A 47-year-old female patient with a previous history of right thyroid lobectomy was admitted to the hospital because of a 3 cm nodule in the thyroid gland. Hormonal evaluation showed subclinical hypothyroidism with serum levels of thyroid stimulating hormone slightly elevated to 4.4 mug/dl (normal: 0.4-4 mug/dl). Thyroid ultrasound showed diffuse irregularity of the gland and the presence of a solitary nodule (30x18 mm in diameter) localized in the left lobe. A fine needle aspiration biopsy was performed. Cytological analysis revealed papillary thyroid carcinoma and Hashimoto's thyroiditis. Total thyroidectomy was performed. During the operation, two of the parathyroid glands were detected to be hyperplastic. Histopathological examination of the thyroid and parathyroid glands revealed Hashimoto's thyroiditis with papillary thyroid carcinoma and synchronous carcinoma of the parathyroid gland. To our knowledge, this association of occult parathyroid carcinoma in a patient with papillary thyroid carcinoma and Hashimoto's thyroiditis has not been reported in the literature. Given the high prevalence of autoimmune diseases in elderly women, a random occurrence of this triad represents the most likely explanation. (C) 2001, Editrice Kurtis.