Abstract
The rapid evolution and increased application of immune checkpoint inhibitors (ICIs) in the oncology setting have introduced novel diagnostic and therapeutic challenges, particularly in the setting of endocrine immune-related adverse events (irAEs). Thyroid dysfunction is among the most common of these, while ICI-type 1 diabetes mellitus is uncommon and can present abruptly with life-threatening complications. A 76-year-old man with extensive-stage small-cell lung cancer being treated with atezolizumab developed new-onset diabetes, which manifested as diabetic ketoacidosis in connection with immunotherapy-induced thyroiditis. Metabolic instability in this patient was compounded by the inappropriate initiation of thyroid hormone replacement during the thyrotoxic phase of the thyroiditis as a result of misinterpretation of his thyroid function tests (TFTs). The unique clinical course of this patient illustrates the broad and unpredictable spectrum of ICI-related endocrinopathies and highlights the importance of prudent interpretation of TFTs, early recognition of ICI-induced type 1 diabetes mellitus, and close multidisciplinary management of irAEs.