Back ground: Cadmium is a widespread and serious material that leads to different harmful cellular 
changes up to death. Se is a well-known element for its antioxidant role in living systems of humans 
and animals. Selenium nanoparticles have the ability to act as an antioxidant with reduced risk of 
ordinary selenium toxicity. Aim of work: is to investigate the effect of Selenium nanoparticles on 
cadmium induced thyrotoxicity in adult male albino rats. Material and methods: A total of 38 adult 
male albino rats were divided into four groups. The control group (8 rats) was given only distilled 
water for 35 days; the cadmium-treated group was given CdCl2 in their drinking water for 35 days; 
the nanoselenium-treated group was given Na2SeO3 in their drinking water for 35 days and the 
nanoselenium-cadmium-treated group was given Na2SeO3 and CdCl2 in their drinking water for 35 
days. The animals of all groups were sacrificed at the same time and their thyroid glands were rapidly 
dissected out. The specimens of each group were processed for light and immunohistochemical 
studies. They were stained with hematoxylin and eosin, masson's trichrome, and inducible nitric oxide 
synthase. Quantitative morphometric, and statistical studies were performed. Results: Picture of 
cytotoxicity was visible in cadmium-treated thyroid sections in the form of disorganized follicles of 
varying diameters, with a predominance of smaller follicles. The follicular epithelial cells appeared to 
be enlarged and had a vacuolated cytoplasm. In addition to reduced or absent colloid. The thyroid 
gland of nanoselenium–cadmium-treated animals showed a nearly normal follicular structure. 
Conclusion: In experimental animal the administration of nanoselenium in response to an increased 
risk of exposure to cadmium compounds has a protective effect against their harmful effects.