Radioactive iodine (RAI) therapy is a radionuclide treatment for hyperthyroidism and well-differentiated thyroid cancer. One of the most popular sources of information for patients on the internet is YouTube. This study aimed to examine the quality of videos about RAI treatment. This cross-sectional study was performed by using videos in YouTube. The terms "radyoaktif iyot tedavisi" and "radioactive iodine treatment" were used to search related videos in June 2021. The quality of the videos was assessed by using the Journal of the American Medical Association (JAMA) benchmark criteria, the DISCERN scale, and the global quality scale (GQS). Of the total 88 videos evaluated, 56 videos (30 in English, 26 in Turkish) were analyzed according to the inclusion and exclusion criteria. Seven (12.5%) videos were assigned to the high-quality group, 16 (28.58%) to the intermediate quality group, and 33 (58.92%) to the low quality group. The findings of this study showed that the most popular videos with the highest video power index (VPI) scores and the highest number of video likes and comments belonged to the intermediate quality group. Contrarily, popularity level, number of video likes, and number of video views were the lowest in the high-quality group. The analysis of video sources revealed that viewers most preferred non-physician-sourced videos, with average total views of 59307.80 standard deviation (SD): 122554.13. The most liked videos were non-physician-made videos, with average total likes of 424.35 (SD: 639.41). The mean VPI scores were the highest in non-physician-made videos, with 25.18 (SD: 25.69). The average JAMA (1.92, SD: 0.50), DISCERN (34.31, SD: 14.33), and GQS scores (2.61, SD: 0.99) were the highest in physician-made videos. Although high-quality videos on YouTube may inform and encourage patients positively, unprofessional, incorrect, and incomplete information can be also uploaded on YouTube and may mislead patients.