Iodine Basics | Exposure Route | Radiation Dose | Health Risks | Protection Standards

Physiologic Effects

• Thyroid nodules, neoplasia, hypothyroidism, and autoimmune thyroiditis with or without hypothyroidism are the main effects of internal exposure to I-131.
• Benign neoplasms are more common than malignant neoplasms.

Persons exposed to releases of I-131 involving melted fuel at nuclear power plants, from production of nuclear weapons, and from fallout from aboveground detonation of atomic bombs have a higher risk for developing thyroid cancer or thyroid disease, or both, than do unexposed populations. In particular, persons exposed during childhood received higher doses, which in many cases were repetitive over time. These persons were more vulnerable than were those exposed as adults.

Radiation causes health effects when either enough cells are killed quickly enough to disrupt tissue function (acute health effects) or damaged cells are incompletely repaired but still viable (carcinogenic, tumorigenic). I-131 radiation might affect cells in the thyroid gland, leading to hypothyroidism or thyroiditis, or might cause benign or malignant thyroid tumors and nodules. The thyroid gland has one of the lowest cell-proliferation rates of body tissues, and its regenerating ability is also low.

Even in the absence of exposure to I-131, thyroid tumors are the most common endocrine neoplasms. Thyroid tumors are usually nodules localized to the thyroid gland, and are often palpable on examination of the anterior neck. I-131 exposure increases the risk of thyroid nodules and cancer. Thyroid cancer is rare. The mean rate of spontaneous thyroid cancer is one in 1 million for children (10 in 1 million for adults), with a female-to-male ratio of 3 to 2. The increased risk for thyroid cancer is especially important for exposures during childhood. The incidence of thyroid nodules increases with age. However, thyroid cancer in children often presents at a more advanced stage than in adults: more distant metastases, more lymph node involvement. The risk of recurrence is higher in children, but the death rate (at least over 20 years) is much lower in children than in adults.

• 5%–10% of palpable nodules are thyroid cancer; the remainder are benign thyroid nodules.

When identifying those nodules that are likely to be malignant, a careful history is crucial. The history should include past medical history, occupational history, environmental exposure history, family history including dietary exposure, and social history. Of particular importance is a history of external radiation exposure to the head, neck, or upper mediastinum in infancy or childhood.

Exposure of the thyroid gland to moderate to high doses (from 6.5 to 2,000 centigray) of I-131 linearly increases the risk for thyroid cancer. Nodular disease occurs in about 20% of these patients, but it might not be apparent until 30 or more years after the initial exposure. The risk for thyroid neoplasm has been correlated directly with younger age at radiation exposure, radiation dose, and sex. (A male patient with a nodule should be regarded with greater suspicion because more women have thyroid cancer—by a ratio of 2:1—and women have more thyroid disease—by a ratio of about 8:1.) Thyroid nodules in children and elderly patients are more likely to be malignant.

Excesses of thyroid nodules and cancer (including 1,800 thyroid cancers) reportedly occurred from the Chernobyl nuclear power plant release. These results might have reached higher levels than expected due to low dietary intake of iodine in the region and high endemic rates of goiter.

Several cohorts of pediatric patients irradiated in head and neck area for thymus hypertrophy, tinea capitis, and chronic tonsillitis have been studied. These studies suggested three findings. First, the thyroid of children is more sensitive to carcinogenesis than is the thyroid of adults. Second, the delay between the external irradiation and the appearance of the cancer is at least 10 years (average delay 20 years). Third, the dose-response curve is linear for persons exposed before 15 years of age (even down to 0.1 Gy [10 rad]). The excess relative risk per unit of exposure (ERR/Gy) for childhood exposure is 7.7 (95% confidence interval, 2.1–28.7).

Reproductive and Developmental Effects From Therapeutic Uses of I-131

I-131 has become a standard treatment for thyroid ablation in persons with hyperthyroidism or thyroid cancer.  At least seven case reports and small case series on adverse reproductive outcomes of medical I-131 use have been published. However, sufficient information was not provided to determine the extent to which the fetus was exposed to I-131 radiation. The authors concluded that although the abnormalities found could not be directly attributed to the therapy, it is prudent to avoid pregnancy for 1 year after radiation treatment.

• Increased risk for thyroid neoplasm remains elevated for at least 40 years after exposure.

Thyroid exposure to either internal or external radiation might trigger an immune response. Changes in thyroid autoimmunity after I-131 therapy have been attributed to the production and release of autoantigens as a result of radiation damage.

Acute radiation thyroiditis occurs within 2 weeks after high exposure to I-131 and is characterized by local pain and tenderness over the gland. Occasionally, significant systemic symptoms have been associated with a massive release of stored thyroid hormone. This syndrome can require treatment with anti-inflammatory agents and beta-adrenergic antagonist agents. Clinically significant acute radiation thyroiditis is unlikely to occur at thyroid I-131 doses below 20,000 rad. Radioactive iodine can accumulate during pregnancy in the fetal thyroid and cause its permanent ablation. Because of the risk for fetal exposure to I-131, women of childbearing age must take a pregnancy test before undergoing medical radioiodine treatment.

Elevated levels of psychologic stress, which can lead to increased risk of depression, anxiety, and posttraumatic stress disorders in some people, can occur after incidents involving nuclear releases.

Most people want to receive information as quickly as possible after notification of a nuclear or chemical release. They want public health officials or their primary care providers to give them advice about potential health risks and what actions to take to prevent serious consequences of their exposure. Timely and correct information is key to preventing stress and relieving its psychosocial effects.

A nuclear release or known past exposure from any type of radiation can lead to increased psychologic stress because of the invisible nature of the event and concern for serious health-related effects from a radiologic contaminant. Results from studies in communities affected by previous nuclear releases, such as Three Mile Island (TMI), showed that area residents experienced long-term elevations of stress with increases in community rates of subclinical depression, anxiety, demoralization, and a heightened perception of risk. Indeed, the high levels of psychosocial stress in communities affected by the TMI release remained elevated for 6 years after the release and did not return to normal until 10 years after the incident (Baum et al. 1983).

Mothers are especially susceptible to psychological effects because of their concern about the effects of radiation on their children’s health. Pregnant women have added worry about risks to their unborn children.

Health care providers may hear concerns from people who might have been exposed to radioactive releases from nuclear tests or facilities. It is common for these people to talk about uncertainties about their health concerning previous or present exposure and the effect on their health. Patients’ health concerns may center around whether they will get ill and—if so—when, what caused the illness, whether the illness can be diagnosed properly, and what its prognosis, treatment, and financial impact will be. Because of fear of cancer and the uncertainty of when or whether it might occur, patients can have emotional stress, risk for developing anxiety, and depression (Vyner 1988).

Social consequences to exposure to radiation can also occur. Many of the men and women exposed in Nagasaki and Hiroshima during World War II were perceived as “damaged” and shunned for marriage because of the potential for “damaged genes.”

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Baum A, Gatchel RJ, Schaeffer MA. 1983. Emotional, behavioral, and physiological effects of chronic stress at Three Mile Island. J Consult Clin Psychol 51(4):565–72.

Vyner HM. 1988. The psychological dimensions of health care for patients exposed to radiation and the other invisible contaminants. Soc Sci Med 27(10):1097–103.