Thyroid dysfunction occurs commonly following immune checkpoint inhibition. The etiology of thyroid immune-related adverse events (irAEs) remains unclear and clinical presentation can be variable. This study sought to define thyroid irAEs following immune checkpoint inhibitor (ICI) treatment and describe their clinical and biochemical associations.
We performed a retrospective cohort study of thyroid dysfunction in patients with melanoma undergoing CTLA-4 and/or PD-1 based ICI treatment from Nov 1, 2009 to Dec 31, 2019. Thyroid function was measured at baseline and at regular intervals following the start of ICI-treatment. Clinical and biochemical features were evaluated for associations with ICI-associated thyroid irAEs. The prevalence of thyroid autoantibodies and the effect of thyroid irAEs on survival were analysed.
A total of 1246 patients were included with a median follow-up of 11.3 months. 518 (42%) patients developed an ICI-associated thyroid irAE. Subclinical thyrotoxicosis (n=234) was the most common thyroid irAE, followed by overt thyrotoxicosis (n=154), subclinical hypothyroidism (n=61), and overt hypothyroidism (n=39). Onset of overt thyrotoxicosis occurred a median of 5 weeks (IQR 2-8) after receipt of first dose of ICI. Combination immunotherapy was strongly associated with development of overt thyrotoxicosis (OR 10.8, 95% CI 4.51-25.6 vs. CTLA-4 monotherapy; p<0.001), as was female sex (OR 2.02, 95% CI 1.37-2.95; p<0.001) and younger age (OR 0.83 per 10-years, 95% CI 0.72-0.95; p=0.007). By comparison, median onset of overt hypothyroidism was 14 weeks (IQR 8-25). The frequency of overt hypothyroidism did not differ between different ICI-types. The strongest associations for hypothyroidism were higher baseline TSH (OR 2.33 per mIU/L, 95% CI 1.61-3.33; p<0.001) and female sex (OR 3.31, 95% CI 1.67-6.56; p=0.01). Overt thyrotoxicosis was associated with longer progression free survival (HR 0.68, 95% CI 0.49-0.94; p=0.02) and overall survival (HR 0.57, 95% CI 0.39-0.84; p=0.005). There was no association between hypothyroidism and cancer outcomes.
Thyroid irAEs are common and there are multiple distinct phenotypes. Different thyroid irAE subtypes have unique clinical and biochemical associations, suggesting potentially distinct etiologies for thyrotoxicosis and hypothyroidism arising in this context.

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