Cushing's syndrome laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Laboratory Findings

When Cushing's is suspected, either a dexamethasone suppression test (administration of dexamethasone and frequent determination of cortisol and ACTH level), or a 24-hour urinary measurement for cortisol offer equal detection rates. Dexamethasone is a glucocorticoid and simulates the effects of cortisol, including negative feedback on the pituitary gland. When dexamethasone is administered and a blood sample is tested, high cortisol would be indicative of Cushing's syndrome because there is an ectopic source of cortisol or ACTH (eg: adrenal adenoma) that is not inhibited by the dexamethasone.

A novel approach, recently cleared by the US FDA, is sampling cortisol in saliva over 24 hours, which may be equally sensitive, as late night levels of salivary cortisol are high in Cushingoid patients. Other pituitary hormone levels may need to be ascertained.

24 hr urine cortisol or midnight salivary cortisol shows hypercortisolism

Measure serum ACTH

<5pg/ml

5-20pg/dl

>20pg/dl

ACTH independent Cushing syndrome

CRH (desmopressin) test

ACTH dependent Cushing syndrome

Adrenal CT/MRI

Evaluate as ACTH independent Cushing syndrome if NO ACTH response

Evaluate as ACTH dependent Cushing syndrome if there is an ACTH response

CRH (desmopressin) AND high dose dexamethasone suppression test

Adequate stimulation and suppression

Mixed or negative response

Cushing disease

Inferior petrosal sinus sampling

References

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