Cause Of Hyponatremia In a Patient With Head Injury

A 21-year-old man was admitted to the intensive care unit after a road traffic accident during which he suffered a severe head injury. He required ventilation.
Investigations are shown.below:

Sodium 128 mmol/l
Potassium 3.6 mmol/l
Creatinine 81 mmol/l
Urea 4 mmol/l
Thyroxine 30 nmol/l
TSH 2 mu/l
Serum cortisol 1000 nmol/l
(NR 170–700 nmol/l)

What is the cause of the hyponatraemia?
a. Hypopituitarism.
b. Addison’s disease.
c. Syndrome of inappropriate ADH secretion.
d. Hypothyroidism.
e. Cushing’s syndrome.

Answer:
c. Syndrome of inappropriate ADH secretion.

Discussion:  The patient has a low sodium concentration in the context of a head injury. The thyroid function tests suggest the possibility of a secondary hypothyroidism, i.e. a low TSH and a low thyroxine concentration, and hence the possibility of damage to the pituitary. However, the very high cortisol level indicates that pituitary function is probably normal (high ACTH production secondary to stress) and therefore the abnormal thyroid function tests represent sick euthyroid syndrome. Low T4, T3 and TSH levels are recognized in critically ill patients with nonthyroid illnesses. Originally such patients were thought to be euthyroid, therefore the term sick euthyroid syndrome was used to describe these biochemical abnormalities. There is evidence now that these abnormalities represent genuine acquired transient central hypothyroidism. Treatment with thyroxine in these situations is not
helpful and may be harmful. It is thought that these changes in thyroid function during severe illness may be protective by preventing excessive tissue catabolism. Thyroid function tests should be repeated after at least six weeks following recovery. Critical illness may also reduce T4 by reducing thyroid binding globulin levels, and T3 is rapidly reduced owing to inhibition of peripheral de iodination of T4.