Nanoparticles have interactions with biological com ponents and thereby affect thyroid gland. The goal of this research was studying the effects of titanium dioxi de nanoparticles on thyroid performance. Forty matu re Syrian mice were divided in four groups. Control group, pl acebo group received only standard food and water w hereas experimental groups received 10 and 100 ppm of tita nium dioxide in water also. After 14 days, blood sa mples were taken and thyroid hormones were measured. Results showed significant reduction in T 4 and TSH amount of all groups which confirm that titanium dioxide is a str ong controller of thyroid hormones and induces hypo thyroidism.