 In a multicentre trial, 260 patients with newly diagnosed Graves disease will be randomized to cholecalciferol 70 mcgday or placebo in a parallel Group design. The intervention will continue during treatment with antithyroid drugs ATD, and for a period of 12 months after cessation of ATD. Blood samples will be collected at study entry, at 3 and 9 months, and at end of study. QoL questionnaires on nine occasions through out the study period. In a subcohort of 80 participants detailed examinations of bone density and geometry, muscle strength and postural balance, immune tests, and measurements of arterial stiffness will be performed at study entry, and at 3 and 9 months after randomisation. 