Immune checkpoint inhibitors (ICIs) have revolutionized cancer therapy and improved outcomes for patients with advanced disease. Pembrolizumab, a monoclonal antibody that acts as a programmed cell death 1 (PD-1(PDCD1)) inhibitor, has been approved for the treatment of advanced melanoma and other solid tumors. Immune-related adverse events (irAEs) including endocrinopathies have been well described with this and other PD-1 inhibitors. While hypothyroidism and hyperthyroidism, and less commonly hypophysitis, are the most common endocrinopathies occurring in patients treated with pembrolizumab, the incidence of type 1 diabetes mellitus (T1DM) was low in clin. trials. This paper report a case of pembrolizumab-induced primary hypothyroidism and T1DM presenting with severe diabetic ketoacidosis (DKA). A 52-yr-old male patient was treated with pembrolizumab for metastatic melanoma. This presented to the emergency department with a 1-day history of nausea and vomiting 2 wk after his seventh dose of pembrolizumab, having complained of polyuria and polydipsia for 2 mo before presentation. Patients had been diagnosed with thyroid peroxidase (TPO) antibody-neg. hypothyroidism, requiring thyroxine replacement, shortly after his fifth dose. Testing revealed a severe DKA (pH: 6.99, glucose: 38.6 mmol/L, capillary ketones: 4.9 and anion gap: 34.7). Patients was treated in the intensive care unit as per the institutional protocol, and subsequently transitioned to s.c. basal-bolus insulin. After his diabetes and thyroid stabilized, pembrolizumab was recommenced to treat his advanced melanoma given his excellent response. This case highlights the importance of blood glucose monitoring as an integral part of cancer treatment protocols composed of pembrolizumab and other ICIs.