An 11-year-old female spayed Toy Poodle was presented to the Cornell University Hospital for Animals’ Emergency Service following referral for thrombocytopenia. The patient had a three-day history of bruising, followed by an acute episode of labored breathing and anorexia. The patient was diagnosed with diabetes mellitus four months prior to presentation, which was historically well-controlled with insulin. On evaluation, the patient was tachycardic and tachypneic with increased respiratory effort and numerous ecchymoses along the dorsum and caudal aspects of the pelvic limbs. Bloodwork revealed severe thrombocytopenia, mild anemia, and an inflammatory leukogram. Thoracic radiographs showed evidence of an alveolar pattern, raising concern for pulmonary hemorrhage. The patient underwent further diagnostic testing and was diagnosed with primary immune-mediated thrombocytopenia (ITP). Initial therapy included dexamethasone, mycophenolate, an insulin constant rate infusion, and fluids supplemented with dextrose. The patient failed to respond to treatment and was euthanized after ten days of hospitalization. This case report will discuss diagnosis, clinicopathologic features, and treatment of primary ITP. The report will also address complications of treating immune-mediated disease while attempting to maintain glycemic control in a diabetic patient.