CUSHING SYNDROME-INVESTIGATIONS

SUMMARY

1. Initial tests are to establish the presence of cortisol excess:
- 24-hour urine free cortisol (UFC). In the plasma, < 5% of cortisol is free and physiologically active. Only the free cortisol is filtered by the glomerulus, so urinary cortisol is always "free" cortisol and reflects plasma free cortisol levels
- Late-night salivary cortisol
- Low-dose dexamethasone suppression test to confirm excess cortisol

2. Is the Cushing syndrome ACTH-dependent or ACTH-independent?
- Check the ACTH level. Normally, a high cortisol completely suppresses ACTH production.

3. If ACTH-dependent Cushing syndrome: order neuro-imaging, chest & abdominal imaging.

4. If ACTH-independent Cushing syndrome: high cortisol and low ACTH-most likely an adrenal tumor (adenoma or carcinoma) that is secreting cortisol. Image the adrenals with contrasted CT.

Reference(s)

Wilkinson, I., Furmedge, D. and Sinharay, R. (2017). Oxford handbook of clinical medicine. Oxford: Oxford University Press. Get it on Amazon.
Feather, A., Randall, D. and Waterhouse, M. (2020). Kumar And Clark’s Clinical Medicine. 10th ed. S.L.: Elsevier Health Sciences. Get it on Amazon.
Hannaman, R. A., Bullock, L., Hatchell, C. A., & Yoffe, M. (2016). Internal medicine review core curriculum, 2017-2018. CO Springs, CO: MedStudy.
Therapeutic Guidelines. Melbourne: Therapeutic Guidelines Limited. https://www.tg.org.au [Accessed 2021].