The thyroid is a small, butterfly-shaped gland located in front of the trachea in the neck. An exquisitely sensitive gland, it enlarges and becomes more active during puberty, pregnancy, or times of great stress. It also alters its size and shape during women’s menstrual cycles.

The three hormones secreted by the thyroid are triiodothyronine (T3), thyroxine (T4), and calcitonin. T3 and T4, which contain iodine, have far-reaching effects on almost all tissues in the body and are intimately involved in physical growth, metabolism, and mental development. Calcitonin helps regulate the amount of calcium in the body. If T3 and T4 levels are low or nonexistent, growth hormone secretion is decreased, and what is released is not effective.

The thyroid’s functioning can be disrupted by radiation to the gland itself or to its regulator—the HPA. The pituitary gland produces TSH that prompts the thyroid to produce the exact amount of hormones needed by the body.

Organ damage

The thyroid is generally not affected by chemotherapy. If damage occurs, radiation is usually the culprit. Children or teens who had total body radiation, mantle radiation for Hodgkin lymphoma, or radiation to the head and/or neck are at the highest risk for a malfunctioning thyroid. Several types of thyroid problems can develop after radiation.

Compensated hypothyroidism. High TSH and normal T4 may occur if the thyroid is working too hard. There are usually no symptoms. An irradiated and/or overstimulated gland is at increased risk for developing tumors, both benign and malignant. Survivors with compensated hypothyroidism are sometimes given supplemental thyroid hormone to allow the gland to rest.

Most often, compensated thyroid dysfunction is found on routine screening of at-risk survivors. An elevation in the TSH is the first sign of thyroid gland dysfunction.

Primary hypothyroidism. Survivors who received more than 1500 cGy of radiation to the neck or more than 750 cGy total body irradiation (TBI) are at risk for primary hypothyroidism (increased TSH and low T4). Survivors of Hodgkin lymphoma, non-Hodgkin lymphoma, head and neck tumors, and those who had TBI prior to a stem cell transplant may develop this problem. Hypothyroidism sometimes occurs in patients treated with craniospinal radiation for leukemia.

Hypothyroidism is very common in Hodgkin lymphoma survivors who received mantle radiation. Treatment at a young age may also increase the likelihood of developing a thyroid problem.

Thyroid dysfunction can occur soon after radiation, but more typically starts 3 to 5 years after treatment.

Thyroid-stimulating hormone deficiency. This late effect, characterized by low TSH and T4 levels, is very uncommon but can occur after radiation to the head or after a stem cell transplant.

Hyperthyroidism. Hyperthyroidism (low TSH and elevated T4) occurs when too much thyroxine is produced, causing the body to use energy faster than it should. It is not well understood, but has been found in very small numbers of survivors who were treated with neck radiation.

Thyroid cancer. Radiation to the neck can result in thyroid cancer later in life, so all survivors at risk need lifelong evaluation of thyroid function.

Signs and symptoms

Hypothyroidism. Signs and symptoms of an underactive thyroid (hypothyroidism) include the following:

• Fatigue or lethargy

• Hoarseness

• Difficulty concentrating

• Depression or mood changes

• Constipation

• Weakness

• Intolerance to cold

• Swelling around the eyes

• Poor growth

• Delayed puberty

• Puffy face and hands

• Weight gain

• Dry or rough skin

• Brittle hair

• Joint or muscle aches

• Slow heart rate

• Low blood pressure

• High cholesterol

• Decreased tolerance for exercise

Hyperthyroidism. The signs and symptoms of an overactive thyroid (hyperthyroidism) include the following:

• Nervousness or anxiety

• Difficulty concentrating

• Fatigue

• Muscle weakness or tremor

• Rapid or irregular heartbeat

• Excessive perspiration

• Heat intolerance

• Diarrhea

• Weight loss

• Menstrual irregularities

• Protruding eyes

• Tenderness in the neck

• Decreased tolerance for exercise

Screening and detection

Free T4 and TSH levels should be checked every year after radiation to the head, chest, or neck, and anytime symptoms develop. Women who take oral contraceptive pills should also have their thyroid levels checked periodically. These are simple blood tests. At some facilities, radioactive iodine uptake by the thyroid is measured. At every yearly follow-up appointment, a survivor’s thyroid should be palpated (felt by hand) and growth of children and young teens should be plotted on a chart. If a healthcare provider can feel a thyroid nodule (bump), an ultrasound of the thyroid will be done to evaluate it. Some institutions now use ultrasounds for screening.

Thyroid problems can occur years or decades after treatment for cancer, so a yearly check is necessary for the rest of your life. If any abnormalities are detected during an examination, referral and follow-up by an endocrinologist or surgeon are necessary.

Medical management

Survivors’ healthcare providers should talk to them about the signs and symptoms of thyroid problems so they will be recognized early. Although thyroid problems are common in survivors who had radiation to the head and neck, treatment is generally easy and effective. Treatments for thyroid problems include:

• Compensated hypothyroidism (high TSH, normal T4).  Daily pill of thryroxine to suppress excessive gland activity.

• Primary hypothyroidism (high TSH, low T4).  Replacement with the hormone thyroxine (daily pill).

• Thyroid-stimulating hormone deficiency (low TSH, low T4).  Daily thyroxine.

• Hyperthyroidism (low TSH, high T4).  Radioactive iodine to destroy the thyroid, then daily replacement with thyroxine.

• Thyroid nodules.   Patients with nodules detected by ultrasound should have a thyroid scan and evaluation by both an endocrinologist and a surgeon. If the scan shows nodules, a biopsy should be performed.

• Thyroid cancer.  A thyroglobulin level (blood test) should be done before the thyroid is removed. If the tumor secretes the thyroglobulin hormone, this simple blood test can be used in the future to screen for recurrence of the cancer. Patients with thyroid cancer usually have the thyroid removed (called a thyroidectomy) and get radioactive iodine afterwards. After surgery, thyroxine replacement is necessary.

Female survivors who are at risk for thyroid problems and are planning to become pregnant should have a blood test done to evaluate their thyroid function. Both the American Association of Clinical Endocrinologists and the American College of Endocrinology recommend that all women planning to become pregnant be screened before they conceive, because mothers with thyroid disease have a higher risk of having children with neurological defects.