Thyroid nodules are very common in the broad-spectrum population, with an estimated prevalence of 4%–7% [1-4]. However, the prevalence of these nodules can vary depending on the size and location, the thickness of the neck, and physician’s skill in detecting them [5-12]. It has been reported that the prevalence of nodules in normal Korean female based on thyroid ultrasonography was 25.3% to 42.2% [13]. However, the number of thyroid nodules increases with age in female [13]. The risk of a malignancy developing from a thyroid nodule is reportedly higher in children, males, adults less than 20 years of age or those greater than 60 years of age, and in patients with a history of head and neck irradiation [14-19].

A wide range of people in many occupations are exposed to radiation. In recent years, there has been a rapid increase in exposure to radiation in the area of medical service due to the rapid developments in diagnosis and therapeutic technologies. Nurses are exposed to radiation due to their tasks in a hospital, although the types of radiation to which they are exposed depend on their assigned departments. Exposure to radiation is dependent on various factors, but the overall exposure rates differ according to the department in which the nurses work and the length of their careers.

It remains controversial as to whether one’s occupation has an effect on the prevalence of thyroid nodules and thyroid cancer. Because radiation is generated from diagnostic equipment and therapeutic procedures in hospitals, there has long been a question whether working in a hospital can affect the prevalence of thyroid nodules and thyroid cancer. It is known that the radiation level detected in a hospital is higher than the radiation level to which the general population is exposed. However, few studies have examined the risk of working in a hospital in comparison to the risks faced by the general population.

In this study, we have investigated the existence of difference in the prevalence of thyroid nodules and thyroid cancer between nurses working at a single hospital and health check-up female, who had undergone a health examination at the same hospital during the same period.

The participants of the study were female nurses, who worked at a single hospital and female who were visiting the doctor for annual examination (who had undergone a health examination at the same hospital) during the period starting from January 2011 to December 2013. We checked both the groups for occupational career, any medical history, family history, height, and weight using questionnaire.

The exclusion criteria included presence of previous history of thyroid cancer or thyroid surgery, and history of head and neck irradiation. From the group of health check-up female, the nurses were excluded.

Finally 1,024 nurses (mean, 41.6±6.7 years; range, 24 to 56 years) and 2,631 health check-up female (mean, 48.3±10.2 years; range, 24 to 67 years) were enrolled for the present investigation.

Thyroid ultrasonography was performed using a real-time ultrasound scanner with a 10 to 12 MHz linear transducer. Any abnormality was detected in the thyroid USG. Thyroid nodule was defined as lumps which arise within an otherwise normal thyroid gland. Thyroid cyst was defined as a fluid-filled cavity.

Nodule size was based on the largest nodule in cases of more than one nodule. USG-guided fine-needle aspiration & cytology (FNAC) was performed in suspicious malignant nodule by one physician. The minimum size for aspirated nodules was 5 mm in diameter. In subjects with nondiagnostic and indeterminate cytology, repeat fine-needle aspiration (FNA) was performed after 3 months. After cytological diagnosis, operation was recommended for suspicious malignant nodule (Fig. 1).

The possibility of chronic exposure to low-dose radiation in the working environment in hospitals has often been described in literature [20-23]. As the number of instruments emitting considerable amounts of radiation is increasing due to the development of medical instruments for the purpose of diagnosis and treatment, radiation from these instruments is a concern for the nurses, who use and maintain them. Vests or cervical guards made of lead are often used to avoid exposure to radiation, but these items are mainly intended to block intentionally emitted radiation. Based on the limited protective measures from all forms of radiations, working in a hospital is occasionally accompanied with fear of exposure to radiations in nursing occupation [21]. In particular, thyroid cancer, which is on a current drastic increase is related to radiation; but female nurses who may perhaps be concerned about their thyroids, have been unable to find an appropriate countermeasure as there have been no investigations on the effects of occupational radiation exposure at hospitals with respect to thyroid nodules and thyroid cancer. Therefore, we conducted a comparative study between nurses and general health check-up female in an effort to investigate whether working at a hospital affects the prevalence of thyroid disease in nurses.

The result of the present study showed that the prevalence of thyroid nodules was increased in health check-up female, however in general; no difference was noted in prevalence of thyroid cancer in both the groups. Interestingly, the rate of prevalence of thyroid cancer versus nodules was significantly high in 30–39 years old nurses. Furthermore, the number of thyroid nodules increased with increase in age. The prevalence of thyroid cancer was not proportional to the ages of the subjects. This indirectly indicates that thyroid nodules or thyroid cancer do not result from radiation, and conceivably indicate appropriate execution of preventive measures, such as shifting the work of a nurse who has worked for a certain period of time in a department associated with radiation or in an operation room where radiation instruments are used (where the possibility of radiation exposure is relatively high). Alternatively, this result may have stemmed from the relatively limited direct radiation exposure. Nonetheless, this issue should be investigated in further studies for an assessment of any correlation between dose and response as the female undergoing a health examination for abnormalities such as a nodule tend to undergo regular medical checkups. The ratios of thyroid cysts, parenchymal disease and thyroid nodule were higher in subjects who underwent a health examination compared to that in the nurse group. On the other hand, because as the nurse group in this study included all the nurses in the single hospital, selection bias was low in the nurse group as compared to the health examination subject group. Therefore, it can be assumed that the working environment in the hospital has no effect on the prevalence of thyroid nodules or thyroid cancer among the nurses.

The main limitation of the current study is the absence of accurate individual estimates of radiation dose. Individual measurements, however, have so far not been adequately computerized and available for epidemiologic analyses. A subsequent study is required to individually measure radiation exposure in a hospital and there is a need for prospective cohort study for determining relationship between radiation and thyroid cancer.

In conclusion, although there are many other parameters that should be taken into account, the result showed that working in a hospital does not increase the prevalence of thyroid nodules or thyroid cancer.