Subclinical hypothyroidism after radioiodine exposure: Ukrainian-American cohort study of thyroid cancer and other thyroid diseases after the Chornobyl accident (1998-2000).

BACKGROUND: Hypothyroidism is the most common thyroid abnormality in patients treated with high doses of iodine-131 (131I). Data on risk of hypothyroidism from low to moderate 131I thyroid doses are limited and inconsistent. OBJECTIVE: This study was conducted to quantify the risk of hypothyroidism prevalence in relation to 131I doses received because of the Chornobyl accident. METHODS: This is a cross-sectional (1998-2000) screening study of thyroid diseases in a cohort of 11,853 individuals < 18 years of age at the time of the accident, with individual thyroid radioactivity measurements taken within 2 months of the accident. We measured thyroid-stimulating hormone (TSH), free thyroxine, and antibodies to thyroid peroxidase (ATPO) in serum. RESULTS: Mean age at examination of the analysis cohort was 21.6 years (range, 12.2-32.5 years), with 49% females. Mean 131I thyroid dose was 0.79 Gy (range, 0-40.7 Gy). There were 719 cases with hypothyroidism (TSH > 4 mIU/L), including 14 with overt hypothyroidism. We found a significant, small association between (131)I thyroid doses and prevalent hypothyroidism, with the excess odds ratio (EOR) per gray of 0.10 (95% confidence interval, 0.03-0.21). EOR per gray was higher in individuals with ATPO < or = 60 U/mL compared with individuals with ATPO > 60 U/mL (p < 0.001). CONCLUSIONS: This is the first study to find a significant relationship between prevalence of hypothyroidism and individual (131)I thyroid doses due to environmental exposure. The radiation increase in hypothyroidism was small (10% per Gy) and limited largely to subclinical hypothyroidism. Prospective data are needed to evaluate the dynamics of radiation-related hypothyroidism and clarify the role of antithyroid antibodies.

The Ukrainian-American study of leukemia and related disorders among Chornobyl cleanup workers from Ukraine: III. Radiation risks.

Leukemia is one of the cancers most susceptible to induction by ionizing radiation, but the effects of lower doses delivered over time have not been quantified adequately. After the Chornobyl (Chernobyl) accident in Ukraine in April 1986, several hundred thousand workers who were involved in cleaning up the site and its surroundings received fractionated exposure, primarily from external gamma radiation. To increase our understanding of the role of protracted low-dose radiation exposure in the etiology of leukemia, we conducted a nested case-control study of leukemia in a cohort of cleanup workers identified from the Chornobyl State Registry of Ukraine. The analysis is based on 71 cases of histologically confirmed leukemia diagnosed in 1986-2000 and 501 age- and residence-matched controls selected from the same cohort. Study subjects or their proxies were interviewed about their cleanup activities and other relevant factors. Individual bone marrow radiation doses were estimated by the RADRUE dose reconstruction method (mean dose = 76.4 mGy, SD = 213.4). We used conditional logistic regression to estimate leukemia risks. The excess relative risk (ERR) of total leukemia was 3.44 per Gy [95% confidence interval (CI) 0.47-9.78, P < 0.01]. The dose response was linear and did not differ significantly by calendar period of first work in the 30-km Chornobyl zone, duration or type of work. We found a similar dose-response relationship for chronic and non-chronic lymphocytic leukemia [ERR = 4.09 per Gy (95% CI < 0-14.41) and 2.73 per Gy (95% CI < 0-13.50), respectively]. To further clarify these issues, we are extending the case-control study to ascertain cases for another 6 years (2001-2006).

A cohort study of thyroid cancer and other thyroid diseases after the Chornobyl accident: dose-response analysis of thyroid follicular adenomas detected during first screening in Ukraine (1998-2000).

The Chornobyl (Chernobyl) accident in 1986 exposed many individuals to radioactive iodines, chiefly (131)I, the effects of which on benign thyroid diseases are largely unknown. To investigate the risk of follicular adenoma in relation to radiation dose after Chornobyl, the authors analyzed the baseline data from a prospective screening cohort study of those exposed as children or adolescents. A stratified random sample was selected from all individuals who were younger than 18 years, had thyroid radioactivity measurements taken within 2 months after the accident, and resided in the three heavily contaminated areas in Ukraine. This analysis is based on the 23 cases diagnosed in 12,504 subjects for whom personal history of thyroid diseases was known. The dose-response relation was linear with an excess relative risk of 2.07 per gray (95% confidence interval: 0.28, 10.31). The risk was significantly higher in women compared with men, with no clear modifying effects of age at exposure. In conclusion, persons exposed to radioactive iodines as children and adolescents have an increased risk of follicular adenoma, though it is smaller than the risk of thyroid cancer in the same cohort. Compared with results from other studies, this estimate is somewhat smaller, but confidence intervals overlap, suggesting compatibility.

Factors associated with elevated serum concentrations of anti-TPO antibodies in subjects with and without diffuse goitre. Results from the Ukrainian-American Cohort Study of thyroid cancer and other thyroid diseases following the Chornobyl accident.

OBJECTIVES: To examine factors associated with the prevalence of elevated anti-thyroid peroxidase antibodies (ATPO) among iodine-deficient adolescents and young adults and test whether associations vary according to the presence of diffuse goitre. DESIGN: Subjects were members of the Ukrainian-American Cohort Study exposed to the Chornobyl accident whose (131)I thyroid dose estimates were below 0.2 Gy. MEASUREMENTS: The odds ratios (ORs) for ATPO above 60 U/ml were estimated using logistic regression models for a number of factors in the total population (N = 5133), and separately for thyroid disease-free subjects (N = 3875), those with diffuse goitre (N = 921), and diffuse goitre without autoimmune thyroiditis (AIT; N = 883). RESULTS: Elevated ATPO was found in 9.9% of the total population and ORs were significantly higher in females, older individuals, those examined in earlier calendar years, residents of Kyiv and Chernihiv oblasts, subjects with a family history of thyroid disease, higher thyroid ultrasound volume, suppressed or elevated TSH, blood collection in March to May, very low thyroglobulin (Tg), and shorter serum storage time. When thyroid disease-free subjects and those with diffuse goitre were compared, there were few differences in antibody prevalence, and after excluding individuals with AIT, the only difference was an increased prevalence of elevated ATPO at low urinary iodine in those with goitre alone. CONCLUSIONS: Although a number of factors are associated with the prevalence of elevated ATPO in our study group, with the exception of urinary iodine these factors are independent of goitre, and differences between thyroid disease-free subjects and those with diffuse goitre are largely due to AIT.

A cohort study of thyroid cancer and other thyroid diseases after the chornobyl accident: thyroid cancer in Ukraine detected during first screening.

BACKGROUND: The Chornobyl accident in 1986 exposed thousands of people to radioactive iodine isotopes, particularly (131)I; this exposure was followed by a large increase in thyroid cancer among those exposed as children and adolescents, particularly in Belarus, the Russian Federation, and Ukraine. Here we report the results of the first cohort study of thyroid cancer among those exposed as children and adolescents following the Chornobyl accident. METHODS: A cohort of 32 385 individuals younger than 18 years of age and resident in the most heavily contaminated areas in Ukraine at the time of the accident was invited to be screened for any thyroid pathology by ultrasound and palpation between 1998 and 2000; 13 127 individuals (44%) were actually screened. Individual estimates of radiation dose to the thyroid were available for all screenees based on radioactivity measurements made shortly after the accident and on interview data. The excess relative risk per gray (Gy) was estimated using individual doses and a linear excess relative risk model. RESULTS: Forty-five pathologically confirmed cases of thyroid cancer were found during the 1998-2000 screening. Thyroid cancer showed a strong, monotonic, and approximately linear relationship with individual thyroid dose estimate (P<.001), yielding an estimated excess relative risk of 5.25 per Gy (95% confidence interval [CI] = 1.70 to 27.5). Greater age at exposure was associated with decreased risk of radiation-related thyroid cancer, although this interaction effect was not statistically significant. CONCLUSION: Exposure to radioactive iodine was strongly associated with increased risk of thyroid cancer among those exposed as children and adolescents. In the absence of Chornobyl radiation, 11.2 thyroid cancer cases would have been expected compared with the 45 observed, i.e., a reduction of 75% (95% CI = 50% to 93%). The study also provides quantitative risk estimates minimally confounded by any screening effects. Caution should be exercised in generalizing these results to any future similar accidents because of the potential differences in the nature of the radioactive iodines involved, the duration and temporal patterns of exposures, and the susceptibility of the exposed population.

A cohort study of thyroid cancer and other thyroid diseases after the Chornobyl accident: objectives, design and methods.

The thyroid gland in children is one of the organs that is most sensitive to external exposure to X and gamma rays. However, data on the risk of thyroid cancer in children after exposure to radioactive iodines are sparse. The Chornobyl accident in Ukraine in 1986 led to the exposure of large populations to radioactive iodines, particularly (131)I. This paper describes an ongoing cohort study being conducted in Belarus and Ukraine that includes 25,161 subjects under the age of 18 years in 1986 who are being screened for thyroid diseases every 2 years. Individual thyroid doses are being estimated for all study subjects based on measurement of the radioactivity of the thyroid gland made in 1986 together with a radioecological model and interview data. Approximately 100 histologically confirmed thyroid cancers were detected as a consequence of the first round of screening. The data will enable fitting appropriate dose-response models, which are important in both radiation epidemiology and public health for prediction of risks from exposure to radioactive iodines from medical sources and any future nuclear accidents. Plans are to continue to follow-up the cohort for at least three screening cycles, which will lead to more precise estimates of risk.