The late effect of subtotal thyroidectomy and radioactive iodine therapy on calcitonin secretion and bone mineral density in women treated for Graves' disease.

This study was designed to evaluate the effect of subtotal thyroidectomy and/or radioactive iodine therapy on plasma immunocalcitonin (iCT) levels and bone densities in patients treated for Graves' disease. Forty-eight women whose ages ranged from 29 to 79 years (mean, 55 years) were evaluated. All were at least 10 years beyond treatment. Fourteen patients had undergone subtotal thyroidectomy, 22 had received radioactive iodine therapy, and 12 had received both. Serum calcitonin levels were measured with the patient fasting and at 30 minutes and 2 hours after the ingestion of 15 mg of calcium in orange juice. Single photon absorptiometry was used to measure bone mineral density of the middle and distal radius. The mean fasting plasma levels of iCT for patients undergoing subtotal thyroidectomy was 27 +/- 2 mumol/L; women treated with radioactive iodine, 26 +/- 2; women undergoing subtotal thyroidectomy followed by radioactive iodine, 24 +/- 2, and for normal control women, 48.5 +/- 4.7. The mean stimulated iCT level of each of the patient groups was significantly lower than that of the normal controls (p = 0.01). There were no significant differences among the groups. Although there was an increased loss of bone mineral density in postmenopausal patients, with age and race as covariates, the bone densities of the distal radius in women undergoing subtotal thyroidectomy and/or receiving radioactive iodine were not significantly lower than those of normal control subjects (p greater than 0.05). These findings are consistent with other observations that patients treated by thyroidectomy and/or radioactive iodine for Graves' disease have lower basal levels of calcitonin and decreased calcitonin response to a provocative stimulus. Whether this loss of calcitonin reserve is a significant factor in development of postmenopausal osteoporosis remains unanswered.