Thyroid cancers account for about 4% of all newly diagnosed cancers in the US, but only 0.3% of cancer deaths, with a 5-year survival rate of 98%. The incidence has risen significantly over the last 10 years, rising at a rate of 4.5% a year.
Patients usually present with a solitary thyroid nodule, either found on clinical examination or as an incidental finding on a diagnostic study, such as CT or ultrasound. Although 95% of nodules are benign, all patients with a thyroid nodule should be evaluated for thyroid cancer. Symptoms that raise suspicion for thyroid cancer include hoarseness, dysphagia or odynophagia, and adenopathy.
Thyroid cancers originate from either follicular or parafollicular cells. Cancers arising from follicular cells can be differentiated (papillary or follicular carcinomas) or anaplastic. Over 90% of thyroid cancers are differentiated, with papillary carcinoma being most common. Medullary carcinoma arises from parafollicular or C cells which produce calcitonin; it is a neuroendocrine tumor. Medullary carcinomas make up 1-2% of thyroid cancers and may be associated with multiple endocrine neoplasia type 2 (MEN 2) syndromes.
Female sex. The female-to-male ratio of thyroid cancer incidence is 2:1.
Radiation exposure. Head and neck radiation, especially during childhood, is a strong risk factor for all thyroid cancers. This has been demonstrated both for therapeutic exposure and from environmental exposures from atomic weapon fallout and nuclear power plant accidents, notably at Chernobyl.
Genetics. Relatives of thyroid cancer patients have a 10-fold greater risk of developing the condition. In addition, medullary carcinoma may be inherited, either as part of MEN 2 syndromes or as an isolated familial disease.
Overweight. A study of approximately 2 million individuals in Norway who were followed for 23 years indicated that body mass index (BMI) > 30 kg/m2 was associated with thyroid cancer incidence. Specifically, risk in women in BMI categories 30-34.9, 35-39.9, and > 40.0 increased by 27%, 33%, and 38%, respectively, compared with risk in women with a BMI of 18.5-24.9. A similar increase was found in risk for men with a BMI > 30.
Ultrasound is used to confirm the presence of thyroid nodules and distinguish solid from cystic lesions. Suspicious lesions, such as solid, hypoechoic nodules, should be further evaluated with fine-needle aspiration biopsy, which will establish the diagnosis in most cases.
Individuals with thyroid nodules should also have their thyroid-stimulating hormone (TSH) levels measured. If the hormone is elevated, a radioactive iodine uptake scan (thyroid scintigraphy) should be performed to distinguish functioning thyroid nodules (those that produce thyroid hormone) from nonfunctioning nodules. Functioning nodules are rarely malignant. Nonfunctioning nodules may be malignant and require a fine-needle aspiration biopsy.
Although serum calcitonin concentration may be elevated in medullary carcinomas, routine measurement of calcitonin in patients with thyroid nodules remains controversial.
Thyroidectomy is the primary therapy for most thyroid cancers. Resection is often followed by postoperative radioactive iodine ablation of residual thyroid tissue and potential metastases.
Lifelong thyroid hormone replacement therapy is necessary for all surgical patients.
For patients with medullary or anaplastic thyroid cancer, radiation and chemotherapy may be used as adjuvants to surgery or for palliation.
In research studies, certain foods have been associated with the risk for thyroid cancer. As is the case with several other forms of cancer, higher risk has been associated with obesity and consumption of certain animal products, and a reduced risk has been associated with higher consumption of plant foods. In individuals who immigrated to the US from certain Asian countries, some evidence also points to the abandonment of traditional diets and the adoption of Western dietary patterns as increasing the risk of thyroid cancer. Here are the nutritional factors associated with reduced risk:
Maintaining a healthy energy intake and healthy weight. A meta-analysis found a 25% greater risk for thyroid cancer among overweight individuals and a 55% greater risk in obese individuals as compared with normal-weight individuals. Paralleling these findings, the European Prospective Investigation into Cancer and Nutrition (EPIC) study found that individuals consuming the highest amount of calories experienced a roughly 30% higher risk for thyroid cancer, compared with those consuming the lowest amount.
Avoiding meat. Persons who consume larger amounts of pork and poultry were found to have a significantly higher risk for thyroid cancer than those who consume little or none of these products. Red meat consumption was associated with a 57% greater thyroid cancer risk in a large study of dietary habits and overall cancer risk.
Favoring vegetables. A diet that includes large amounts of vegetables appears to reduce thyroid cancer risk by roughly 25%. Research does not at this time indicate a significant reduction in thyroid cancer risk with higher fruit intakes.
Tea drinking. A meta-analysis found a roughly 25% lower risk for thyroid cancer in high versus low tea consumers, although only European and American populations (not Asians) benefitted.
Moderate alcohol consumption. Consuming two drinks per day was associated with a roughly 45% lower risk for thyroid cancer in the NIH-AARP study when compared with no alcohol intake. However, alcohol use increases risk of other cancer forms.
Low-iodine diets may not be needed or beneficial. Low-iodine diets have previously been recommended prior to thyroid ablation as a means of increasing the uptake of radioactive iodine by the thyroid gland. However, evidence has not been convincing of the superiority of this approach when compared with a less strict iodine intake.
See Basic Diet Orders chapter.
What to Tell the Family
Thyroid cancer, like many other cancers, is a disease that may be influenced by diet and lifestyle, although evidence does not yet permit a firm conclusion. Limited evidence suggests that avoiding fatty foods and increasing fruits and vegetables reduces the risk that this disease will develop. These diet changes are easier when the whole family makes them together.
The role of diet and lifestyle after diagnosis has not been well studied. Surgery remains the treatment of choice, followed by appropriate chemotherapy or radiation. Overall, the 5-year survival rate for those diagnosed with thyroid cancer is 98%.
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