Potential Adverse Health Effects from Exposure to Radioactive Iodine (¹³¹I) Released from Hanford

The primary consequence from exposure to radioactive iodine ¹³¹I is injury to or destruction of the thyroid gland.

The Thyroid Gland

The thyroid gland is part of the endocrine system of the human body. The endocrine system maintains homeostasis (i.e. bodily equilibrium) and processes by regulating the release of hormones. The endocrine system controls such body processes as growth and development, reproduction, and metabolism.

Where is it?

The thyroid gland is located in the lower front part of the neck, at the base of the throat, below the Adam's apple. It has two connected lobes, and sits with one lobe on either side of the windpipe. The right lobe is usually higher than the left lobe. Each lobe is about the size of a teaspoon.

What does it do?

Although small in size, the thyroid gland significantly effects bodily functions. The primary role of the thyroid gland is to help regulate the body's metabolism. The gland produces thyroid hormone which is actually two active thyroid hormones (levo-thyroxin [T4] and triiodothyronine [T3]) which are important in the manufacture of proteins, the regulation of body temperature, and in overall energy production and regulation. Thyroid hormone is often referred to as the body's major metabolic hormone. It controls the rate at which glucose is "burned" or oxidized, and converted to body heat and chemical energy. All body cells depend upon a continuous supply of chemical energy to fuel their activities, making every cell in the body a target for thyroid hormone. Thyroid hormone is also important for normal tissue growth and development especially in the reproductive and nervous systems. The thyroid gland requires adequate amounts of dietary iodine to produce thyroid hormones. Iodine is concentrated in the thyroid gland and combined with other substances to form T3 and T4.

A second important hormone product of the thyroid gland is calcitonin, or thyrocalcitonin. This hormone decreases blood calcium levels, causing calcium to be deposited in the bones. Calcitonin counteracts to the parathyroid hormone, which is produced by the parathyroid glands. The parathyroid glands are discussed below.

Women are more frequently affected by thyroid dysfunction than men are. In addition, because the thyroid gland plays a role in the reproductive system, women are affected differently by thyroid dysfunction than men are.

Radiation and the Thyroid

Radiation can affect cells in the thyroid gland so that they do not function properly, or may cause them to grow abnormally. Radioactive iodine can be especially harmful to the thyroid, because iodine concentrates in the thyroid gland.

Radiation Injury and Exposure to Radiation

Iodine 131

The thyroid gland needs iodine to produce thyroid hormones. The thyroid does not recognize the difference between ordinary iodine, like that found in iodized salt, and radioactive iodine, like Iodine 131 (¹³¹I), which was released from the Hanford Nuclear Reservation during nuclear weapon production. If a person ingests ¹³¹I, the thyroid gland absorbs it or, takes it up, just as it would iodine from iodized salt. If ¹³¹I is ingested or inhaled, it may cause permanent, irreversible damage to the thyroid gland, resulting in hypothyroidism or thyroid nodules. Thyroid cancer can also be caused by exposure to ionizing radiation.

Children are at a higher risk of thyroid disease from exposure to ¹³¹I because one major source of exposure is drinking milk from cows or goats who have grazed on contaminated grass, and children often drink more milk than adults. In addition, a child's smaller thyroid gland receives a bigger dose per unit of ingested radio-iodine. For example, a newborn's thyroid dose is about 16 times higher per ingested amount of ¹³¹I than an adult's dose. Accordingly, persons who were in their childhood years during Hanfords' release of ¹³¹I likely had the highest exposure, and may have been most harmed by the releases.

Thyroid Disorders

Thyroid Neoplasms and Thyroid Cancer

Thyroid neoplasms are abnormal masses of the thyroid, often called nodules. They occur more often in women than in men. Most thyroid nodules (approximately 90%) are benign or non-cancerous. However, they may produce difficulty in swallowing, speaking or breathing if they become too large and put pressure on important structures in the neck.

Thyroid nodules may be cancerous, or can become malignant. For people who develop uncommon but dangerous forms of the disease, or if the thyroid cancer is undetected until an advanced stage, the disease may be fatal. However, the treatment for thyroid cancer usually produces excellent cure rates (in excess of 90%; 5-year survival rate is 95%). In 1997, an estimated 16,100 Americans were diagnosed with thyroid cancer. An estimated 1,230 died from the disease. The incidence rate for women is more than twice as high as that for men.

Thyroid cancer is normally a slow-growing cancer and often there are no obvious signs until the tumor is large enough to protrude from the neck or is large enough or situated in such a way that it obstructs the windpipe or esophagus and causes difficulty swallowing, breathing, and/or speaking (hoarseness). Even though a person may be unaware of a lump in the thyroid, it may be readily palpable by a health care provider.

Thyroid tumors can cause or result in hyperthyroidism. There are two types of benign thyroid nodules: "hot" and "cold." Hot nodules are those that produce significant quantities of thyroid hormone, and the increased elevation of thyroid hormone in the blood may be detected by blood tests. The individual with a hot nodule will also likely have symptoms consistent with hyperthyroidism (see below). Cold nodules are also called non-functioning nodules. When a nodule is a cold nodule, the thyroid hormone level in the blood may test normal, so the patient may not have symptoms of thyroid disease.

Diseases of Thyroid Function

Hypothyroidism

Radiation exposure, including exposure to I-131 (radioactive iodine) can cause abnormalities of thyroid function, primarily hypothyroidism. Hypothyroidism involves the production of too little thyroid hormone.

Hypothyroidism is the insufficient synthesis of the thyroid hormone. This can cause a variety of health problems, including fatigue, weight problems, lethargy, and impairment of memory and mental function. Hypothyroidism can also cause hypersecretion of TSH (thyroid stimulating hormone). This can lead to goiter, or enlargement of the thyroid gland.

Untreated hypothyroidism can advance with increasing loss of mental function and physical energy, and in extreme cases can result in coma and death. Very young children are at higher risk of irreversible damage from untreated hypothyroidism. If hypothyroidism occurs in the first two years of life without prompt treatment, the afflicted child can develop irreversible brain damage.

You may have thyroid problems that have not been diagnosed by a medical provider. For example, there are many symptoms of hypothyroidism which sometimes go undiagnosed because they are attributed to advancing age or other health problems. Some symptoms of thyroid problems worsen over time. Symptoms caused by injury or damage to the thyroid may not occur until many years later, for example, many years after exposure to ¹³¹I.

Thyroid Problems and Women

Typically, more women than men are affected by hypothyroidism and other thyroid disorders. Women are affected differently by thyroid disease then men, because the thyroid gland is involved in the reproductive system.

Hypothyroidism in adolescent women is most commonly associated with delayed puberty. Once a woman has begun menstruation, hypothyroidism may cause abnormally heavy periods, sometimes with prolonged bleeding that may result in anemia. Hypothyroidism may prevent ovulation. However, despite the failure to ovulate, menses may still occur, so that the only evident symptom of hypothyroidism may be infertility. In women with severe hypothyroidism, the level of a pituitary hormone called prolactin may be increased. High levels of prolactin can cause milk production unrelated to pregnancy and childbirth. In addition, high levels of prolactin may also prevent ovulation and cause decreased fertility. Untreated hypothyroidism has been associated with early miscarriage, premature delivery, and an increased risk of stillbirth.

Hyperthyroidism

Hyperthyroidism is a thyroid disorder involving overproduction of thyroid hormone. Thyroid tumors, which are linked to radiation exposure, can cause hyperfunctioning of the thyroid and increased levels of thyroid hormone in the blood. Symptoms of hyperthyroidism include nervousness, fatigue, heart palpitations, weight loss, heat intolerance, irritability, tremor, muscle weakness, decreased menstrual flow, sleep disturbance, increased perspiration, increased frequency of bowel movements, change in appetite, and thyroid enlargement. Hyperthyroidism is also a symptom of Graves' Disease, a disease characterized by goiter, protruding eyes, photophobia, double vision, and decreased visual acuity.

Relationship Between Hypothyroidism and Hyperthyroidism

In some cases, individuals with hyperthyroidism may have (or may have had) hypothyroidism, and vice versa. In some cases, this can result from the method of treatment. For example, an individual who receives synthetic thyroid hormone as a treatment of hypothyroidism may receive too high a dose, resulting in hyperthyroidism. A change in dose level will typically resolve this transient hyperthyroidism.

In some cases, however, an individual who has transient hypothyroidism may then develop Grave's hyperthyroidism without medical intervention such as treatment with thyroid hormone (replacement therapy). This condition is probably more common than is generally recognized. Although the exact mechanism by which hypothyroidism transforms into hyperthyroidism has not been identified, some findings strongly suggest that autoimmune mechanisms play an important role.

The Parathyroid Glands

The parathyroid glands are tiny masses of glandular tissue found on the back (posterior side) of the thyroid gland. Typically, there are two glands on each thyroid lobe, or a total of four parathyroid glands.

These glands secrete parathyroid hormone (PTH) also called parathormone. This hormone is the most important regulator of calcium equilibrium in the blood. When blood calcium levels drop below a certain level, PTH is released, which stimulates bone destruction cells to release calcium from bone into the blood. Although the skeleton is the major target of PTH, this hormone also stimulates the kidneys and intestine to absorb more calcium.

PTH operates in balance with calcitonin, a thyroid hormone. PTH acts to increase calcium levels in blood, and decrease bone levels of calcium, whereas calcitonin increases calcium levels in bone and decreases calcium levels in the blood. It is important that PTH and calcitonin act in equilibrium.

Hyperparathyroidism

Studies done on people who have received radiation treatments to the neck demonstrate a relationship between radiation and a disorder of the parathyroid glands known as hyperparathyroidism. Hyperparathyroidism, which occurs more frequently in women than in men, is a disorder resulting from uncontrolled secretion of parathyroid hormone by one or more parathyroid gland, causing increased levels of calcium in the blood (hypercalcemia). Severe hyperparathyroidism can cause bone destruction, resulting in bone fragility.

¹³¹I in the thyroid gland may cause tumors in the adjacent parathyroid glands. Tumors in the parathyroid glands often cause hyperparathyroidism and hypercalcemia.  Those with hyperparathyroidism may be at higher risk for thyroid cancer.

Symptoms are typically vague, and the disorder is usually diagnosed when blood tests indicate too much calcium in the blood. Where symptoms do occur, they are usually related to high levels of calcium in the blood: fatigue, weakness, drowsiness, nausea and vomiting, high blood pressure, slow heart rate, and frequent urination. Because these symptoms are also common to other diseases, a person displaying these symptoms may not necessarily have hyperparathyroidism. If you have any symptoms, please see your medical provider.

Sources: Harrison's Principles of Internal Medicine (13th ed., 1994); Hanford Thyroid Disease Study (HTDS) Review of Thyroid Disease and Approach to Diagnosis (February, 1997); HTDS: Questions & Answers about Radiation & Thyroid Disease (February 1997); Radio-Iodine: From Hanford to Chernobyl . . . And Beyond? John W. Gofman, M.D., Ph.D., Spring, 1993; Hanford Health Information Network (HHIN) Training Manual -- Responses to Frequently Asked Questions; Essentials of Human Anatomy and Physiology (5th ed, 1997); Thyroid Foundation of America - The Bridge, Volume 10 Number 1 (February 1998); The Thyroid and Reproductive System: Disorders of Menstruation, Fertility and Pregnancy, Loren Wissner Greene, M.D., F.A.C.P., F.A.C.E. (In TFA - The Bridge, Vol.10(1), pages 3-6); National Institutes of Health, National Cancer Institute Friday, July 25, 1997 press release; R. Wilson, J.H. McKillop, C. Jenkins and J.A. Thomson, In Vivo and In Vitro Studies into the Immunological Changes Following Iodine 131 Therapy for Graves' Disease, Eur. J. Nucl. Med. (1991) 18:265-268; V. Fatourechi, H. Gharib, Hyperthyroidism following Hypothyroidism, Arch. Intern. Med. (1988) 148:976-978.; Peter A. Singer, David S. Cooper, Elliot G. Levy, Paul M. Ladenson, Lewis E. Braverman, Gilbert Daniels, Francis S. Greenspan, I. Ross McDougall, Thomas F. Nokolai, Treatment Guidelines for Patients with Hyperthyroidism and Hypothyroidism, Journal of the American Medical Association (1995) Vol. 273(10) pages 808-812.

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