Thyroid Clear Cell Adenoma with Marked Dilatation of Membranous Structures: Electron-Microscopic Study

The electron-microscopic features of a thyroid follicular adenoma with clear cell change is described. The tumor was 7 mm in size and located in the upper portion of the left lobe of the thyroid of a 29-year-old Japanese female. Its cut surface was pale brown in color and well demarcated. The lesion consisted of solid nests with pinpoint lumina. The cytoplasm of the tumor cells was completely clear, but slightly vesicular without oncocytic change. Electron-microscopic examination revealed that the cytoplasm of the tumor cells was filled with numerous empty vacuoles. The vacuoles consisted of dilated membranous structures, indicating that they had been formed from rough endoplasmic reticulum and Golgi apparatus. There was no accumulation of glycogen, mucin, or lipid. The dilated membranous structures in the cytoplasm may be related to the increased thyroglobulin synthesis by the tumor.