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.[1]

Patients usually present with a solitary painless 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.[2]

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.[3]

Risk Factors

Female sex. The female-to-male ratio of thyroid cancer incidence is 2:1.

Radiation exposure. Head and neck radiation, especially before age 20, is a strong risk factor for all thyroid cancers. This has been demonstrated both for therapeutic exposure, such as treatment for acne and tonsillitis, and from environmental exposures from atomic weapon fallout and nuclear power plant accidents, notably at Chernobyl.[4]

Genetics. Relatives of thyroid cancer patients have a 10-fold greater risk of developing the condition.[5] In addition, medullary carcinoma may be inherited, either as part of MEN 2 syndromes or as an isolated familial disease. In 2014, The Cancer Genome Atlas published comprehensive genomic alterations. BRAF is the most frequently mutated gene, occurring in 60% of papillary thyroid carcinoma cases. RAS (HRAS, NRAS, and KRAS) occurs in 13%, particularly in the follicular variant.[6][7]

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.[8]

Diagnosis

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.[9]

Treatment

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.[10]

Nutritional Considerations

Dietary habits appear to modify the risk of thyroid carcinoma. A diet with a limited consumption of starchy foods and products rich in salt, fat, and sugar, and a higher consumption of cruciferous and non-cruciferous vegetables, milk and dairy products, and seafood could be protective towards thyroid cancer. These results warrant further investigations and, if confirmed, might have important public health implications for the reduction of thyroid cancer through the improvement of dietary habits.[11]

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.[12] 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.[13]

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.[14] Red meat consumption was associated with a 57% greater thyroid cancer risk in a large study of dietary habits and overall cancer risk.[15]

Favoring vegetables. A diet that includes large amounts of vegetables appears to reduce thyroid cancer risk by roughly 25%.[5]

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 Asian) benefitted.[16]

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.[17] Alcohol use, however, increases the risk of other cancers.

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.[6]

Orders

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%.

References