The dexamethasone inhibition test is an examination that is performed when hypercortisolism is suspected. Hypercortisolism, also known as Cushing’s syndrome, is a condition that is associated with elevated cortisol levels. The increased cortisol level has a negative influence on the metabolism of the human body and it becomes unbalanced. Various symptoms can occur, such as trunk obesity, impaired glucose tolerance, high blood pressure and muscle weakness.
The dexamethasone inhibition test is carried out if Cushing’s syndrome is suspected. This test is intended to confirm the suspicion. Patients who suffer from the following symptoms require further clarification: full moon face, truncal obesity, impaired glucose tolerance, high blood pressure, hypogonadism of the gonads (in men potency disorders, in women cycle disorders), muscle weakness and psychological upset. These are the classic symptoms caused by this disease. They can occur individually or in combination and should be taken seriously and examined.
The short test
The test principle is based on a suppression of cortisol. For this purpose the patient takes dexamethasone. Dexamethosone is a synthetically produced glucocorticoid which has the same effect as cortisol.
However, in order to understand the further principle of the examination, one should understand the underlying physiology. For the body to produce cortisol, it needs a stimulus. This stimulus is provided by the hormone ACTH (adrenocorticotropin), which is produced in the pituitary gland and from there is released into the bloodstream.
The ACTH now reaches the adrenal cortex and stimulates the cells to produce cortisol. Consequently, the cortisol level in the blood rises. However, since too much cortisol is harmful, the body has developed a feedback mechanism.
A high cortisol level inhibits the release of ACTH. As a result, less cortisol is produced. However, when this level drops again, the ACTH level rises and the cells in the adrenal cortex produce more cortisol.
For the short test, blood is taken from the patient in the morning and the cortisol level is determined. On the same day, the patient must now take dexamethasone around midnight. The next day, a new blood sample is taken.
Here the cortisol level is determined and evaluated. In healthy people there should be a suppression. If this is not the case, the test is positive and further clarification is required. For this purpose, the dexamethasone long test should be carried out, among others.
The long test
The dexamethasone long test is performed after the short test. It differs in the duration of the test. It usually takes 3 days and includes several doses of dexamethasone.
The test principle is again based on a suppression of cortisol. If this is not the case, this indicates a disturbed mechanism. A temporal suppression speaks for a pathology of the central nervous system – i.e. the pituitary gland or the hypothalamus.
If there is no suppression at all, however, this indicates that there is independent hormone production. In other words, the feedback mechanism that would normally occur with high cortisol levels ceases to exist and the hormone is constantly being produced. This independent production can usually be observed in tumours. In this case, further diagnostics are absolutely necessary – including imaging procedures.