The electron microscopic results confirmed the results obtained by the light microscope. Some follicles were lined with low prismatic, cuboidal or flattened epithelium with apparent decrease in epithelial height. That was attributed to hypoactivity of the thyroid gland. This was confirmed by biochemical changes in this work in the form of decrease in T3 and T4 level and increase in the TSH level [36]. Others showed stratification of their lining epithelium. The nuclei of thyrocytes appeared shrunked hyperchromatic nuclei, fragmented and dilated rER, swollen mitochondria with degenerated cristae, accumulation of colloid vesicles inside the thyrocytes, even complete loss of all the cytoplasmic organelles. Similar results observed previously in the form of disrupted basal laminae of the follicles, regressed nuclei and disrupted cell organelles [24]. Also, follicular hyperplasia may be connected with a cascade of cellular events involving oxidative stress, genomic DNA damage, and modulation of apoptotic regulatory gene (P53) after exposure to chromium [37]