Agency for Toxic Substances and Disease Registry
Radiation Exposure from Iodine 131
Answers to Pretest and Challenge Questions

Pretest Answers

1. The thyroid gland is the critical organ for I-131 exposure. Essentially all of the iodine entering the body quickly becomes systemic (EPA 1988), with approximately 30% depositing in the thyroid.
2. The main route of human internal I-131 exposure of humans is ingestion of contaminated fresh dairy products, eggs, and leafy vegetables, depending on downwind distance from the release; the major source of internal exposure is milk consumption. Goat's and sheep's milk contain approximately 10 times the concentration of radioiodine found in cow's milk.
3. I-131 in large amounts can produce thyroiditis. Hypothyroidism and thyroid cancer can result from smaller exposures of I-131 and its accumulation in the thyroid gland.
4. Infants and children are the groups most sensitive to I-131 exposure. The dose to children is much higher than for adults because the thyroid mass in children is smaller than that for adults; the first week of life is an especially vulnerable period. Populations in the former Soviet Union exposed to much larger radiation doses (especially those affected by the Chernobyl nuclear release) showed an increased incidence of thyroid cancer for children younger than 15 years old at the time of the release.

Challenge Answers

1. The main sources of I-131 in the environment have been from nuclear power plant releases and nuclear weapons production and testing. The main sources in the United States have been the Nevada test site and the Hanford Nuclear Reservation.
2. Dietary intake of iodine before exposure is important because a relative iodine deficiency increases the thyroid uptake of I-131. After exposure, the most critical dietary information needed is the amount and type of milk and milk products consumed, their I-131 concentrations, and the time they were consumed relative to the time of the release. The concentration of I-131 in goat's and sheep's milk is 10 times that of cow's milk. Fresh milk drunk directly on the farm has higher amounts of I-131 than the amount in milk that has been sent from the farm to the processing plant and then to a store. This variation in amount of I-131 is related to the short half-life of I-131 and the decay that occurs in the time the milk is processed. Although cheese and other aged-milk products tend to have lower amounts of I-131, it is also important to determine how much of these products have been consumed and what their I 131 concentrations were when they were consumed.
3. Infants and children are the groups most sensitive to I-131 exposure. The dose to children is much higher than the dose to adults because the thyroid mass in children is smaller than that of adults. The first week of life is an especially vulnerable period. Populations in the former Soviet Union exposed to much larger radiation doses (especially those affected by the Chernobyl nuclear release) showed an increased incidence of thyroid cancer for children younger than 15 years old at the time of the release.
4. I-131 has a strong affinity for the thyroid gland, which is the critical target organ for exposure. Essentially all of the iodine entering the body quickly becomes systemic (EPA 1988), with approximately 30% distributing to the thyroid.
5. This patient's exposure history should include previous childhood head, neck, and upper mediastinum radiation exposure; previous residences (proximity to nuclear testing or release sites); dietary habits since childhood, including milk consumption and source (fresh vs. processed milk; whether milk was from a cow, sheep, or goat); source of drinking water; occupational history; and hobbies. Patients who consumed goat's milk contaminated with I-131 have a higher radiation dose of exposure than if they drank contaminated cow's milk from the same pasture. The patient also should be asked about symptoms consistent with hypothyroidism, hyperthyroidism, and disorders of calcium metabolism.
6. Fine needle aspiration biopsy (FNAB) is the procedure of choice for evaluating a palpable nodule and determining whether or not it is malignant.
7. Serum TSH level is a useful initial screening assay because it can identify patients with either thyrotoxicosis or hypothyroidism. Chronic autoimmune thyroiditis can present with an increased TSH level and a thyroid nodule.
8. Serum FT4 and T3 should be measured if TSH is abnormal. A serum calcitonin level should be obtained if either a medullary thyroid carcinoma or a multiple endocrine neoplasia type II is suspected. Also, because of the risk of hyperparathyroidism after exposure to I-131, serum calcium, phosphorus, and parathyroid hormone should be assessed.
9. If the nodule is found to be benign by FNAB, the patient could be treated with T4 in a dose sufficient to suppress serum TSH, which will limit glandular growth. If the nodule decreases in size, the patient should be maintained on T4 indefinitely and the nodule monitored with ultrasound. If the nodule persists while on T4 therapy, the patient will need a repeat FNAB. If the nodule grows during T4 therapy, a surgical resection is indicated.
10. The patient and her husband are at higher risk for developing thyroid disease or thyroid cancer because they received higher levels of exposure to I-131 than persons who did not live near the Hanford Reservation during the time of the highest I-131 emissions. The daughter is not at higher risk because she was born in 1963, and emissions decreased after 1962. No higher risk for any other disease is known. Children in the United States are screened at birth for thyroid function; no further thyroid tests are needed if the child is growing normally without other medical problems. Good prenatal care would be highly advised for your patient's daughter. However, you could reasonably reassure your patient that an abnormal pregnancy outcome as a result of exposure to I-131 is unlikely because all of the potential exposures for the patient and her daughter were in the past.
11. If you live within 50 miles of a nuclear facility that produces or is capable of releasing I-131, you should work with your medical association, local or state public health department, emergency response organizations, and elected representatives to ensure that a stockpile of KI is available and a distribution plan is in place to distribute it if required.
12. The perception of risk by individuals or communities can be affected by several factors other than how the risk is communicated. This includes culture, social and economic level, geographic location, previous experiences, and other variables inherent to the individual. In matters of high concern and low trust, perception equals reality. Peter Sandman (1993) developed the following framework for dealing with risk:

    “Risks that are: natural are more readily accepted than those that are man-made visible and avoidable are more readily accepted than those that are unseen and imposed by others voluntarily assumed are more acceptable than those that are involuntarily imposed familiar are more acceptable than risks that are exotic or unfamiliar.”

13. The health care provider can encourage trust and credibility by getting the facts of the exposure straight, being forthcoming with information that meets the needs of the individuals, coordinating efforts with public health agencies, and avoiding giving mixed messages. It is important to deal with the uncertainty; listen to and deal with specific concerns; convey the same information to all segments of the audience; and explain risk in language people understand, simplifying language and presentation, but not content.

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