Ectopic intrathyroidal thymus accompanied by intrathyroidal parathyroid as a cause of a solitary thyroid nodule in adult.

Ectopic intrathyroidal thymic tissue can form a mass lesion within the thyroid gland and has rarely been reported in children. We report a case of intrathyroidal thymus accompanied by an intrathyroidal parathyroid gland in a 29-year-old woman who visited our hospital due to right neck mass diagnosed as metastatic papillary carcinoma by aspiration cytology in an outside hospital. She had a history of right thyroidectomy due to papillary thyroid carcinoma three years ago. Ultrasonographic examination of the left thyroid gland showed a well-defined hyperechoic nodule, which did not show change in size compared with the previous study. She underwent complete thyroidectomy with right neck dissection. Grossly, the left thyroid had a well demarcated pale yellow nodule measuring 0.9 cm in diameter. On microscopic examination, the nodule was composed of mature fat tissue, ectopic thymic tissue with Hassall's corpuscles, and an intrathyroidal parathyroid gland. This is an exceedingly rare case of ectopic intrathyroidal thymus with intrathyroidal parathyroid gland, in an adult.