Distinguishing immune checkpoint inhibitor-induced hypothyroid myopathy from myositis: a case report and review

Immune checkpoint inhibitors (ICIs) have revolutionized cancer treatment; however, they can induce immune-related adverse events (irAEs), including hypothyroidism, the most common endocrine irAE. ICI-induced hypothyroid myopathy (ir-HM) is a rare condition that can be mistaken for ICI-induced myositis (ir-myositis). We encountered a case of ir-HM that was similar to ir-myositis. A 72-year-old male with lung cancer developed hypothyroidism and myopathy 21 weeks after initiating pembrolizumab treatment. Muscle biopsy revealed myofibrillar network disruption without inflammatory changes. A literature review identified 9 ir-HM cases, typically occurring more than 2 months after the initiation of ICI therapy. Key distinctions between ir-HM and ir-myositis include onset time, treatment approach, and the presence of the myoedema. Proper diagnosis of ir-HM is crucial for appropriate management, potentially allowing continuation of ICI therapy. This study highlights the importance of considering ir-HM in patients with myopathy symptoms occurring a longer period after the initiation of ICI treatment.