Background:  Harmful effects of ionizing radiation exposure on the thyroid gland are well established. However, there is lack of information on long-term thyroid exposure effects in the population affected by the nuclear weapons testing at the Semipalatinsk Nuclear Test Site.

Objectives: This study aimed to assess the prevalence of thyroid nodules and thyroid cancer in a cohort of 3,183 exposed individuals who underwent initial ultrasound thyroid examination in 1998, and out of whom 1,057 individuals were still alive in 2023 and were repeatedly examined to detect changes in the thyroid gland to investigate effects of non-radiation and radiation factors on the risk thyroid nodules and cancer development.
 

Methods: Thyroid nodules (benign, cyst and suspicions for malignancy) prevalence was assessed using thyroid ultrasound examination. Thyroid cancer cases were identified through retrospective cohort analysis and linkage of cohort data with medical records from various sources. Individual thyroid radiation doses, both from external and internal irradiation, were reconstructed based on fallout deposition data, residential history, and dietary patterns, particularly childhood milk consumption. Information on thyroid risk factors were collected using study questionnaire in personal interviews. Unconditional logistic regression models were used to evaluate associations between thyroid diseases and potential risk factors calculating Odds Ratio (OR) with respective 95% confidence intervals (95% CI) and p-values. Sensitivity analyses were performed to assess possible impact of selected parameters on the study results.
 

Results: Among cohort members alive as of 01/01/2023, the mean individual thyroid dose estimates were 318.4 mGy (range 0–7,550 mGy) for total (external + internal) exposure. The prevalence of ultrasound-detected thyroid nodules was 66.3% (n=701). 16 cancer cases were identified in the cohort. Individuals exposed to higher thyroid radiation doses (>50.0 mGy) had a significantly increased risk of thyroid nodules (OR: 1.30; 95% CI: 1.01–1.68). Female gender, advanced attained age (≥80 years), use of thyroid hormone medications, and history of thyroid disease were also significantly associated with increased thyroid nodule risk, while engaging in mental work was significantly associated with a reduced risk. After adjusting for non-radiation factors, the association between thyroid radiation dose and thyroid nodules remained of similar magnitude but lack the statistical significance. Nodules suspicious for malignancy showed a non-significant increased association with radiation dose (OR: 1.30; 95% CI: 0.87–1.93; p = 0.197). Benign nodules also showed an increased OR, while cystic nodules demonstrated no association with thyroid dose. The presence of thyroid nodules was strongly associated with an increased risk of thyroid cancer (OR: 10.6; 95% CI: 2.57–64.3; p = 0.001). Little evidence was found for an association between thyroid cancer and radiation dose.
 

Conclusions: This study provides strong evidence for associations between several non-radiation and radiation risk factors and the presence of thyroid nodules. Thyroid cancer risk was significantly elevated in persons with earlier detected thyroid nodules. No significant association was found between thyroid cancer and radiation exposure dose, possibly due to thyroid cancer incomplete ascertainment done retrospectively. Further research on thyroid nodules and thyroid cancer incidence in this population is warranted.