INS and Polydipsia: A 66-year-old male with non-small cell lung cancer presented with fatigue, polyuria, and polydipsia following the third dose of pembrolizumab. Blood tests showed elevated anion gap, ketonemia, and ketonuria. The patient was treated with insulin IV therapy and discharged on insulin aspart and subcutaneous insulin glargine. The patient continued their pembrolizumab therapy.