The thyroid is a butterfly-shaped gland in the neck that secretes thyroid hormones which regulate a number of physiologic processes, including growth, development and metabolism. With respect to Ionising Radiation, the thyroid is vulnerable to intakes of radioactive Iodine, for example I-125, by either Inhalation or Ingestion. In the event that an intake of I-125 occurs the thyroid will be the organ or accumulation and will preferentially uptake the iodine from the systemic system. This creates an Internal Radiation hazard.

Whilst an intake of radioactive iodine into the thyroid occupationally is undesirable, it is also used in medicine. For example, I-131 can be used in radioiodine therapy for thyroid cancer where somewhere between 1-5 GBq will be used. It can also be used in thyroid diagnostic scans where much less activity is used. In medical use the radiation exposure is justified so that the risk of the exposure is outweighed by the benefits (of treatment and diagnostic information).