.Hypothyroidism comes up often as a differential for various things, but only Question 11 from the second paper of 2002 addresses it directly (diagnosis, features, management). The hypothyroidism considered in this chapter lays somewhere between the mild "sick euthyroid" syndrome and frank myxoedema. Myxoedema coma comes up much more often in the exam, and has been allocated to its own chapter.
Causes of Hypothyroidism
An excellent article is available, which treats this topic with the detail it deserves. Using this as a substrate, I will make an attempt to summarise the causes of hypothyroidism in a table.
Features of hypothyrodism
One can summarise these by saying that everything goes slower.
Management of hypothyroidism
Well, one could do no better than to replace the missing hormone. Its not clever, but it is the right solution. In myxoedema coma, one may wish to replace it intravenously.
A dose of 50 to 200mcg is usually enough. One traces the efficacy of one's management by observing the decrease in TSH (which should be elevated in "proper" hypothyroidism, and which will fall to normal levels as thyroid function is restored).
The addition of corticosteroids into the mix tends to be advocated because until test results are available, one is never sure whether one is dealing with some sort of hypoadrenal syndrome.
Issues specific to hypothyroidism in the ICU
Need for intubation due to a decreased level of consciousness
These people may have an abnormal level of consciousness. Myxoedema coma is the most severe manifestation of hypothyroidism, and it maybe the reason the patient is admitted to the ICU in the first place. However, even without it, hypothyroid patients may require intubation because their threshold for unconsciousness is decreased.
Increased sensitivity to anaesthetics
When one is going to administer the anaesthetic for intubation, one would be well advised to down-titrate one's doses, because hypothyroid patients will be more sensitive to anaesthetic agents. There has been several case series of hypothyroid patients who were diagnosed after they reacted badly to routine anaesthesia;
Prolonged ventilation and delayed weaning
After a course of ventilation, hypothyroidism may delay weaning because of respiratory muscle weakness, and may be one of the causes of weaning failure.
Poor response to vasopressors and inotropes
The responses to catecholamine vasopressors will be blunted in hypothyroidism. Initially, tests performed on patients before and after treatment demonstrated no such relationship. However, later publications confirmed that there is indeed a dampening of the response to sympathomimetics, and that this response is by no means uniform. Hypothyroidism results in a decreased expression of adrenergic receptors, and a dysfunction of posterceptor signalling. However, it seems there is usually still enough "spare receptors" around to maintain a normal response to infused catecholamines. A few unlucky patients may be so receptor-depleted that their catecholamine requirements may increase.
Severe hypothyroidism seems to promote bleeding by increasing the rate of fibrinolysis. This is in contrast to the increase in fibrinolytic activity which is seen with mild and moderate hypothyroidism. The mechanism for this is poorly understood.