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Relevance of, and Potentiality for, in Vivo Intrathyroidal Iodine Determination

Within the last decades, information has appeared about the relationship between excessive iodine intake and thyroid diseases, including thyrotoxicosis, thyroiditis, and thyroid cancer. The problem of excess iodine is not restricted only to those countries with a traditionally high intake of sea products. In the second half of the twentieth century, iodine became used intensively in agriculture in developed countries as well as in new technologies for food products and drinks. The spectrum of iodinecontaining medicines also increased greatly. As a result, in some developed countries, the average daily iodine intake is now several times higher than the physiological norm.1 Clinical and experimental studies were undertaken to estimate the importance of excessive iodine intake in the induction of thyroid tumors. Concentration and total content of iodine in the thyroid were determined by two techniques: instrumental neutron activation analysis (INAA)2 and energy-dispersive X-ray fluorescent analysis (EDXRF).3 Necropsy samples were studied from 70 men and 20 women, aged 2 to 87, who were residents of the region where the daily iodine intake was at the lower limit of the norm. Age-dynamics regularities of intrathyroidal iodine concentration were found, and the mean value for normal subjects averaged 345 ± 21 μg/g wet tissue (mean ± SEM). Age dynamics of intrathyroidal iodine concentration agreed well with age distribution of occult papillary carcinoma at necropsy and in resected thyroids.4 Iodine concentrations of the intact tissue in thyroids with benign (n = 90) and malignant (n = 45) nodules were considerably higher than the level of the age norm.5,6 It is known that the risk of thyroid cancer for subjects with oncologic (nonthyroidal) diseases is 2to 4-fold higher than that in healthy persons of the same age. It was shown that intrathyroidal iodine concentration of oncologic patients (n = 16) was about 2.5-fold higher than the norm. Intrathyroidal iodine concentration of patients with cardiovascular diseases (n = 18), renal insufficiency, liver cirrhosis, pneumonia, tuberculosis, and sepsis (n = 8) was within or lower than the age norm.7 However, the results of clinical studies did not reveal a cause-and-effect relationship. Thus, some tests with excessive iodine intake were carried out on rats. The 10fold increase in dietary iodine intake of the 7-month-old rats (n = 126) resulted in over 5-fold growth of the thyroid tumor incidence compared to the control (p < 0.05).8 The same diet given to 1-month-old rats (n = 240) after the thyroid was irradiated with 50 Gy radioiodine-131 caused almost a 1.5-fold rise in the radioiodine blastomogenic effect (p < 0.05).9 Among all the parameters studied, such as body and thyroid mass,