Hyperfunction of the adrenal cortex

Hyperfunction of the adrenal cortex

Hyperfunction of the adrenal cortex can be conveniently discussed in terms of the overproduction of the three main products:

adrenal androgens

Cortisol excess

Prolonged exposure of body tissues to cortisol or other glucocorticoids gives rise to the clinical features that collectively are known as Cushing’s syndrome (Fig 49.1), after the American neurosurgeon Harvey Cushing. It most often results from prolonged use of steroid medications (iatrogenic). Much less frequently, it is caused by tumours that secrete either cortisol or ACTH (see below); these can sometimes be very difficult to diagnose.

Fig 49.1 Some clinical features of Cushing’s syndrome.

In any investigation of Cushing’s syndrome the clinician should ask two questions:

‘Does the patient actually have Cushing’s syndrome?’ The possibility that a patient may have Cushing’s syndrome frequently arises because they are obese or hypertensive, conditions frequently encountered in the population at large. Initial investigations will in most cases exclude the diagnosis of Cushing’s syndrome.

Once the diagnosis of Cushing’s syndrome is established, then a second question may be asked: ‘What is the cause of the excess cortisol secretion?’ Tests used in the differential diagnosis are different from those used to confirm the presence of cortisol overproduction.

Confirming the diagnosis

Iatrogenic Cushing’s syndrome is usually obvious – the patient is on steroid medications. The steroid may have been taken orally, inhaled or applied topically. Iatrogenic Cushing’s syndrome is not usually a diagnostic dilemma and will not be considered further here.

Cortisol, secreted in excess by the adrenal cortex, will rapidly exceed the available capacity of the plasma binding protein, cortisol binding globulin. Unbound cortisol is filtered readily into the urine. ‘Urinary free cortisol’ in a 24-hour collection, or assessed as a cortisol : creatinine ratio in an early morning urine sample, is one of the initial screening tests in a patient with suspected adrenocortical hyperfunction. The latter measurement can be made on a small aliquot of urine. Repeatedly high early morning urine cortisol : creatinine ratios are evidence enough to proceed with further investigations of the patient. If the test is negative on three occasions, Cushing’s syndrome may be excluded from the differential diagnosis.

Cortisol concentrations measured at 08:00 and 22:00 normally show a circadian rhythm with the evening sample having a lower value than that in the morning. This difference is usually not apparent in the patient with Cushing’s syndrome. It is essential that the patients are not stressed when such measurements are made.

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Tags: Clinical Biochemistry An Illustrated Colour Text 5e

Jun 18, 2016 | Posted by admin in BIOCHEMISTRY | Comments Off

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