The COVID-19 pandemic has changed healthcare systems around the world. Medical personnel concentrated on infectious disease management and treatments for non-emergency diseases and scheduled surgeries were delayed. We aimed to investigate the change in the severity of thyroid cancer before and after the outbreak of COVID-19 in Korea. We collected three years of data (2019, 2020, and 2021) on patients who received thyroid surgery in a university hospital in South Korea and grouped them as “Before COVID-19”, “After COVID-19 1-year” and “After COVID-19 2-years”. The total number of annual outpatients declined significantly after the outbreak of COVID-19 in both new (1303, 939, and 1098 patients) and follow-up patients (5584, 4609, and 4739 patients). Clinical characteristics, including age, sex, BMI, preoperative cytology results, surgical extent, and final pathologic diagnosis, were not significantly changed after the outbreak of COVID-19. However, the number of days from the first visit to surgery was significantly increased (38.3 ± 32.2, 58.3 ± 105.2, 47.8 ± 124.7 days, p
= 0.027). Papillary thyroid carcinoma (PTC) patients showed increased proportions of extrathyroidal extension, lymphatic invasion, vascular invasion, and cervical lymph node metastasis. Increased tumor size was observed in patients with follicular tumor (3.5 ± 2.2, 4.0 ± 1.9, 4.3 ± 2.3 cm, p
= 0.019). After the COVID-19 outbreak, poor prognostic factors for thyroid cancer increased, and an increase in the size of follicular tumors was observed. Due to our study being confined to a single tertiary institution in Incheon city, Korea, nationwide studies that include primary clinics should be required to identify the actual impact of COVID-19 on thyroid disease treatment.