To further investigate the relationship between calcitonin deficiency and osteoporosis, we have measured bone mineral content (BMC) by single photon absorptiometry in patients made iatrogenically calcitonin deficient by prior total thyroidectomy for thyroid cancer. Compared to sex-, age-, height-, and weight-matched normal controls, male patients had a significantly lower mean BMC at the midradius (1.162 ± 0.02 vs. 1.301 ± 0.05 g/cm; P < 0.02) and the distal radius (1.180 ± 0.04 vs. 1.338 ± 0.04 g/cm; P < 0.01). Female patients also had a significantly lower BMC at the midradius compared to those of a similarly matched group of normal controls and a group of patients on L-T₄ suppression for nodular goiters (0.791 ± 0.04 vs. 0.896 ± 0.05 vs. 0.891 ± 0.03 g/cm; P < 0.025). We conclude that calcitonin deficiency from surgical thyroidectomy is associated with significant decreases in bone mineral content in both sexes. This lends further support to the concept that calcitonin deficiency may be an important causative factor in the development of osteoporosis. (J Clin Endocrinol Metab56 936,1983)