Effect of decicurie doses of radioactive iodine 131 on parathyroid function.

Although parathyroid deficiency has been reported after administration of relatively small doses of iodine 131, reports of such deficiency after the much larger doses given in the management of thyroid cancer are notable by their absence in the literature. We observed one such patient, then instituted a prospective study. We observed a 58 percent incidence of apparently diminished parathyroid reserve among 53 patients given high doses of I 131. Susceptibility does not appear to be related to age, sex, extent of thyroid operation, operative technique, the amount of thyroidal I 131 uptake, the use of external radiation to the neck, or to the presence of parathyroid glands or of Hashimoto's disease in the operative specimen; however, susceptibility may be related to the anatomic location of the parathyroid glands in relation to the thyroid gland. The administered dose of I 131 almost reached significant levels (p = 0.1) in the present study. The initial pretreatment serum calcium levels were consistently lower among affected patients (p = 0.01), suggesting the presence of a preexisting latent impairment of parathyroid reserve which was subsequently unmasked after the administration of I 131. Thyroid cancer is compatible with very long-term survival, and therefore, there is a considerable risk of delayed chronic hypocalcemia among patients who have received large doses of I 131, particularly female patients in the premenopausal age group. These patients deserve special management surveillance.