The American Cancer Society estimates that among the 62,450 people who will be diagnosed with thyroid cancer in 2016, some will be treated with thyroidectomy, either a partial or total removal of the thyroid gland.
Patients treated with a near-total or total thyroidectomy will be unable to produce thyroid hormone naturally, and will require treatment with thyroid hormone replacement therapy (THRT) for the rest of their lives to help maintain target levels of thyroid hormone. For these patients, the American Thyroid Association guidelines indicate that use of remnant ablation involving administration of radioactive iodine (RAI) to identify and eliminate remaining thyroid cells including cancer cells can be considered, even among a select group of low-risk patients.
However, what is often missed in planning for this course of treatment is the risk that patients may become symptomatically hypothyroid during the ablation process. Longer term, this same risk can occur when patients undergo certain tests requiring thyroid hormone (TSH or thyrotropin) stimulation during routine monitoring to determine whether they have had a recurrence of thyroid cancer.
Understanding Hypothyroidism During RAI
Hypothyroidism, where the body lacks sufficient levels of thyroid hormone, has a variety of symptoms including fatigue, mental fog, sensitivity to cold, dry skin, and constipation. Some studies have shown that hypothyroidism can affect cognitive and motor skills, making it difficult for patients to perform functions that might require a rapid response, such as driving.
In the RAI ablation procedure following a thyroidectomy, patients must have sufficient levels of TSH in the blood. This stimulates thyroid tissue to take up the radioactive iodine more effectively. Clinicians monitor TSH levels and wait until they reach approximately 25-30 mU/L. Radioactive iodine is then administered at doses ranging from 30-100 mCi.
After three to seven days, patients undergo a whole-body scan (WBS) to identify remaining thyroid tissue or remaining thyroid cancer. In cases where clinicians either discontinue or delay initiation of thyroid hormone replacement therapy during this process, patients will become chemically hypothyroid, leading to an increased risk that they might develop symptoms associated with hypothyroidism.
Long-term Patient Monitoring
Following initial treatment for thyroid cancer, depending on their level of risk most patients are monitored regularly (usually annually) for recurrence of disease. For low-risk patients monitoring typically includes neck examination, ultrasound and blood tests (thyroglobulin). For patients at higher risk, monitoring might also include WBS, CT scan, MRI, or PET scan. In high-risk patients who require TSH stimulated thyroglobulin measurement and/or WBS, clinicians can use either thyroid hormone withdrawal (THW) or rhTSH to provide TSH stimulation to optimize the effectiveness of the RAI uptake. As in the remnant ablation process, THW in patients during monitoring can increase the risk of hypothyroidism with each occurrence.
Recognizing the Impact of Hypothyroidism on Patients
In thyroid cancer, the prospects of long-term survival, overall for most patients are encouraging. Studies have shown that about 98% of patients diagnosed with localized thyroid cancer will survive for five years. Survival rates at 10 and 15 years are 97% and 95%, respectively, according to an article published in the journal Surgery in 2013.
While the treatment of thyroid cancer must focus primarily on eradication of disease, the potential for patients to live for many years following a diagnosis also requires clinicians to carefully consider the impact of different decisions related to both treatment and long-term monitoring on patient health and quality of life.
With hypothyroidism, patients can experience potentially debilitating symptoms that can range in both severity and duration. Recent research has shown that many of the symptoms of hypothyroidism, including fatigue, depression, memory loss, and mobility impairment, can have a significant impact on patient health. It is important for both clinicians and patients to understand the impact of risks associated with treatment for thyroid cancer such as hypothyroidism, and to make appropriate determinations regarding strategies to reduce these risks for appropriate patients in a course of care that can last for many years or decades.
Endocrine Society member Richard Weiss, MD, is the executive director, global medical affairs–osteoporosis, at Radius Health, Inc. and an associate professor at the University of Central Florida School of Medicine. Prior to his current position, he was formerly the Global Medical Director, Endocrinology-Rare Diseases, Sanofi-Genzyme.
 Schlumberger, et. al, NEJM, 366;18.