A 12-year-old spayed female Shetland Sheepdog weighing 11.4 kg was evaluated at the University of Tennessee Veterinary Teaching Hospital for polyuria, polydipsia, and panting. Concurrent problems included atopy and hypothyroidism. Medications included biweekly allergen immunotherapy, a chlorpheniramine b (0.5 mg/kg PO q12h), and levothyroxine c (0.02 mg/kg PO q12h).Physical examination was normal except for indirect systolic blood pressure of 200 mmHg, measured with Doppler. The dog had thrombocytosis with 623,000 platelets/L (reference range, 200,000-500,000/L) and increased alkaline phosphatase (ALP) activity of 237 U/L (reference range, 12-122 U/L). Urine specific gravity was 1.014 with 2ϩ protein, and the urine protein to creatinine ratio was 2.4 (reference range, Ͻ0.5). Aerobic urine culture was negative for bacterial growth.The patient had an exaggerated response to an ACTH stimulation test, with baseline cortisol of 3.4 g/dL (reference range, 1.5-6 g/dL) and poststimulation cortisol of 22.3 g/dL (reference range, 8-16 g/dL). Cortisol also failed to suppress after administration of a low dose of dexamethasone (LDDS), with baseline cortisol of 2.5 g/ dL (reference range, 1.5-6 g/dL), 4-hour cortisol of 1.3 g/dL (reference range, Ͻ1 g/dL), and 8-hour cortisol of 1.5 g/dL (reference range, Ͻ1 g/dL), which was less than 50% suppression of baseline. Endogenous ACTH concentration was low at 2.5 pmol (reference range, 6.7-25 pmol/L). These results were considered consistent with adrenal-dependent hyperadrenocorticism. Sonographic evaluation of the abdomen identified mineralization and enlargement of the right adrenal gland (Ͼ20 mm) and an irregularly enlarged cranial pole of the left adrenal gland (12.5 mm). Differential diagnoses for bilateral adrenal tumors included some combination of functional adrenal adenoma, carcinoma, pheochromocytoma, metastatic neoplasia, or bilateral nodular adrenocortical hyperplasia secondary to pituitary-dependent hyperadrenocorticism (PDH). Despite finding bilateral adrenal tumors sonographically, analysis of the results of a high-dose dexamethasone suppression test (HDDS) revealed baseline cortisol of 2.2 g/dL (reference range, 1.5-6 g/dL), 4-hour cortisol of 1.1 g/dL (reference range, Ͻ1 g/dL), and 8-hour cortisol of 1 g/dL