Thyroxine and calcitonin secreted by the thyroid gland are reduced in 26 cases patients with thyroid cancer after total or subtotal thyroidectomy. Since calcitonin participates in bone mineral metabolisim, bone mineral density (BMD) was examined in these patients treated in the past 10 years. It was found that serum calcitonin level was decreased, and the degree of reduction positively correlated with BMD (r = 0.78, P < 0.005). Osteoporosis developed about 10 years of age earlier. Most patients did not constantly receive thyroid tablets after excision of the thyroid. The resulting hypothyroidism led to ovarian dysfunction in female patients as shown by lowered level of estradiol. It is therefore indicated to administer thyroid tablet and calcitonin to prevent osteoporosis.