Tests for Cushings Syndrome

Related Subjects: |Adrenal Physiology |Addisons Disease |Phaeochromocytoma |Adrenal Adenomas |Adrenal Cancer |Cushing Syndrome |Cushing Disease |Cushings Syndrome due to Ectopic ACTH |Congenital Adrenal hyperplasia |Primary hyperaldosteronism (Conn's syndrome) |ACTH |McCune Albright syndrome

Overnight Dexamethasone suppresion test: 1st line screening test :Establish diagnosis of Cushings syndrome

• Outpatient test to screen for Cushing's Syndrome is present
• Dexamethasone 1mg PO at midnight
• Check serum cortisol at 8AM.
• Normal : Cortisol <50nmol/L; Cortisol > 50 nmol/L Look for Cushing’s syndrome
• False positves: depression, obesity, alcohol excess, liver inducers e.g. phenytoin

24h urinary free cortisol: 1st line screening test

• Collect urine and normal: <280nmol/24h
• Elevated then check second line tests

Low Dose 48 hr test: 2nd line test :Confirm diagnosis of Cushings syndrome

• To confirm Cushing's Syndrome is present
• Cushings syndrome fails to suppress cortisol levels
• Method
  • Dexamethasone 0.5 mg PO given 6 hourly for 48 hours (4 mg in total)
  • Cortisol measured at 0 and 48 hours
• Investigations
  • Cortisol levels undetectable at 48 hours then Cushing's syndrome is unlikely
  • If levels are detectable proceed to determine the cause

High Dose 48 hr test: 2nd line test Determine source of cortisol

• Evaluation of the Causes of Cushing's Syndrome
• High Cortisol levels already been shown
• Failure to suppress cortisol can divide pituitary and non pituitary
• Method
  • Dexamethasone 2 mg 6 hourly PO is given for 48 hours (16 mg in total)
  • Cortisol measured at 0 hours and at 48 hours
• Investigations
  • Pituitary disease seens Cortisol reduced by over 50%
  • Other causes: Cortisol reduced slightly or not at all

Plasma ACTH

• Plasma ACTH : If undetectable, an adrenal tumour is likely
• Plasma ACTH : high can be ectopic or pituitary so use High dose supporession test
• Plasma ACTH: high ectopic vs pituitary; Give corticotropin-releasing hormone (CRH) test: 100mcg ovine or human CRH IV. Measure cortisol at 120min. Cortisol rises with pituitary disease but not with ectopic ACTH production.