Anaplastic thyroid cancer is one of the rarest but most aggressive forms of cancer affecting the thyroid gland. This disease grows exceptionally fast and often spreads to other parts of the body before it is even diagnosed. While it accounts for only a small percentage of all thyroid cancers, it requires urgent medical attention and specialized treatment. Understanding this condition can help patients and their families navigate this challenging diagnosis.

Epidemiology

Anaplastic thyroid cancer, also called anaplastic thyroid carcinoma (ATC), is extremely rare. It makes up less than 2 percent of all thyroid cancer cases diagnosed worldwide. This means that out of every hundred people who develop thyroid cancer, fewer than two will have this particular aggressive type.^[1][2]

Despite being rare, anaplastic thyroid cancer is responsible for a disproportionate number of deaths from thyroid cancer. While it represents only 1 to 2 percent of cases, it causes between 20 and 50 percent of all thyroid cancer deaths, making it one of the most lethal forms of the disease.^[6] In the United States, slightly more than 1,000 new cases are diagnosed each year, and approximately 600 new cases occur annually based on different reporting estimates.^[5][8]

This cancer occurs most commonly in older adults. The average age at diagnosis is around 60 years, and it is most frequently seen in people over the age of 60. Some sources indicate it can occur in individuals ages 40 to 70, but it is notably uncommon in younger people.^[1][2][4] Women are affected more often than men, although papillary thyroid cancer shows an even stronger female predominance. The gender difference in anaplastic thyroid cancer is somewhat less pronounced than in other thyroid cancer types.^[8]

The incidence of anaplastic thyroid cancer has remained relatively constant over the past several decades. Unlike some other thyroid cancers, which have seen increases in diagnosis rates, anaplastic thyroid cancer rates have not changed significantly. This consistency suggests that better diagnostic techniques have not increased detection of this aggressive cancer type.^[2]

Geographically, anaplastic thyroid cancer tends to be more common in areas where people have insufficient iodine in their diets. These iodine-deficient regions often have higher rates of thyroid problems in general, including goiters and various types of thyroid cancer.^[8]

Causes

Scientists do not know the exact cause of anaplastic thyroid cancer. Despite extensive research into this aggressive disease, the specific triggers that cause normal thyroid cells to transform into anaplastic cancer cells remain unclear. Researchers continue to study what may cause this transformation, but definitive answers are still being sought.^[1][2]

One important observation is that anaplastic thyroid cancer often develops in people who already have other types of thyroid cancer. In about half of all cases, anaplastic thyroid cancer occurs alongside pre-existing differentiated thyroid cancer, such as papillary or follicular thyroid cancer. This suggests that many anaplastic thyroid cancers may actually start as a less aggressive form of thyroid cancer that later transforms into the more dangerous anaplastic type. When examined under a microscope, between 20 and 50 percent of anaplastic thyroid cancer samples show evidence of coexisting differentiated thyroid cancer.^[2][6]

This transformation process is called dedifferentiation. In this process, cancer cells that once looked and behaved somewhat like normal thyroid cells lose those characteristics and become highly abnormal and aggressive. The cells no longer have the organized structure or function of typical thyroid cells, which is why anaplastic thyroid cancer is also called undifferentiated thyroid cancer.^[1][2]

Anaplastic thyroid cancer generally occurs in areas where people have had long-standing thyroid problems. A history of pre-existing goiter (an enlarged thyroid gland) or other thyroid disease increases the likelihood of developing this cancer. The connection between chronic thyroid conditions and anaplastic thyroid cancer suggests that ongoing irritation or abnormal growth in the thyroid gland may contribute to cancer development over time.^[8]

Risk Factors

Several factors have been identified that increase a person’s risk of developing anaplastic thyroid cancer. While having one or more risk factors does not mean someone will definitely develop the disease, understanding these factors can help with early detection and awareness.

Age is one of the most significant risk factors. Being older than 60 years substantially increases the risk of developing anaplastic thyroid cancer. The vast majority of cases occur in people who are in their 60s or older. This age-related risk is important because it helps doctors know which patients may need more careful monitoring of thyroid abnormalities.^[1][2]

Gender also plays a role, as being female increases the risk. Women are more likely than men to develop anaplastic thyroid cancer, although the difference is not as dramatic as it is for other types of thyroid cancer. This gender difference may be related to hormonal factors, though the exact mechanism is not fully understood.^[1]

Having a history of other thyroid cancers is a major risk factor. People who have previously been diagnosed with papillary or follicular thyroid cancer are at higher risk of developing anaplastic thyroid cancer. This is because anaplastic cancer can arise from these less aggressive cancer types through the dedifferentiation process described earlier. This means that anyone with a previous thyroid cancer diagnosis should remain vigilant about new or changing symptoms in the thyroid area.^[1][2]

A long-standing goiter or history of thyroid disease increases risk. People who have had an enlarged thyroid gland for many years or who have chronic thyroid conditions are more susceptible to developing anaplastic thyroid cancer. The longer someone has had thyroid problems, the greater the accumulated risk may be.^[1][4]

Radiation exposure to the chest or neck area is another recognized risk factor. People who have received radiation treatment to the neck region, whether for medical treatment of other conditions or through environmental exposure, have an increased risk. This includes radiation therapy given during childhood or adolescence for other medical problems. The risk from radiation exposure can persist for many years after the exposure occurred.^[4][6]

Living in iodine-deficient areas may increase risk. Populations that do not get enough iodine in their diet through food or water have higher rates of various thyroid problems, including anaplastic thyroid cancer. Iodine is essential for normal thyroid function, and chronic deficiency can lead to thyroid abnormalities that may eventually become cancerous.^[8]

Symptoms

Anaplastic thyroid cancer typically causes noticeable symptoms that develop rapidly, often over just a few weeks. This is very different from other types of thyroid cancer, which usually grow slowly and may not cause any symptoms for a long time. The fast-growing nature of anaplastic thyroid cancer means symptoms often appear suddenly and worsen quickly.

The most common initial symptom is a rapidly enlarging lump or mass in the front of the neck. This growth usually appears in a matter of days to weeks, and both patients and family members can typically see and feel it. The lump is usually hard, like a rock, and may be painful or tender to the touch. Sometimes the overlying skin becomes red or swollen. Unlike many thyroid nodules that are discovered incidentally, anaplastic thyroid cancer lumps are hard to ignore because of their size and rapid appearance.^[1][4][6]

As the tumor grows larger, it begins to press on structures in the neck, causing what doctors call compressive symptoms. These symptoms result from the cancer physically squeezing or pushing against nearby organs and tissues. Patients may experience difficulty swallowing, with food or pills getting “stuck” when trying to swallow. This happens because the growing tumor presses against the esophagus, which is the tube that carries food from the mouth to the stomach.^[1][4]

Breathing problems are another common and concerning symptom. As the tumor grows, it can compress the trachea, which is the windpipe that carries air to the lungs. This compression causes difficulty breathing, especially when lying flat. Some patients develop loud breathing, which doctors call stridor—a harsh, high-pitched sound that occurs when the airway is partially blocked. In severe cases, the airway can become so compressed that it becomes a medical emergency requiring immediate treatment.^[1][5]

Changes in voice are frequent symptoms. Many patients develop hoarseness or notice that their voice sounds different. This occurs when the tumor affects the nerves that control the vocal cords, called the recurrent laryngeal nerves. In some cases, these nerves can become paralyzed, leading to vocal cord paralysis, which makes speaking difficult and can contribute to breathing problems.^[1][4]

A persistent cough that does not go away is another possible symptom. This cough is not related to a cold or infection but is caused by irritation from the tumor pressing on the airway. Some patients may also cough up blood, though this is less common and usually indicates more advanced disease.^[1][5]

Neck pain can occur, particularly as the tumor grows larger and invades surrounding tissues. The pain may be constant or may worsen with certain movements or when swallowing. This is different from the occasional neck stiffness or soreness that many people experience, as cancer-related pain tends to be more persistent and progressive.^[5]

When the cancer has spread beyond the neck to other parts of the body, additional symptoms may appear. Swollen lymph nodes in the neck indicate that cancer has spread to nearby lymphatic tissue. Bone pain can occur if the cancer has spread to bones. Neurological issues may develop if cancer spreads to the brain or spinal cord. General symptoms like weakness and fatigue may also be present, especially as the disease progresses.^[1][5]

Prevention

Unfortunately, there are no proven methods to prevent anaplastic thyroid cancer specifically. Because scientists do not fully understand what causes this cancer to develop, it is difficult to recommend specific prevention strategies that would guarantee protection against it. However, there are some general approaches that may help reduce risk or enable earlier detection.

One important preventive measure is ensuring adequate iodine intake. Since anaplastic thyroid cancer is more common in iodine-deficient areas, making sure you get enough iodine in your diet may help reduce overall thyroid disease risk. In many countries, table salt is fortified with iodine specifically to prevent iodine deficiency. Eating iodine-rich foods like seafood, dairy products, and eggs can also help maintain healthy iodine levels. However, people should not take iodine supplements without medical supervision, as too much iodine can also cause thyroid problems.

For people who have been diagnosed with other types of thyroid cancer, particularly papillary or follicular thyroid cancer, close monitoring and appropriate treatment are essential. Since anaplastic thyroid cancer can arise from these less aggressive cancer types, properly treating the initial cancer and following up regularly with healthcare providers may help prevent transformation to anaplastic cancer. This includes following recommended treatment plans, taking prescribed thyroid hormone medications, and attending all scheduled follow-up appointments.

Minimizing unnecessary radiation exposure to the neck and chest area is advisable when possible. While medical radiation is sometimes necessary for treating certain conditions, avoiding unnecessary radiation exposure may help reduce risk. People who need radiation therapy should discuss with their doctors the benefits versus risks and whether protective measures for the thyroid can be used.

Being aware of risk factors and symptoms is crucial for early detection. People who have a long-standing goiter or history of thyroid disease should be vigilant about any sudden changes in their thyroid area. If a lump that has been stable for years suddenly starts growing rapidly, this warrants immediate medical evaluation. Similarly, anyone who develops new neck masses, especially if they are hard and growing quickly, should seek medical attention promptly.

Regular health check-ups with a primary care provider can help identify thyroid problems early. During routine physical examinations, doctors often feel the neck to check for thyroid enlargement or lumps. If any abnormalities are detected, further testing can be done to determine their nature. Early detection of thyroid problems, even if they are not yet cancerous, allows for monitoring and intervention if needed.

Pathophysiology

Anaplastic thyroid cancer causes profound changes in how the thyroid gland and surrounding tissues function. Understanding what happens in the body when this cancer develops helps explain why it is so aggressive and why it causes the symptoms it does.

The cancer begins when cells in the thyroid gland undergo abnormal changes. These cells lose their normal structure and appearance, becoming what doctors call undifferentiated. This means the cancer cells no longer look like or behave like typical thyroid cells. Under a microscope, anaplastic thyroid cancer cells appear highly abnormal, with irregular shapes and sizes. They divide and multiply at an extremely rapid rate, much faster than normal cells or even other types of cancer cells.^[1][2]

The cells of anaplastic thyroid cancer have a very high mitotic rate, meaning they are constantly dividing to create new cancer cells. This rapid cell division is what causes the tumor to grow so quickly—sometimes visibly enlarging over days or weeks. The cancer tissue also shows extensive areas of dead cells, called necrosis, because the cells grow so fast that they outgrow their blood supply.^[6]

At the molecular level, anaplastic thyroid cancer cells accumulate multiple genetic mutations that drive their aggressive behavior. These cells often have changes in genes that normally control cell growth, cell death, and DNA repair. Common genetic alterations include mutations in genes called BRAF, RAS, TP53, PIK3CA, and others. These mutations work together to make the cancer cells grow uncontrollably, resist normal signals that would tell them to stop dividing or die, and evade the body’s immune system.^[3][6]

One particularly important genetic change involves a gene called TP53. This gene normally acts as a “guardian” that prevents damaged cells from becoming cancerous. When TP53 is mutated, as it often is in anaplastic thyroid cancer, cells lose this protective mechanism and can accumulate more DNA damage without dying. This contributes to the cancer’s aggressive nature and resistance to treatment.^[8]

The cancer is highly locally invasive, meaning it aggressively grows into nearby structures. Anaplastic thyroid cancer does not simply push surrounding tissues aside as it grows; instead, it actively invades and destroys them. The cancer commonly grows into muscles, the trachea (windpipe), the esophagus (food pipe), blood vessels, and nerves in the neck. This invasion explains many of the symptoms patients experience, such as difficulty breathing when the trachea is invaded or voice changes when nerves to the vocal cords are affected.^[3][8]

The cancer also spreads through the blood vessels and lymphatic system. It has a strong tendency to spread to lymph nodes in the neck early in its course. Cancer cells break off from the main tumor and travel through lymphatic channels to nearby lymph nodes, where they establish new areas of cancer growth. This is why swollen lymph nodes are a common finding in patients with anaplastic thyroid cancer.^[3]

Distant spread of the cancer, called metastasis, occurs frequently and early. In more than 50 percent of cases, the cancer has already spread to distant parts of the body by the time it is diagnosed. The most common sites of distant spread are the lungs and bones, though the cancer can spread to virtually any organ. Cancer cells travel through the bloodstream to these distant sites and establish new tumors there.^[1][8]

Within the tumor itself, there is often evidence of inflammation. The body’s immune system recognizes the cancer as abnormal and sends immune cells, such as T cells and macrophages (cells that try to destroy foreign material), to the tumor site. However, despite this immune response, the cancer cells are able to evade destruction. In fact, the cancer cells often express high levels of a protein called PD-L1 on their surface, which helps them hide from immune system attacks.^[6]

Blood vessels within and around the tumor are also abnormal. The rapidly growing cancer stimulates the formation of new blood vessels, a process called angiogenesis, to supply itself with nutrients and oxygen. However, these blood vessels are poorly formed and leaky, which contributes to the chaotic growth pattern of the cancer and can make treatment more challenging.

The cancer disrupts normal thyroid hormone production. Although thyroid function tests are usually normal at diagnosis, the cancer cells do not produce thyroid hormones the way normal thyroid cells do. The cancer tissue replaces functional thyroid tissue, and if enough of the gland is destroyed or removed during treatment, patients will need thyroid hormone replacement medication for the rest of their lives.^[5]

Because of all these changes—rapid uncontrolled growth, local invasion, early spread to lymph nodes and distant organs, genetic instability, and resistance to normal growth controls—anaplastic thyroid cancer behaves very differently from other thyroid cancers. These pathophysiological features make it one of the most aggressive and difficult-to-treat cancers known to medicine.