Thyroid Dysfunction Induced by Pembrolizumab: Five Case Reports.

Introduction: Although pembrolizumab, as a programmed cell death receptor-1 (PD-1) inhibitor, has revolutionized cancer therapy, it is frequently accompanied by immune-related adverse events. Thyroid dysfunction represents a common irAE, yet its clinical trajectory, optimal management approaches, and long-term outcomes remain incompletely characterized. Case Presentation: We conducted a retrospective analysis of 5 cancer patients who developed thyroid dysfunction following pembrolizumab therapy. All patients ultimately progressed to hypothyroidism, manifesting heterogeneous patterns: biphasic thyroid dysfunction, persistent hypothyroidism, and delayed presentation after treatment cessation. Patients with baseline positivity for thyroid antibodies (thyroglobulin/thyroid peroxidase antibodies) and elevated thyrotropin levels demonstrated higher susceptibility to severe hypothyroidism. Longitudinal follow-up revealed that thyroid function achieved stabilization following early initiation of hormone replacement therapy, whereas delayed intervention was associated with increased requirements for hormone dose titration. Conclusion: Thyroid dysfunction induced by pembrolizumab is characterized by diverse clinical manifestations and heterogeneous timing of onset. Combination regimens and baseline thyroid status may be significantly associated with its course. Re-treatment risk assessment and dynamic monitoring during therapy should be strengthened in clinical practice as early intervention may improve patient outcomes.