Investigations

Question 1

What is the dexamethasone suppression test?

Answer 1

• Exogenous steroid that suppresses the pituitary through negative feedback
• It binds to glucocorticoid receptors in the pituitary and inhibits ACTH secretion by pituitary gland

Question 2

What is the result of a dexamethasone test in Cushing’s disease?

Answer 2

• low dose dexamethasone: no reduction in cortisol output

- High-dose dexamethasone: inhibition of cortisol

Question 3

What is the result of a dexamethasone test in adrenal tumour or ectopic ACTH?

Answer 3

No reduction in low or high dose dexamethasone suppression tests.

Question 4

How do you diagnose SIADH?

Answer 4

• Serum osmolality is low

- Urine osmolality is high (Na is being lost in urine)

Question 5

What are the 1st line tests for Cushing’s syndrome?

Answer 5

• Overnight dexamethasone suppression test
• 24hr urinary free cortisol (normal: <280nmol/24hr) is alternative

Question 6

What are the 2nd line tests for Cushing’s syndrome?

Answer 6

• If 1st line abnormal: 48hr dexamethasone suppression test

- 48hr high dose dexamethasone may distinguish pituitary from other causes (no suppression)

Question 7

What are the localization tests for Cushing’s syndrome?

Answer 7

• Plasma ACTH: if ACTH is undetectable, an adrenal tumour is likely > CT/MRI adrenal glands
• If ACTH is detectable, distinguish a pituitary cause from ectopic ACTH production by high-dose suppression or CRH test

Question 8

What are the investigations for Addison’s disease?

Answer 8

• Decreased Na, increased K, urea and Ca
• Decreased glucose
• Eosinophilia
• Anaemia
• Short ACTH stimulation test (synacthen)