Diagnostic testing for adrenal insufficiency (short Synacthen test, ACTH)
Core concept
- Two questions, in order
- 1. Is there cortisol deficiency? - 0900 h cortisol, then short Synacthen test
- 2. Where is the lesion? - plasma ACTH drawn at the same time
| Primary (Addison's) | Secondary/tertiary | |
|---|---|---|
| ACTH | HIGH | Low or inappropriately normal |
| Aldosterone / renin | dec aldosterone, inc renin | Normal (RAAS intact) |
| K+ | inc | Normal |
| Na+ | dec | dec (SIADH-like, from cortisol deficiency) |
| Pigmentation | Present (POMC/MSH) | Absent - pale |
| Other pituitary axes | Normal | Often abnormal |
- *Aldosterone is preserved in secondary adrenal insufficiency* - ACTH is only a minor, transient regulator of the zona glomerulosa
- -> no hyperkalaemia, no salt craving, less haemodynamic collapse; fludrocortisone is not required
3 more sections, plus exam facts
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