Cushing Syndrome

The main common cause of Cushing syndrome is long term steroid therapy [like patients with renal transplant, or who are on immunosuppressant, Lupus]. If this case is excluded, you should think about 3 sources:

• Pituitary Cushings
• Adrenal Cushings
• Ectopic Cushings

Which of the three above mentioned will have the highest ACTH levels? = Ectopic [Small cell carcinoma]
Which of the three above mentioned will have the lowest ACTH levels? = Adrenal [ The main reason is because its making cortisol, which would suppress the ACTH]

For Cushing, there are 2 screening tests:

1. 24hr urine test for free cortisol
• This is the best screening test for Cushings because this distingushes Cushing syndrome from Cushingoid obesity. This test got high sensitivity than specificity.
2. Dexamethasone suppresion test
• Dexmeth is a cortisol analog [so act like cortisol]
• If you give it to a normal person, it will suppress ACTH.
• if you give a LOW dose of dexmeth to a patient with cushings, it will NOT suppress their cortisol.
• Remember there are 2 endocrine diseases that you can suppress ACTH - PITUITARY Cushings and PROLCATINOMA. So if you give HIGH dose of dexmethasone, you are able to suppress the ACTH release by the pituitary and cortisol goes down. It will not be suppresed in adrenal and ectopic Cushings [small cell carcinoma].