Medullary thyroid carcinoma (MTC) is a tumor that arises from parafollicular cells of the thyroid gland. MTC can occur sporadically (75%) or as part of inherited cancer syndromes (25%). In most cases, hereditary MTC evolves from preneoplastic C cell hyperplasia (CCH), so early detection of this pathology would evidently be critical. A recent study reports that alterations in succinate dehydrogenase (SDH) D are responsible for familial non-RET CCH. First, we studied SDHD in two families with hereditary non-RET CCH and found no alterations related to the inheritance of this disease. Then, we investigated whether the H50R variant could be a risk factor in the sporadic development of MTC in both Spanish and English patients. We found no evidence that the presence of the H50R is strongly associated with the risk of sporadic MTC, although we did observe an association with age at diagnosis of MTC in Spanish H50R carriers that we did not find in English patients. Finally, we looked for evidence of CCH or any other thyroid disease in a panel of germ-line SDH (B or D) mutation carriers and found none. We conclude that SDHD variants do not constitute a risk factor for developing CCH or sporadic MTC.