—Dr Netelenbos mentions three cases of primary HPT after 9, 11, and 20 years of 131 I treatment of hyperthyroidism, composed of one case from a previously published series1 and two from his experience, but he provides no evidence for a causal relationship. A history of hyperthyroidism, with or without 131 I treatment, occurs more commonly than expected by chance in patients with primary HPT.2 If this is a genuine association, then, conversely, primary HPT must occur more commonly than expected in hyperthyroidism,3 although no data are available. In four series with a high proportion of radiation-induced primary HPT,1,4,5 the combined incidence of hyperthyroidism was 1.7% (8/466) compared with the reported 1% concurrence of these two diseases,2 but this difference is probably not important. In our own experience, there have been no cases of primary HPT occurring in 131I-treated patients. The experimental data in rats
Rao SD, Frame B, Miller MJ, Kleerekoper M. Hyperparathyroidism After Radioactive Iodine Therapy-Reply. Arch Intern Med. 1981;141(11):1556. doi:10.1001/archinte.1981.00340120164041
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