AbstractIntroductionVitamin C (ascorbic acid) is a water‐soluble vitamin, that plays a key role in the prevention and treatment of scurvy. As vitamin C is an antioxidant and thyroid function may be affected and may affect vitamin C levels, for the first time, we aimed to provide a detailed review of all human studies evaluating the different roles of vitamin C in the thyroid gland. Thyroid cancers, goitre, Graves' disease and other causes of hyperthyroidism and hypothyroidism were the conditions discussed in this study. Furthermore, vitamin C addition to other medications such as levothyroxine was also reviewed.MethodsIn this study, we reviewed the relevant literature regarding the association between vitamin C and thyroid diseases using original studies from PubMed, Scopus, Embase, and Web of Science.ResultsIn this review, we found anti‐cancer effects for intravenous (IV) administration of vitamin C in addition to the beneficial effects of using it in combination with radiotherapy and chemotherapy. As autoimmune diseases affect some antioxidant markers, some studies reported a significant difference in blood vitamin C levels in patients with autoimmune thyroid diseases such as Graves' disease. Despite many studies evaluating the effects of IV administration of vitamin C in mentioned diseases, there is a lack of evidence for oral consumption of vitamin C.ConclusionsTo conclude, there is a lack of evidence, especially clinical trials, for the therapeutic effects of vitamin C on thyroid diseases; however, promising results were reported in some studies in the literature.