Radiation Exposure
From Iodine 131

Clinical Evaluation

“Early detection of a change in health status is the most effective way to lessen the burden of more advanced disease and enhance survival.”

Dr. Barry L. Johnson, assistant surgeon general and ATSDR assistant administrator, congressional testimony on the National Cancer Institute's Management of Radiation Studies (Congressional testimony 1998).

Because I-131 concentrates in the thyroid gland, evaluation of a patient exposed to I-131 centers on diseases of the thyroid. Exposure to I-131 can cause thyroiditis, hypothyroidism, and thyroid neoplasms. The patient might have a variety of symptoms related to exposure or might have health-related concerns about past exposure. The occurrence of thyroid diseases caused by exposure is indistinguishable from those that occur spontaneously. The patient might not have specific knowledge of the nature of the exposure, which might have occurred years earlier.

A history and appropriate physical examination supplemented with laboratory investigation, imaging studies, and fine-needle aspiration biopsy (FNAB) of the nodules in question should provide the clinician with sufficient information to assess the likelihood of malignancy and to advise his or her patients of appropriate treatment options.

Ultrasound can find many nodules not palpable during the physical examination. Ultrasound is being used for thyroid monitoring programs in other countries where some of the population has been exposed to I-131 releases. However, the use of thyroid ultrasound in mass screenings for thyroid nodules is controversial because of its high sensitivity and low specificity.

If a nodule is identified, fine-needle aspiration biopsy (FNAB) performed by an experienced physician with appropriate training and experience is the procedure of choice. If the cytology of the nodule is malignant or nondiagnostic, the patient should be referred to a specialist for surgical resection.

Patient History

• History and physical exam should focus specifically on signs and symptoms related to the thyroid gland.

The medical history should include prior endocrine, thyroid, or parathyroid problems; prior thyroid diagnostic tests and treatments; and history of thyroid or neck surgery. Information about changes in the size of the nodule or nodules can assist in determining the etiology. Nodules that are unchanged for years are probably benign, but nodules that grow rapidly demand careful evaluation and are more likely to be associated with parathyroid disorders.

A family history of Hashimoto thyroiditis, benign thyroid nodule, or goiter favors a diagnosis of benign disease. Other history that suggests benign disease includes symptoms of hypothyroidism or hyperthyroidism, and pain or tenderness of the nodule. Risk factors for malignant disease can include a family history of thyroid carcinoma or multiple endocrine neoplasia type II; the patient's age (<20 years or >70 years); the patient's gender (male); recent changes in voice, breathing, or ability to swallow; and a childhood history of head, neck, or upper mediastinum radiation exposure.

Exposure History

An exposure history includes previous childhood head, neck, and upper mediastinum radiation exposure; previous residences (downwind from or proximity to nuclear testing or release sites); dietary habits since childhood; source of drinking water; occupational history; and hobbies. Milk consumption and source are important risk factors (for example, fresh versus processed milk; milk from a cow, sheep, or goat). The patient should be asked about symptoms consistent with hypothyroidism, hyperthyroidism, and disorders of calcium metabolism.

Exposure to I-131 could be indicated by the patient's answers to questions in the exposure history relating to the following:

• previous childhood head, neck, and upper mediastinum radiation exposure
• previous residences
• dietary habits since childhood
• milk consumption and source.

Populations exposed to I-131 can have a higher prevalence rate for thyroid nodules than populations that have not been exposed. Patients, especially infants and children who have been exposed to significant doses of I-131, are more susceptible to the associated negative health effects. The major clinical concerns after significant I-131 exposure include hypothyroidism and thyroid cancer.

Physical Examination

Physical examination of the neck and thyroid should evaluate the gland's size, presence of nodules, and the cervical lymph nodes. The thyroid gland should be inspected for shape, consistency, and areas of tenderness. Local examination of the neck is best accomplished with the patient seated in good light with the neck moderately extended. To facilitate the examination, the patient should be given a glass of water to assist swallowing. Auscultation of the neck provides some indication of the vascularity of the gland. A systolic or continuous bruit is usually associated with hyperthyroidism. The parathyroid glands are also susceptible to the effects of I-131 exposure. The presence of cervical lymphadenopathy, especially in children, might be the first sign of thyroid cancer. In general, a nodule 1 centimeter (cm) or greater should be palpable on physical examination.

Signs and symptoms that should prompt concern include rapid enlargement of a previous or new thyroid nodule, unilateral vocal cord paralysis, dysphagia, and dyspnea. A solitary nodule in an otherwise normal gland should raise the suspicion of thyroid carcinoma. A lesion is probably malignant if it is adherent to the surrounding structures (trachea or strap muscles). Palpable cervical lymphadenopathy adjacent to a thyroid nodule is suspicious for a carcinoma, or it might be the only indication of metastatic thyroid cancer when no thyroid nodule is palpable.

It would be appropriate to consult an internist, endocrinologist, a surgeon specializing in thyroid surgery, or an interventional radiologist when assessing a patient with a suspicious thyroid nodule and an abnormal screening evaluation. These specialists can either assist with the interpretation of the screening results or formulate a management plan for the patient. (Information about specialists is available from the American Board of Medical Specialties, which has a Web site at URL: http://www.abms.org/).

Case Study (continued)

The woman is a well-developed, mildly overweight, well-nourished female who looks her stated age of 55 years. Palpation of her neck reveals an ill-defined thyroid that is slightly tender diffusely with a homogenous, rubbery texture. A 1-cm nodule is just palpable in the left lobe. Auscultation of the neck reveals no bruits, either over the carotids or over the thyroid. No cervical nodes are palpable. Chvostek and Trousseau signs are negative. Hair and skin appear unremarkable, with perhaps the exception of some puffiness of the face. No evidence of mental dullness is seen. Deep tendon reflexes are normal without prolongation of relaxation phase. The rest of the examination is unremarkable.

When requestioned about specific symptoms of hypothyroidism, your patient admits that she has felt a bit more tired lately. She has been constipated occasionally and intolerant of cold. She has gained weight despite eating less. She attributed these symptoms to aging and had not thought much about them.

Challenge questions

7. Which diagnostic tests are recommended for routine initial screening of thyroid function?

8. What additional tests could be obtained for the evaluation of a thyroid nodule?

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