What is thyroid cancer?

Thyroid cancer¹ is a disease that occurs when malignant (cancerous) cells form in the tissues of the thyroid.

The thyroid is a small, butterfly-shaped gland found in the front of the neck. This gland produces hormones that regulate numerous body functions, including heart rate, blood pressure, body temperature, and weight.

There are multiple forms of thyroid cancer, generally categorized as Well-Differentiated and Undifferentiated Thyroid Cancers (DTC and UTC). Most cases of thyroid cancer are curable with treatment.

Papillary thyroid cancer

Papillary thyroid cancer², considered a DTC, is the most common type of thyroid cancer. It is also one of the fastest-growing types of cancer, with over 20,000 new cases each year. This form of cancer arises from follicular cells, which produce and store thyroid hormones.

Most patients with papillary thyroid cancer will not die from the disease. Papillary thyroid cancer is the thyroid cancer with the best prognosis (likelihood of survival), and most patients can be cured of the disease if they receive the appropriate treatment in its early stages.

While papillary thyroid cancer can affect people of all ages, most patients present with the disease before the age of 40. In up to 20% of patients with papillary thyroid cancer, the lymph nodes (small glands that filter lymph, the clear fluid that circulates through the lymphatic system) will already be involved by the time of diagnosis. However, unlike other cancers, involved lymph nodes usually have almost no impact on thyroid cancer survival.

Follicular thyroid cancer

As its name suggests, follicular thyroid cancer³ also affects the thyroid’s follicular cells. This form of thyroid cancer is also categorized as a Well-Differentiated Thyroid Cancer (DTC).

Follicular cancer can spread to the lymph nodes in the neck, though this is much less common than in papillary cancer. Conversely, follicular cancer is more likely than papillary cancer to spread to distant organs (or metastasize), particularly to the lungs and bones.

Medullary thyroid cancer

Medullary thyroid cancer⁴ is different from other types of thyroid cancers in that it originates from the parafollicular C cells of the thyroid gland. These cells do not make thyroid hormone—instead, they make a different hormone called calcitonin. MTC frequently spreads to lymph nodes and can also spread to other distant organs.

MTC is likely to be inherited (run in a family) in up to 25% of diagnoses of the disease. Whether MTC is inherited or sporadic (not familial) can be determined by a blood test that detects mutations in the RET proto-oncogene. Inherited forms of MTC can be associated with other endocrine tumors in the syndromes Multiple Endocrine Neoplasia (MEN) 2A and 2B.

Anaplastic thyroid cancer

Anaplastic thyroid cancer⁵ is the most advanced and aggressive form of thyroid cancer. It is also one of the fastest-growing of all cancers. This form of thyroid cancer is also called undifferentiated thyroid cancer because the thyroid cells of people with this disease do not look or behave like typical thyroid cells.

The cause of anaplastic thyroid cancer is unknown. In some patients, it starts with differentiated thyroid cancers (such as papillary or follicular thyroid cancers). This form of thyroid cancer most commonly occurs in people over age 60.

How common is thyroid cancer?

Thyroid cancer is relatively uncommon compared to other cancers.

Women are 3 times more likely than men to develop thyroid cancer. According to the American Cancer Society⁶, there will be about 52,890 new cases of thyroid cancer (12,720 in men and 40,170 in women) and 2,180 deaths from thyroid cancer (1,040 men and 1,140 women) in the United States in 2020.

Papillary thyroid cancer, the most common form of the disease, makes up about 80% of all cases⁷ of thyroid cancer. It is the 8th most common cancer among women in general and the most common cancer in women younger than 25 years old.

Follicular thyroid cancer⁸ accounts for roughly 10–15% of all thyroid cancers in the United States. Medullary thyroid cancer makes up just 1–2% of cases⁹, while anaplastic thyroid cancer accounts for less than 2%¹⁰ of thyroid cancers.

Until recently, thyroid cancer was the most rapidly increasing cancer in the United States. This is largely due to improved technologies and more sensitive diagnostic procedures (like CT or MRI scans) detecting small thyroid cancers that might have gone unnoticed in the past.

Thyroid cancer causes

Thyroid cancer is more common¹¹ in people who have had a history of exposure to high doses of radiation, have a family history of thyroid cancer, and are older than 40 years of age. However, it is not clear exactly what causes¹² thyroid cancer in most patients.

Like any cancer, thyroid cancer occurs when cells undergo genetic changes (called mutations) and become malignant (cancerous). Mutated cells grow and multiply rapidly and lose the ability to die like normal cells. Accumulated abnormal cells form a tumor. They can also invade nearby tissues and spread throughout the body (metastasize).

Thyroid cancer risk factors

Certain risk factors¹³ may increase your risk of thyroid cancer. Some of these risk factors—like smoking—can be changed, while others (like age or family history) cannot.

Risk factors for thyroid cancer that cannot be changed include:

• Female sex: Women are 3 times more likely¹⁴ than men to develop thyroid cancer.
• Age: The risk of thyroid cancer peaks earlier for women (who are most often diagnosed in their 40s or 50s) than for men (usually diagnosed in their 60s or 70s).
• Inherited genetic syndromes: Some genetic syndromes that increase the risk of thyroid cancer include familial medullary thyroid cancer, Cowden’s disease, and multiple endocrine neoplasia.
• Family history: Having a first-degree relative—such as a parent, brother, sister, or child—with thyroid cancer increases your risk of the disease.

Some risk factors for thyroid cancer that can be changed include:

• Radiation exposure: Exposure to radiation is a proven risk factor for thyroid cancer. Examples of exposure to high levels of radiation include radiation treatments to the head or neck and fallout from sources like nuclear power plant accidents or weapons testing.
• Being overweight or obese: People who are overweight or obese have an increased risk for developing thyroid cancer. This risk appears to go up as one’s body mass index (BMI) increases.
• Iodine in the diet: While follicular thyroid cancers are more common in areas of the world where people’s diets are low in iodine, eating a diet that’s high in iodine may increase your risk for papillary thyroid cancer.

Thyroid cancer symptoms

Typically, early-stage thyroid cancer doesn’t cause any noticeable signs or symptoms. As cancer grows, however, it may cause:

• A lump that can be felt through the skin on the neck
• Changes in your voice, including hoarseness
• Difficulty swallowing
• Pain in the neck and throat
• Swollen lymph nodes in the neck

It is also possible to develop benign (non-cancerous) thyroid conditions. Sometimes, the thyroid gland becomes enlarged (called a goiter). Goiters can be diffuse (meaning that the whole thyroid gland is enlarged) or nodular (the gland is large and has one or more nodules, or bumps, in it).

Changes to a thyroid’s size and shape can often be felt—or even seen—by a patient or healthcare provider. Benign thyroid nodules can sometimes be left untreated and monitored with watchful waiting, as long as they aren’t growing larger or causing any uncomfortable symptoms. Other more severe cases may require medical treatment.

There are many reasons why a thyroid gland might become enlarged. Most of the time, an enlarged thyroid is not indicative of cancer.

Lumps or bumps (called nodules) may also grow of the thyroid gland. Most thyroid nodules are benign; roughly 2 or 3 out of 20 are malignant. Sometimes thyroid nodules make too much thyroid hormone, leading to hyperthyroidism. In most cases, these nodules are benign.

Most thyroid nodules are cysts filled with fluid or a storm form of the thyroid hormone colloid. Solid nodules with little fluid or colloid are more likely to be cancerous; however, most are benign.

If you experience any of the signs or symptoms of thyroid cancer or feel any changes in the size or shape of your thyroid, contact your healthcare provider. He or she will be able to determine whether your condition is benign and provide you with the best course of treatment, if necessary.

Spread of thyroid cancer

Cancer can spread¹⁵ from where it originated to other parts of the body. In this process (called metastasis), cancer breaks away from the primary tumor and travels through other body structures. There are three ways that cancer can spread:

• Through tissue: Cancer spreads by growing into nearby areas.
• Through the lymph system: Cancer enters the lymph system, traveling through the lymph vessels to other parts of the body.
• Through the blood: Cancer enters the blood, then travels through the body in blood vessels.

Metastatic tumors are always the same type of cancer as the primary tumor. If, for example, thyroid cancer cells spread to the lungs, the resulting disease is not lung cancer but is considered metastatic thyroid cancer (as the cells remain thyroid cancer cells no matter where they are).

After thyroid cancer has been diagnosed, tests can be conducted to determine whether cancer cells have spread within the thyroid or to other parts of the body. This testing process is called staging, as it is used to determine the stage of the disease (generally, Stage I to Stage IV, IVA, or IVB).

Thyroid cancer complications

The most common complication¹⁶ of thyroid cancer is recurrence (cancer comes back). Thyroid cancer can return after treatment, even if you’ve had your thyroid removed (thyroidectomy). This can occur when microscopic cancer cells spread beyond the thyroid before the gland is removed.

Thyroid cancer may recur in other parts of the body, but commonly returns in:

• The lymph nodes in the neck
• Small pieces of thyroid tissue left behind after surgery

Recurrent thyroid cancer can be treated. If your thyroid cancer does come back, your healthcare provider may recommend that you have periodic blood testing or thyroid scans to check for signs of recurrence.