Introduction: Hepatocellular carcinoma (HCC) is the fourth leading cause of cancer mortality worldwide. Recent animal studies suggest that thyroid hormone treatment improves HCC prognosis. The aim of this study was to describe the association between thyroid disease and HCC prognosis in humans. Methods: We performed a nationwide cohort study including all persons with an HCC diagnosis from 2000 to 2018. Patients' age, sex, HCC treatment, and diagnoses of thyrotoxicosis, nontoxic goiter, and myxedema were obtained from Danish national healthcare registries. We used regression models to examine the association between thyroid disease and mortality hazard and restricted mean survival time after HCC diagnosis, adjusting for confounding by sex and age. Results: We included 4,812 patients with HCC and 107 patients with thyroid disease. Median follow-up time was 5 months (total 5,985 person-years). The adjusted mortality hazard ratio was 0.68 (95% CI: 0.47-0.96) for thyrotoxicosis and 0.60 (95% CI: 0.41-0.88) for nontoxic goiter. The restricted mean survival time during the 5 years following HCC diagnosis was 6.8 months (95% CI: 1.1-12.6) longer for HCC patients with thyrotoxicosis than for patients without thyroid disease, and it was 6.9 months (95% CI: 0.9-12.9) longer for HCC patients with nontoxic goiter than for patients without thyroid disease. Conclusions: In this large nationwide cohort study, thyrotoxicosis and nontoxic goiter were associated with prolonged HCC survival.