Although immune checkpoint inhibitor (ICI) therapy has improved the prognosis of unresectable hepatocellular carcinoma (HCC), it has also resulted in unique immune-related adverse events (irAEs). The relationship between irAE and treatment outcomes in ICI-treated unresectable HCC patients remains unknown.To elucidate the correlation between immune-related toxic effects and prognosis in patients with unresectable HCC treated with pembrolizumab.From March 2019 to February 2021, a total of 190 unresectable HCC (Barcelona Clinic Liver Cancer C) patients receiving pembrolizumab treatment were retrospectively reviewed. Overall survival (OS) was the primary endpoint, while objective response rate (ORR), disease control rate (DCR), and time to progression (TTP) were secondary evaluation indexes. We assessed demographics, irAEs, and outcomes by retrospective review.One hundred and forty-three males and 47 females were included in the study. The ORR and DCR were 12.1% (23/190) and 52.1% (99/190), respectively. The median OS was 376 d [95% confidence interval (CI): 340-411 d] and the median TTP was 98 d (95%CI: 75-124 d). The overall incidence of treatment-related adverse events was 72.6% (138/190) and 10.0% of them were severe irAEs (grade ≥ 3). Child-Pugh B class, portal vein tumor thrombus, extrahepatic metastasis, and hypothyroidism were the independent risk factors for survival. Patients with hypothyroidism showed a longer OS [517 d (95%CI: 423-562) vs 431 d (95%CI: 412-485), P = 0.011] and TTP [125 d (95%CI: 89-154) vs 87 d (95%CI: 61-98), P = 0.004] than those without irAEs.Pembrolizumab-treated patients with unresectable HCC who experienced hypothyroidism have promising ORR and durable response. Hypothyroidism, an irAE, may be used as a clinical evaluation parameter of response to ICIs in unresectable HCC.©The Author(s) 2023. Published by Baishideng Publishing Group Inc. All rights reserved.