<prompt> What is the physiological role of calcitonin in metabolism?Lowers Ca2 levelsIncreases Ca2 levelsIncreases PO4 levelsNone of the above  <response>: Calcitonin lowers circulating calcium and phosphate levels. It exes its calcium-lowering effect by inhibiting bone resorption. This action is direct, and calcitonin inhibits the activity of osteoclasts in vitro. It also increases Ca2 excretion in the urine. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. 2012. Chapter 21. Hormonal Control of Calcium Phosphate Metabolism the Physiology of Bone. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks Eds, Ganongs Review of Medical Physiology, 24e. 