Atezolizumab-Induced Thyroiditis and Immune Checkpoint Inhibitor (ICI)-Type 1 Diabetes Mellitus: Diagnostic and Therapeutic Challenges in ICI-Associated Endocrinopathies

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.