Objectives This study sought to ascertain the risk of thyroid cancer in relation to occupational exposure to ionizing radiation and extremely low-frequency magnetic fields (ELFMF) in a cohort representative of Sweden’s gainfully employed population.

Methods A historical cohort of 2 992 166 gainfully employed Swedish male and female workers was followed up from 1971 through 1989. Exposure to ELFMF and ionizing radiation was assessed using three job exposure matrices based on industrial branch or occupational codes. Relative risks (RR) for male and female workers, adjusted for age and geographic area, were computed using log-linear Poisson models.

Results Occupational ELFMF exposure showed no effect on the risk of thyroid cancer in the study. However, female workers exposed to high intensities of ionizing radiation registered a marked excess risk (RR 1.85, 95% confidence interval (95% CI) 1.02–3.35]. This trend was not in evidence among the men.

Conclusions While the study confirms the etiologic role of ionizing radiation, with a higher incidence of thyroid cancer being recorded for the most-exposed female workers, our results do not support the possibility of occupational exposure to ELFMF being a risk factor for the development of thyroid cancer.

Key terms cohort; electromagnetic fields; ionizing radiation; occupation; occupational exposure; risk; Sweden; thyroid cancer; thyroid neoplasm