What is the recommended diagnostic workup for differentiating PDH from adrenal-dependent hyperadrenocorticism in dogs?

Question

Accepted Answer

The recommended differentiation workup combines the low-dose dexamethasone suppression test (LDDST), endogenous ACTH concentration, and abdominal ultrasonography, with computed tomography (CT) reserved for surgical planning or suspected macroadenoma.17 The LDDST is the first-line test and serves dual purpose — it both confirms hyperadrenocorticism (HAC) and differentiates pituitary-dependent hyperadrenocorticism (PDH) from adrenal-dependent hyperadrenocorticism (ADH) in many cases.47 A decrease of ≥50% in plasma cortisol concentration or urinary corticoid-to-creatinine ratio (UCCR) after dexamethasone administration is consistent with PDH.7 Resistance to suppression occurs in approximately 25% of PDH dogs on the high-dose dexamethasone suppression test (HDDST), limiting its reliability as a sole differentiating test.2 Endogenous ACTH concentration is a key differentiating test. In PDH, inappropriate and unregulated ACTH secretion causes intermittent elevation in plasma ACTH concentration, whereas in ADH, autonomous cortisol secretion suppresses ACTH via negative feedback at the hypothalamic-pituitary axis.7 The corticotropin-releasing hormone test (CRHT) offers an alternative approach: at a cutoff of 26.3 pg/mL for endogenous ACTH concentration (EAC), sensitivity is 90.62% and specificity is 87.50%; at a cutoff of 54.5 pg/mL for post-stimulation ACTH concentration (PAC), sensitivity is 100% and specificity is 66.67%.2 The 95% reference interval for EAC in healthy dogs is 5.00–79.8 pg/mL, and for PAC is 1.92–153.42 pg/mL.2 Abdominal ultrasonography is a standard component of the differentiation workup. Bilateral adrenal enlargement supports PDH, while a unilateral adrenal mass with contralateral atrophy supports ADH.7 The sensitivity of ultrasonography to detect adrenomegaly in dogs with HAC is 95.6% when weight-adjusted reference intervals are applied.9 Weight-adjusted upper limits for left adrenal gland thickness in healthy dogs are: 5.1 mm (2.5–5 kg), 5.5 mm (>5–1