Final answer:
The major risk factor for thyroid cancer is exposure to radiation. Inadequate iodine intake can cause goiter, but it is not the primary risk factor for developing thyroid cancer. Other lifestyle and environmental factors contribute to cancer risk but are not specific to thyroid cancer.
Step-by-step explanation:
The major risk factor for thyroid cancer is c. Exposure to radiation. Although inadequate iodine intake can lead to goiter, goiter itself can be caused by a variety of factors, not all of which increase the risk of thyroid cancer significantly. Radiation, particularly in childhood, is a well-established risk factor for thyroid cancer. For example, following the Chernobyl nuclear disaster, there was a noticeable increase in thyroid cancers, particularly in children who were exposed to the radioactive fallout.
Goiter can be caused by insufficient iodine intake as the thyroid gland enlarges to compensate for the lack of thyroid hormone production. Goiters can be prevented by ensuring adequate iodine intake, such as through iodized salt. However, while goiters may not directly lead to thyroid cancer, they can be associated with both benign and malignant thyroid conditions.
Other factors that contribute to cancer development include poor diet and physical inactivity, and environmental exposures to elements like radon and UV radiation, but these are not as directly associated with thyroid cancer as radiation exposure is.