A 6 year old male castrated mixed breed dog presented to the Internal Medicine Service at the Cornell University Hospital for Animals for evaluation of thrombocytopenia diagnosed the day previously at his primary veterinarian. Upon presentation, multiple areas of petechiation were noted along the oral mucosa and inner pinna. A full diagnostic workup (complete blood count, biochemical analysis, infectious disease testing, thoracic radiographs, abdominal ultrasound, bone marrow aspirate) subsequently diagnosed the patient with primary immune-mediated thrombocytopenia (IMT). The patient was initially treated with immunosuppressive doses of corticosteroids, azathioprine, a single dose of vincristine, and famotidine. Furthermore, the patient was also a newly diagnosed diabetic, and as such, treatment had to focus on glycemic control with the possible insulin antagonistic effects of glucocorticoid treatment. During treatment, the patient suffered severe side effects from glucocorticoid treatment, which altered his treatment plan. The following report describes the diagnostic workup of thrombocytopenia, treatment of primary IMT, and management of treatment complications.