The Thyroid gland with two symmetrical lobes has an important role in metabolism of the body and regulating of calcium. Any factor making structural and hormonal changes in this gland can produce metabolic disorders. To investigate the functional changes of the thyroid gland following coadministration of soy extract and Vitamin D3, 42 mature female mice in 7 groups were studied for 35 days. Two doses of soy extract (5 and 10 g/kgBW/day); two doses of Vitamin D3 (100 and 200 ?g/kgBW/day); and, a combination of both soy extract and Vitamin D3 with two doses were fed to each mouse by gavage. At the end of the feeding trial, following anesthetizing by diethyl ether, mice were bled. Serum levels of calcium were determined by method Colorimetry, and serum concentrations of T3, T4 ,TSH were determined by method Radio Immuno Assay. Data was statistically analyzed by the one way ANOVAtest and significant differences were observed between groups (p<0.001). Results showed the occurrence of a dose-dependent hypothyroidism in mice receiving only soy extract. In mice receiving only vitamin D3, significant and dosedependent increases of calcium levels, significant and dose-dependent decreases of TSH levels and, insignificant decreases in serum concentrations of T3 and T4 were observed. Finally, groups receiving a combination of high doses of soy extract and Vitamin D3, showed hypothyroidism. In conclusion, this study suggests that co-administration of soy extract and Vitamin D3, only in low doses, can balance the effects of individual use of these components on thyroid function and calcium homeostasis.