Cushing syndrome is due to the effects of excessive glucocorticoids which may be exogenous or endogenous.
Endogenous sources of excess cortisol production include:
- adrenal adenoma: 20%
- primary pigmented nodular adrenal dysplasia (PPNAD): rare 2
- ACTH secreting tumour: 80%
- adrenocorticotropin (ACTH)-independent macronodular adrenocortical hyperplasia (AIMAH): rare cause of ACTH independent Cushing syndrome 4
The work up of Cushing syndrome requires measurement both of cortisol as well as ACTH. Measuring cortisol typically needs to be over a 24 hour period because release is intermittent.
Imaging of the suspected region is then required:
If ACTH is elevated but no microadenoma can be identified, and no ectopic source can be found, then inferior petrosal sinus sampling can be undertaken. Bilateral adrenal hyperplasia is one of the most common findings on abdominal CT.
ACTH secreting pituitary microadenomas may be unapparent on imaging in 40-50% of cases.
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