Endogenous ACTH: pituitary or adrenal Cushing's in the dog
10-year-old female Dachshund with hyperadrenocorticism confirmed by low-dose dexamethasone suppression, with no suppression at 4 h. On ultrasound, the left adrenal is 7.5 mm thick and the right 6.8 mm, with no masses. The ACTH sample was collected in EDTA, chilled and centrifuged immediately.
| Parameter | Result | Reference |
|---|---|---|
| Cortisol at 4 h (dexamethasone) | 3.9 µg/dL | < 1.4 |
| Cortisol at 8 h (dexamethasone) | 4.2 µg/dL | < 1.4 |
| Endogenous ACTH | 88 pg/mL | 10–45 |
What is the most appropriate interpretation?
chevron_right Show answer and explanation
A. Adrenal origin, because there was no suppression at 4 h
Lack of suppression does not differentiate: it occurs in all adrenal tumours but also in roughly a third of pituitary cases. A functional adrenal tumour would suppress ACTH, not raise it.
B. Pituitary-dependent hyperadrenocorticism
Correct. A normal-high or high endogenous ACTH, properly handled, indicates a pituitary origin, and fits with symmetrical adrenals at the upper limit with no masses. With an adrenal tumour ACTH would be low or undetectable.
C. Iatrogenic hypercortisolism due to undisclosed corticosteroids
Exogenous corticosteroids suppress ACTH and cause adrenal atrophy, and endogenous cortisol would be low. This is the opposite of what is seen.
D. It cannot be differentiated without a high-dose dexamethasone suppression test
The high dose only helps if suppression occurs; many pituitary cases do not suppress with it either. Endogenous ACTH and ultrasound are the preferred tools today.
lightbulb Key takeaway
To determine the origin, combine endogenous ACTH and adrenal ultrasound: normal-high ACTH with symmetrical adrenals indicates a pituitary origin; low or undetectable ACTH with a mass and a small contralateral gland, an adrenal tumour. ACTH is very labile: EDTA, cold and prompt centrifugation.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.