The syndrome may be conveniently classified into ACTH-dependent disease (pituitary or ectopic) and non-ACTH-dependent disease (adrenal tumours).
ACTH-dependent and ACTH-independent disease can usually be distinguished by the measurement of serum ACTH (Fig. 8.13), ACTH being undetectable with adrenal tumours. Abdominal CT will confirm the presence of an adrenal tumour, but care must be taken to distinguish between an adenoma in one part of atrophic glands and one or more nodules in hyperplastic adrenals (nodular hyperplasia). ACTH levels tend to be higher with ectopic sources than with pituitary sources, but there is considerable overlap.
