Background: Thyroiditis and Graves’ disease have been reported after COVID-19 vaccination. We evaluated the risks of adverse events after COVID-19 vaccination among patients treated for hypothyroidism. Methods: In this retrospective population-based cohort study of Hong Kong Hospital Authority electronic health records with Department of Health vaccination records linkage, levothyroxine users were categorised into unvaccinated, vaccinated with BNT162b2 (mRNA vaccine) or CoronaVac (inactivated vaccine) between 23 February and 9 September 2021. Study outcomes were dosage reduction or escalation in levothyroxine, emergency department visit, unscheduled hospitalisation, adverse events of special interest (AESI) according to World Health Organization’s Global Advisory Committee on Vaccine Safety, and all-cause mortality. Inverse probability of treatment weighting for propensity score was applied to balance baseline patient characteristics among the three groups. Hazard ratios were estimated using Cox regression models. Patients were observed from the index date until the occurrence of study outcome, death, or censored on 30 September 2021, whichever came first. Results: In total, 47,086 levothyroxine users were identified (BNT162b2: n=12,310; CoronaVac: n=11,353; unvaccinated: n=23,423). COVID-19 vaccination was not associated with increased risks of levothyroxine dosage reduction (BNT162b2: HR=0.971, 95%CI 0.892–1.058; CoronaVac: HR=0.968, 95%CI 0.904–1.037) or escalation (BNT162b2: HR=0.779, 95%CI 0.519–1.169; CoronaVac: HR=0.715, 95%CI 0.481–1.062). Besides, COVID-19 vaccination was not associated with a higher risk of emergency department visits (BNT162b2: HR=0.944, 95%CI 0.700-1.273; CoronaVac: HR=0.851, 95%CI 0.647-1.120) or unscheduled hospitalisation (BNT162b2: HR=0.905, 95%CI 0.539-1.520; CoronaVac: HR=0.735, 95%CI 0.448-1.207). There were two (0.016%) deaths and six (0.062%) AESI recorded for BNT162b2 recipients, and one (0.009%) and three (0.035%) for CoronaVac recipients, respectively. Conclusions: BNT162b2 or CoronaVac vaccination is not associated with unstable thyroid status or an increased risk of adverse outcomes among patients treated for hypothyroidism in general. These reassuring data should encourage them to get vaccinated against COVID-19 for protection from potentially worse COVID-19-related outcomes.