Clinical and Radiobiological Observations on Latent Effects of X-irradiation on the Thyroid Gland

Abstract

The function of the human thyroid which had been given 3700–6500 rads of x-rays (fractionated over 3 to 4 weeks) during the treatment of early laryngeal cancer was studied 1½ to 12 yr later. No cases of hypothyroidism were found in 20 patients, and radioactive iodine tracer tests, serum protein-bound iodine values and serum cholesterol levels were normal. Experiments in 168 rats showed that x-ray doses up to 1600 rads (single delivery) had no effect on the iodide trapping function of the thyroid for periods up to 10 months after irradiation. In contrast, similar doses of x-rays permanently impaired the capacity of the rat thyroid cells to multiply in response to a goitrogenic challenge (thyroid cell reproductive integrity). The hypothesis is advanced that, while inhibition of thyroid cell turnover may be of little consequence in the normal thyroid because it has a low mitotic index, it is probably the mechanism underlying the pathogenesis of the high incidence of hypothyroidism which follows treatment of thyrotoxicosis (in which the gland has a high mitotic index) with radioactive iodine(¹³¹I).