A case of functioning plurifocal papillary carcinoma causing a clinical condition of hyperthyroidism is presented. Histologically, the carcinoma presented a dominant macrofocal nodule of an approximately 5 cm diameter, surrounded by chronic thyroiditis. Its occurrence in a patient under 20 years of age within a hot area in the thyroid bed and the lack of histological findings of hyperplastic tissue led the authors to consider this a rare case of hyperfunctioning differentiated thyroid carcinoma, and not a pure association of hyperthyroidism and carcinoma. Autonomously functioning thyroid nodules that are differentiated carcinomas are extremely rare with only 18 such cases reported in literature. While in the past it was commonly believed that a hot nodule is unlikely to be malignant, the possibility of an hyperfunctioning carcinoma should not be overlooked.