A case of Marine Lenhart syndrome with Hashimoto's thyroiditis that mimicked thyroid carcinoma.

Background: Marine Lenhart syndrome is a rare disease and causes refractory hyperthyroidism. So far, little evidence on the combination of both Marine Lenhart syndrome and Hashimoto's thyroiditis is available. We suspect that Marine Lenhart syndrome when combined with Hashimoto's thyroiditis might have its particular features, which are not exactly the same as those of the isolated Marine Lenhart syndrome.

Case Presentation: A 56-year-old middle-aged man presented with recurrent hyperthyroidism, and Graves' disease combined with Hashimoto's thyroiditis was considered. Radionuclide imaging showed a hot nodule, but ultrasonography suggested the possibility of malignancy with a category of 4B according to the Chinese-Thyroid Imaging-Reporting and Data System (C-TIRADS) model. Fine needle aspiration cytology (FNAC) revealed eosinophilic follicular lesions with papillary features, and prompted that papillary thyroid carcinoma could not be excluded. Partial thyroidectomy was performed and the nodule was proven to be benign by histopathology. The final diagnosis was atypical Marine Lenhart syndrome with Hashimoto's thyroiditis.

Conclusions: Marine Lenhart syndrome combined with Hashimoto's thyroiditis has its particular characteristics, showing some signs mimicking malignancy. Limitations of ultrasonography and FNAC in diagnosis should be noted in these scenarios.