Addison's disease due to bilateral adrenal malignancy in a dog.

A 12-year-old Rottweiler cross Labrador was presented with anorexia and weakness. Adrenal insufficiency was diagnosed with hyponatraemia, hyperkalaemia and undetectable resting and post-ACTH cortisol and aldosterone concentrations. The only abnormal diagnostic imaging result was bilateral adrenomegaly. Cytologic findings of liver, spleen and peripheral lymph nodes were normal. The dog responded initially to standard replacement therapy but relapsed shortly afterwards. The owners opted for euthanasia and allowed only removal of both adrenal glands. Microscopically, infiltrative polymorphic proliferations of densely packed tumour cells arranged as nests, intermingled with multifocal areas of necrosis and inflammatory cells were found. Silver staining revealed a few non-neoplastic adrenomedullary cells, whereas neoplastic cells did not stain. Immunohistochemistry was negative for neuron-specific enolase, vimentin, cytokeratin, synaptophysin, chromogranin A, S-100 protein, CD 56, 79 and 3. The final diagnosis was highly anaplastic bilateral adrenal neoplasia. This is the first report of bilateral adrenal malignancy presenting as clinical hypoadrenocorticism in a dog.