Thyroid-stimulating hormone levels in children from Chernobyl.

This study assesses parameters of thyroid function in persons who resided in Ukraine, Belarus, and southern Russia and exposed at 0 to 16 years of age to radioiodine contamination from the Chernobyl accident. Six to eight years after the accident a group of 300 young people who had immigrated to Israel were interviewed, underwent physical and ultrasound thyroid examination, and had their serum tested for thyroid-stimulating hormones (TSH), thyroid hormones, thyroglobulin, and antithyroid antibodies. Comparative groups came from areas with high (>1 Ci/km2) or low (< 1 Ci/km2) 137Cs ground contamination. Girls from high contamination areas, when compared to girls from areas with low ground contamination, showed significant upward shifts in levels of serum TSH (p = 0.023) although remaining within normal limits. Boys showed no significant differences. There was no evidence for differences in thyroid size or nodularity between the two groups of girls. A working hypothesis is proposed by which the shift in TSH levels in girls from high radiocontamination areas was associated with subclinical radiation damage from environmental radioiodine at the time of the accident.


Introduction
Children exposed to Chernobyl radioiodine in 1986 are a population of special concern because of high past consumption of radioiodine-containing dairy products, iodine deficiencies and endemic goiter in Belarus and Ukraine (1)(2)(3), the presumed higher sensitivity of children to ionizing radiation, and recent reports of elevated rates of childhood thyroid cancer, mainly in Belarus but also in Ukraine and the southern part of the Russian republic (4)(5)(6).
Many immigrants came to Israel from such areas in a recent wave of immigration from-the countries of the former Soviet Union (FSU). Of the approximately 700,000 people who came to Israel from the FSU between 1990 and 1996, our estimate is that at least 20% came from regions of Belarus, Russia, and Ukraine that were contaminated with various amounts of radionuclides (7) as a result of the Chernobyl accident. This preliminary study assesses thyroid function in immigrants who were children (0 to 16 years of age) at the time of the Chernobyl accident and reports on preliminary results of assessments of thyroidstimulating hormone (TSH) serum levels in these young people.

Methods
Three hundred immigrant children from Belarus, Russia, and Ukraine were examined through face-to-face interviews, physical examinations (induding the thyroid), blood sampling, and ultrasound examinations from 1992 to 1994. Total serum triiodothyronine, tetraiodothyronine, TSH, thyroglobulin, antithyroid, and antimicrosomal antibodies were measured. TSH levels were assessed using a chemoluminescent thirdgeneration assay (8). The 129 boys and 171 girls, who were 0 to 16 years of age at time of the reactor accident, ranged in age from 5 to 24 at the time of examination. The children and young adults were brought in by their parents or were referred themselves and appeared in the clinic on a voluntary basis.
A natural comparative experiment was suggested by identifying subjects coming from more contaminated and less contaminated areas as defined by the 1991 International Atomic Energy Agency (IAEA) ground radiocontamination maps (9). The group was therefore divided into those who originally resided in regions of higher and lower 137Cs exposures because the dispersal and deposition of 131I would be expected to have a similar distribution. High-and low-exposure areas are defined here as those areas with 137Cs concentrations greater or less than 37 kBq/m2 (1 Ci/km2), respectively, as expressed on the IAEA 137Cs ground contamination maps.
The 94 girls from low-contamination regions included 35 from Kiev, 18 from Minsk, 9 from Vinnitsa, 5 each from Bobruysk and Zitomir, 3

Results and Discussion
The assumption that the thyroid radiation dose due to radioactive iodine can be directly correlated with 137Cs deposition is useful as an indication of those regions that probably were contaminated with radioiodine, but it is recognized to be an oversimplification. Short-lived radioiodines (induding 1321, 1331, and 1351), which have been estimated to be more destructive per rad than 131I (10), may have caused radiation exposure related to the distance from Chernobyl, dependent on the degree of their physical decay by the time the radioactive cloud passed some distance from its Environmental Health Perspectives * Vol 105, Supplement 6 * December 1997 origin. Hence, there is reason to expect that children living nearer the site of the accident and those living downwind had relatively higher exposures to radioiodines, which may have led to increases in the incidence of childhood thyroid cancer reported in these regions. Development of thyroid hypofunction therefore may also be anticipated. Indeed, Larsen et al. (10) showed that a high prevalence of thyroid hypofunction was a delayed complication of exposure to early fallout from a hydrogen bomb explosion.
Our results (Figure 1) indicate that the distribution of TSH levels in girls was higher in children from high-exposure areas, although almost all values lay within the normal range (0.4 to 4.0 pIU/ml). The shift in TSH levels was statistically significant at p< 0.02 by the X2 test. Although boys showed a similar trend the increase among boys was not statistically significant. Combining boys and girls into a single group for statistical evaluation did not improve the results of the analysis. The apparently greater sensitivity of girls may reflect known gender differences for clinical problems of the thyroid (11). Possible explanations for the TSH effect include either radiation damage to the thyroid with a resulting homeostatic increase of TSH or greater iodine deficiency in the more exposed areas.
To test for the second possibility we examined the incidence of enlarged thyroids by direct palpation and ultrasound. There was no difference in the prevalence of enlarged thyroids between children coming from low and high radiocontamination areas. Therefore, iodine deficiency does not appear to be an explanation for the elevated TSH levels among children in areas more highly exposed to nudear fallout.
Our working hypothesis, therefore, is that some children who came from the higher contamination areas had subclinical radiation damage that led to compensatory homeostatic increases in TSH. Hence, elevated TSH in a population group could be a marker for prior exposure of the thyroid to environmental radioiodine. Other evidence for past radiation exposure (based on elevated clastogenic factors) has been reported in children who came from areas radiocontaminated by the Chernobyl accident (12).
It must be pointed out, however, that the effects observed do not provide any quantitative indication of radiation doses received by individuals within the group.
We are unaware of any previous studies of such an effect on TSH levels in children who lived in regions contaminated by the Chernobyl accident. Therefore we present these preliminary results anticipating that large-scale studies in FSU countries may be able to test the hypothesis with larger numbers of subjects from radiocontaminated areas.