What features help distinguish thyroid myopathy from immune checkpoint inhibitor-associated myopathy?

Immune checkpoint inhibitors (ICIs) can cause myositis (ICI-myositis). Since ICIs can also induce hypothyroidism, myopathy secondary from hypothyroidism can also be associated with ICI therapy. Different from thyroid myopathy, patients with ICI-myositis barely have myoedema or muscle pseudohypertrophy. Importantly, ICI-myositis is frequently (> 50%) overlapped with myasthenia gravis and/or myocarditis (Aldrich et al., PMID 33258544). Taken together, if cancer patients have muscle pain and/or weakness after ICI therapy, detailed history and physical exam are critical. ESR, CRP, CK, aldolase, cardiac enzymes, TSH, and free T4 should be checked as an initial work up. If needed, cardiology and/or neurology should be involved at the early stage ...