An 11-year-old, spayed female Labrador Retriever dog was referred for evaluation of chronic, progressive inappetence and decreased hindlimb function. Preliminary diagnostics were consistent with primary hyperparathyroidism (PHPT). The diagnosis was confirmed ultrasonographically prior to surgical parathyroidectomy, and a histopathologic diagnosis of parathyroid carcinoma was made. The dog was monitored for complications and its clinical signs resolved or improved markedly after treatment. This paper discusses the signs, diagnostic approach, and histopathology of the case.