a low free T4. Uncommonly secondary or tertiary hypothyroidism (inappropriately low level of TSH for T4). Study of other pituitary or hypothalamic function may be required ± imaging.
• Specific treatment involves replacement of thyroid hormone (usually as T4 50 – 200 mcg/day). Elderly, especially with heart disease require a more gradual introduction (eg. 25 mcg). Intravenous T3 (5-20mcg initially) may also be used in the treatment of myxoedema coma. Other treatment involves supportive care (ventilation, fluid and electrolyte management, temperature control) and corticosteroids (eg. hydrocortisone 100 mg tds) in severe cases until adrenal insufficiency excluded.
Hypothyroidism in critical illness receives proper treatment in another chapter.
In order to render the process of revision simpler, I reproduce the table of clinical manifestations here:
The diagnosis of hypothyroidism rests on the measurement of TSH, T3 and T4 which can lead one to differentiate between the causes of hypothyroidism (eg. is it the pitutary, or is it the thyroid gland itself?). One may wish to test for rT3 - the "reverse" form of T3 - which is increased in "sick euthyroid" syndrome.
Lastly, the management is not clever, and consists of thyroxine replacement. The college insists we mentione corticosteroid replacement. One may wish to also mention the following features, unique to the intensive care setting:
- need for intubation in the context of a decreased level of consciousness
- delay in weaning from ventilation due to untreated hypothyroidism
- increased sensitivity to sedating agents
- decreased sensitivity to inotropes and vasopressors
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