Cushing's syndrome laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Furqan M M. M.B.B.S[2]

Overview

When Cushing's syndrome 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. 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.

Laboratory Findings

The laboratory findings associated with Cushing's syndrome are:

Diagnosis of hypercortisolism

Hypercortisolism can be established by any of the following tests:

• 24-hour urine cortisol
• Midnight salivary cortisol
• Low dose dexamethasone suppression test; high cortisol level after the dexamethasone test is suggestive of hypercortisolism.

Tests to establish the cause of hypercortisolism

• Measure Adrenocorticotrophic hormone (ACTH) levels.
• If ACTH is less than 5 pg/dl, perform an adrenal CT scan or MRI to search for possible adrenal causes.
• If ACTH is higher than 5 pg/dl Corticotrophic releasing hormone (CRH) test also called desmopressin test can be used to differentiate between Cushing disease and ectopic ACTH secretion.

Approach to the diagnosis of Cushing's syndrome

24 hr urine cortisol or midnight salivary cortisol shows hypercortisolism

Measure serum ACTH

<5pg/ml

5-20pg/dl

>20pg/dl

ACTH independent Cushing syndrome

Corticotrophic releasing hormone (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

Corticotrophic releasing hormone (CRH) (desmopressin) AND high dose dexamethasone suppression test OR MRI brain

Adequate stimulation and suppression OR pituitarty mass >6mm

Mixed/negative response OR no pituitary mass OR mass < 6mm

Cushing disease

Inferior petrosal sinus sampling

Cushing disease if step-up in ACTH

Ectopic ACTH secretion if no step-up in ACTH

Other Laboratory findings

• Hypokalemia

References

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