Replying to Jane Sedgewick 14/05/2025 - 12:07
Hi Jane,
Thank you so much for the great questions. You are definitely not a pest. It’s fantastic that you’re getting so much out of the course.
You’re absolutely right that when you have a collapsed dog with marked hyperkalaemia and hyponatraemia, Addison’s becomes the top differential, especially in the presence of classic clinical signs like lethargy, vomiting, bradycardia, and hypovolaemia. Very few diseases give you that combination in a sick dog. Once sodium is low and potassium is genuinely high (not an artefact), Addison’s should be considered the most likely diagnosis unless something else clearly explains it, like urinary obstruction or acute kidney injury.
That said, regarding your question about differentials for a low basal cortisol, there are a few scenarios where this might occur without true Addison’s:
Critical illness-related corticosteroid insufficiency (CIRCI) is increasingly recognised in dogs with sepsis or SIRS. These dogs may have a flat or blunted ACTH stim but are not Addisonian. A 2021 study by Marchetti et al. found that CIRCI occurred in nearly 50% of dogs with SIRS and was associated with increased CRP and band neutrophils, but not necessarily with hypotension or mortality. Importantly, these dogs may have low delta cortisol on ACTH testing, so we need to interpret results carefully in the ICU setting.
Reference: Marchetti M, Pierini A, Favilla G, Marchetti V. Critical illness-related corticosteroid insufficiency in dogs with systemic inflammatory response syndrome: A pilot study in 21 dogs. Vet J. 2021;273:105677. doi:10.1016/j.tvjl.2021.105677
Iatrogenic suppression from prior steroid treatment, even topical creams or ear medications.
Sampling variation. Basal cortisol is highly dynamic and a low result can sometimes occur transiently in otherwise healthy or mildly stressed animals.
Hepatic dysfunction or severe systemic illness, which can affect cortisol metabolism or binding proteins.
That’s why, even in the face of compatible electrolytes and clinical signs, a low basal cortisol doesn’t confirm Addison’s. It just tells us we can’t rule it out. That’s where the ACTH stim is still essential. On the flip side, a basal cortisol above 55 nmol/L (around 2 µg/dL) is very helpful to exclude Addison’s.
The combination of all three in a collapsed dog dose make Addison’s the most likely thing! It’s questions like this that make the course better, so please do keep them coming.
Happy Sunday!
Scott 🙂
