Thyroid Cancer Facts: What You Need to Know
Thyroid cancer is one of the more commonly diagnosed cancers in the United States. The American Cancer Society estimates that about 45,240 people will be diagnosed with thyroid cancer in 2026, and about 2,320 people will die from it. Thyroid cancer is diagnosed at a younger average age than most other adult cancers, with the typical age at diagnosis around 51. It is also almost three times more common in women than in men, for reasons researchers do not fully understand. If you or someone you love just heard the words "thyroid cancer," you are far from alone, and there is a large, active community of survivors and specialists ready to help.
One of the most important things to understand right away is that "thyroid cancer" is not one single disease. It is a group of several distinct cancer types that all start in the thyroid gland (a small gland in your neck) but behave in very different ways. Some grow slowly and respond very well to treatment. Others grow quickly and need urgent, specialized care. Knowing which type you or your family member has changes almost everything about what comes next, from the treatment plan to the outlook.
The good news is that most thyroid cancer, specifically the papillary and follicular types (together called differentiated thyroid cancer, or DTC), tends to have an excellent prognosis. Many people treated for these types go on to live long, full lives. At the same time, this guide wants to be honest with you: a good prognosis does not mean this diagnosis is small or easy. Living with a cancer diagnosis, a surgical scar on your neck, lifelong medication, or ongoing worry about recurrence is real and can be hard, even when your doctor tells you the outlook is good. Your feelings about this diagnosis are valid no matter how "treatable" your cancer type is said to be.
Two other types of thyroid cancer, medullary thyroid carcinoma and anaplastic thyroid carcinoma, work very differently from papillary and follicular thyroid cancer. They come from different cells, are found and treated differently, and carry different outlooks. Because they are so different, this guide covers what makes them unique in clearly marked sections throughout, and HealthTree Foundation covers them in more depth in other guides in this series. If you have been told you have medullary or anaplastic thyroid cancer, look for those specially marked sections, since general information about "thyroid cancer" often does not apply to you in the same way.
This guide walks through the basics: what the thyroid gland does, how thyroid cancer starts and spreads, the different types you may hear about, related conditions that are not cancer, and the genetic and lifestyle factors linked to risk. For more on symptoms, diagnosis, staging, and treatment, see the other guides in this series, including thyroid cancer symptoms, how thyroid cancer is diagnosed, and thyroid cancer treatment.
What Is the Thyroid?
The thyroid is a small, butterfly-shaped gland located at the base of the front of your neck, just below your Adam's apple (the thyroid cartilage). Most people cannot see or feel their own thyroid gland when it is healthy. It has two halves, called lobes, one on the right side of your windpipe and one on the left. A thin strip of tissue called the isthmus connects the two lobes, which is what gives the gland its butterfly shape.
Even though it is small, the thyroid has a big job. It makes two thyroid hormones, called T3 and T4 (their full chemical names are triiodothyronine and thyroxine), using iodine that your body absorbs from food. These hormones travel through your blood to nearly every tissue in your body. They help control your metabolism (how your body turns food into energy), your heart rate, your body temperature, and your overall energy level. Too much or too little thyroid hormone can affect how you feel every single day, from your weight to your mood to how tired you feel.
The thyroid does not decide on its own how much hormone to make. That job belongs to the pituitary gland, a pea-sized gland at the base of your brain. The pituitary gland releases a hormone called TSH (thyroid-stimulating hormone), which tells the thyroid how much T3 and T4 to produce. This feedback system is why doctors often check TSH levels first when checking on thyroid health, and why TSH becomes such an important number to track after thyroid cancer treatment.
The thyroid gland is made up mostly of follicular cells, which are the cells responsible for absorbing iodine and making thyroid hormone. But the thyroid also contains a separate, much smaller population of cells called C cells, also known as parafollicular cells. Instead of making thyroid hormone, C cells make a different hormone called calcitonin (a hormone that helps regulate calcium levels in the blood and can act as a tumor marker for certain thyroid cancers).
This distinction between follicular cells and C cells matters enormously for understanding thyroid cancer. Papillary thyroid carcinoma, follicular thyroid carcinoma, and anaplastic thyroid carcinoma all arise from follicular cells or their descendants. Medullary thyroid carcinoma is different. It arises from C cells, not follicular cells. That single difference explains many of the ways medullary thyroid cancer is diagnosed, monitored, and treated differently from the other types, which you will see repeated throughout this guide.
(Source: American Cancer Society)
How Does Thyroid Cancer Start?
In a healthy body, cells grow, divide, and die in an orderly, controlled way. Cancer starts when something goes wrong in that process, usually a change (called a mutation) in the DNA of a cell that tells it to keep growing and dividing when it should not. Over time, these abnormal cells can build up into a tumor. In the thyroid, this process most often begins in follicular cells, which is why papillary and follicular thyroid carcinoma make up the large majority of thyroid cancer cases.
Researchers have identified several key molecular drivers behind papillary thyroid carcinoma. The most common is a mutation called BRAF V600E, which is found in a large share of papillary thyroid cancers and pushes cells to grow uncontrollably. Another important driver is a RET/PTC rearrangement, where pieces of DNA get rearranged in a way that activates growth signals inside the cell. Follicular thyroid carcinoma tends to have a different molecular fingerprint, most often involving RAS gene mutations, along with rearrangements involving the PAX8 and PPAR gamma genes in some cases.
It is important to know that for the vast majority of people, thyroid cancer is not inherited. Most cases happen because of these random, acquired mutations rather than a gene passed down from a parent. Only a small portion of thyroid cancer cases are linked to inherited genetic syndromes, which are covered later in this guide.
How Medullary Thyroid Carcinoma Starts Differently
Medullary thyroid carcinoma works differently from the very beginning. Instead of starting in follicular cells, it starts in C cells, the calcitonin-producing cells described above. The central driver of medullary thyroid carcinoma is a mutation in the RET gene, the same gene involved in some papillary thyroid cancers, but affecting a different cell type and behaving differently. Unlike most thyroid cancers, a meaningful share of medullary thyroid carcinoma, roughly one quarter of cases, is inherited and linked to a RET mutation passed down through families. This is one of the most important facts to understand if you or a family member has been diagnosed with medullary thyroid cancer, since it changes how the rest of the family may need to be evaluated. Genetic testing and counseling are discussed in more detail later in this guide.
A Note on Anaplastic Thyroid Carcinoma
Anaplastic thyroid carcinoma often does not start from scratch. Research suggests that many anaplastic thyroid cancers actually develop from an existing papillary or follicular thyroid cancer that acquires additional mutations over time, such as mutations in the TERT promoter region, and becomes far more aggressive and fast-growing as a result. This process, sometimes called dedifferentiation, is one reason anaplastic thyroid carcinoma is treated as such a different and urgent disease from the differentiated thyroid cancers it can arise from.
Where Does Thyroid Cancer Start?
Thyroid cancer usually starts as a single nodule (a lump or growth) in one lobe of the thyroid gland, though it can occasionally involve both lobes. Most people never feel this nodule themselves. It is far more often found during a routine neck exam by a doctor or discovered unexpectedly on an imaging test, such as a CT scan or ultrasound, done for a completely unrelated reason.
For differentiated thyroid cancer (papillary and follicular thyroid carcinoma), the most common pattern of spread is to the lymph nodes in the neck, especially for papillary thyroid cancer. This can sound alarming, but it is important to know that even when papillary thyroid cancer has spread to nearby lymph nodes, the outlook generally remains very good. Less often, differentiated thyroid cancer, particularly follicular thyroid carcinoma, can spread through the bloodstream to more distant sites such as the lungs or bones.
Because differentiated thyroid cancer tends to grow slowly, it is often caught before it has spread very far, which is one reason the overall outlook for this group of cancers is favorable. Your care team will use imaging tests and, if needed, biopsies of any suspicious lymph nodes to understand exactly where your cancer is at the time of diagnosis. You can learn more about this process in how thyroid cancer is diagnosed and how the results are used for thyroid cancer stages.
How Medullary and Anaplastic Thyroid Cancer Spread Differently
Medullary thyroid carcinoma has its own pattern of spread. Because it can be more aggressive than differentiated thyroid cancer, it is somewhat more likely to have already reached the lymph nodes or, less commonly, distant organs like the liver, lungs, or bones by the time it is diagnosed. This is one reason genetic testing and careful staging are so important for everyone diagnosed with medullary thyroid cancer.
Anaplastic thyroid carcinoma behaves quite differently from either of the other types. It tends to grow and invade nearby neck structures very quickly and often spreads to distant parts of the body early in its course, most commonly the lungs. Because of this aggressive behavior, anaplastic thyroid carcinoma is treated as a medical emergency that requires urgent evaluation by a multidisciplinary care team as soon as it is suspected.
How Does Thyroid Cancer Affect the Body?
The most common way thyroid cancer affects the body is by causing a lump or nodule that grows within the thyroid gland itself. Most people do not feel pain or discomfort from this. If a nodule grows large enough, it can occasionally press on nearby structures in the neck, which may cause difficulty swallowing, a feeling of tightness, or, less commonly, a change in voice if the tumor affects the nearby nerve that controls the vocal cords (the recurrent laryngeal nerve).
Beyond the thyroid itself, the most common site of spread for differentiated thyroid cancer is the lymph nodes in the neck. This is common enough that it is considered a normal part of many papillary thyroid cancer diagnoses and does not necessarily change the excellent long-term outlook for most patients. Less commonly, thyroid cancer can spread further to the lungs or to bone, which your care team will look for during staging, especially if your cancer is a more aggressive type.
One detail that surprises many patients is that having thyroid cancer usually does not affect how you feel day to day before treatment. Because thyroid cancer is typically confined to one small area of the gland rather than involving the entire thyroid, most people with thyroid cancer have completely normal thyroid hormone levels and do not feel hormonally unwell. Blood tests like TSH are often normal even when a cancer is present, which is part of why regular neck exams and imaging, rather than blood tests alone, are so important for finding thyroid nodules.
This can feel confusing for patients who expect to "feel sick" from a cancer diagnosis. It is completely normal to feel physically fine while still processing a serious diagnosis, and that mismatch between how you feel physically and how you feel emotionally is a common experience worth talking about with your care team or a support community. You can read more in thyroid cancer symptoms.
After treatment, especially if your entire thyroid gland is removed, your body will need thyroid hormone medication for the rest of your life, since you can no longer make it on your own. This lifelong medication need, along with regular monitoring, becomes an important ongoing part of life after thyroid cancer, which is covered in more detail in the treatment guides in this series.
Types of Thyroid Cancer
Thyroid cancer includes several distinct subtypes, and knowing which one you have is one of the most important pieces of information in your diagnosis. Here is an overview of each.
Papillary thyroid carcinoma: The most common type of thyroid cancer, making up about 8 out of 10 cases (roughly 80%). It usually grows slowly, often spreads to lymph nodes in the neck, and generally has an excellent long-term outlook. There are several subtypes, called variants, including the follicular variant (the most common, with a similarly good outlook) and less common, faster-growing variants such as tall cell, columnar, and diffuse sclerosing.
Follicular thyroid carcinoma: The second most common type, making up roughly 1 out of 10 thyroid cancers (about 10% to 15%). It is more likely than papillary thyroid cancer to spread through the bloodstream to distant sites like the lungs or bones, though its overall outlook remains good in most cases. Oncocytic carcinoma, previously called Hurthle cell carcinoma, is a variant of follicular thyroid cancer that makes up about 3% of thyroid cancers and can be harder to find and treat.
Medullary thyroid carcinoma (MTC): A less common type, making up roughly 1% to 2% of thyroid cancers (some estimates go as high as 5%). Unlike the types above, MTC arises from C cells rather than follicular cells, which means it is not treated with radioactive iodine and is monitored using different tumor markers, calcitonin and CEA, rather than thyroglobulin. About one quarter of MTC cases are inherited.
Poorly differentiated thyroid carcinoma: An uncommon, intermediate category that falls between differentiated thyroid cancer and anaplastic thyroid cancer in how it behaves under the microscope and clinically. It tends to grow and spread more aggressively than papillary or follicular thyroid cancer but is not as aggressive as anaplastic thyroid cancer.
Anaplastic thyroid carcinoma (ATC): A rare type, making up roughly 1% to 2% of thyroid cancers, but one that is far more aggressive than the others. It grows quickly, is considered at least stage IV at diagnosis regardless of size, and requires urgent, specialized treatment. Despite being rare, it accounts for a disproportionate share of thyroid cancer deaths.
Thyroid lymphoma: A distinct and uncommon type of cancer that starts in immune system cells within the thyroid rather than in thyroid hormone-producing cells. It is treated very differently from the other types described here, generally with chemotherapy and sometimes radiation rather than the surgery-based approach used for most thyroid cancers.
Related Conditions to Thyroid Cancer
Many people land on this page after a thyroid nodule was found on a routine exam or an imaging test done for something else entirely, and the word "nodule" can feel just as scary as the word "cancer." It helps to know that thyroid nodules are extremely common. By age 60, about half of all people have a thyroid nodule that can be found through examination or imaging, and over 90% of these nodules turn out to be benign (not cancerous). When a biopsy is done, a benign result is far more common than a cancerous one.
Goiter is another common, usually benign condition in which the thyroid gland becomes enlarged, sometimes because of iodine deficiency, an autoimmune condition, or simply an overgrowth of thyroid tissue over time. A goiter can occur with normal, high, or low thyroid hormone levels, and most goiters are not cancerous, though your doctor may still recommend imaging or biopsy of specific nodules within an enlarged gland.
Hashimoto thyroiditis is an autoimmune condition in which the immune system gradually attacks the thyroid gland, most often leading to an underactive thyroid (hypothyroidism) over time. It is the most common cause of hypothyroidism and is also associated with a higher chance of developing thyroid nodules. Graves disease is a different autoimmune condition in which the immune system overstimulates the thyroid, leading to an overactive thyroid (hyperthyroidism). Both Hashimoto thyroiditis and Graves disease are benign thyroid conditions, not cancer, though they do require their own ongoing medical management.
Thyroid adenoma, including follicular adenoma, is a benign (noncancerous) growth of thyroid tissue that can look and feel similar to a cancerous nodule, which is exactly why nodules need proper evaluation with ultrasound and, often, a needle biopsy rather than assumptions based on how they feel. Follicular adenoma is actually more common than follicular carcinoma.
The overall message here is one of reassurance grounded in real numbers: the great majority of thyroid nodules and thyroid conditions you encounter, in yourself or when researching online, are not cancer. Still, because differentiating benign from cancerous nodules cannot be done by feel alone, proper evaluation matters, and you should always follow through on your doctor's recommended workup rather than assuming a lump is nothing.
What Are the Genetic and Risk Factors?
Radiation exposure is one of the most well-established risk factors for thyroid cancer, particularly exposure that happens during childhood. This includes radiation therapy used to treat other cancers, such as Hodgkin lymphoma, when the treatment involves the head, neck, or chest, as well as radiation exposure from nuclear accidents or weapons testing. The risk depends on the dose and the age at exposure, with younger children generally facing higher risk. Everyday medical imaging like dental X-rays, mammograms, and routine chest X-rays carries only very low radiation doses and is not considered a meaningful risk factor.
Having a first-degree relative (a parent, sibling, or child) with thyroid cancer increases your own risk, even when there is no known inherited syndrome running in the family. That said, most people diagnosed with thyroid cancer do not have a family history of the disease. A small number of inherited genetic syndromes are also linked to a higher risk of thyroid cancer. Cowden syndrome, caused by changes in the PTEN gene, raises the risk of papillary or follicular thyroid cancer along with breast, endometrial, and kidney cancers. Familial adenomatous polyposis, including its variant Gardner syndrome, is caused by changes in the APC gene and raises the risk of papillary thyroid cancer along with a very high risk of colon cancer. Carney complex, caused by changes in the PRKAR1A gene, raises the risk of papillary and follicular thyroid cancer, often at a younger age, along with distinctive skin changes.
The evidence on iodine and thyroid cancer risk is mixed and worth describing honestly rather than oversimplifying. Iodine deficiency has been linked to a higher risk of follicular thyroid carcinoma, which is part of why this type is more common in parts of the world with low dietary iodine. On the other hand, some research suggests that a diet very high in iodine may be linked to a higher risk of papillary thyroid carcinoma. In the United States, where iodine is added to table salt, and most people get enough of it through their diet, dramatic swings in either direction are uncommon, and there is no evidence that specific supplements or extreme diet changes can meaningfully lower your thyroid cancer risk.
Female sex and age are also well-documented risk factors. Thyroid cancer is almost three times more common in women than in men, for reasons that are not fully understood, and it tends to occur at a somewhat younger average age than most other adult cancers. Excess body weight has also been associated with a modestly increased risk of thyroid cancer in research studies.
Medullary Thyroid Carcinoma Genetics
Genetics deserve special attention for medullary thyroid carcinoma, because roughly one quarter of all MTC cases are hereditary and linked to mutations in the RET gene. These inherited cases fall into a few related conditions: multiple endocrine neoplasia type 2A (MEN2A), multiple endocrine neoplasia type 2B (MEN2B), and familial medullary thyroid carcinoma (a form in which MTC occurs without the other tumors seen in MEN2A or MEN2B). Because the specific RET mutation a person carries is linked to how early and how aggressively MTC tends to develop, genetic testing is a real, actionable recommendation for every single person diagnosed with medullary thyroid cancer, not an optional extra step. If a RET mutation is found, genetic counseling can help you understand what it means for you and can guide testing recommendations for your family members, since finding a mutation early in a relative can lead to lifesaving monitoring or preventive treatment. Your specialist care team, often including both an endocrinologist and a genetic counselor, will set the exact testing and monitoring plan based on your specific mutation.
What Are the Related Cancers to Thyroid Cancer?
For people with MEN2A, the same RET mutation that raises the risk of medullary thyroid carcinoma also raises the risk of two other tumors: pheochromocytoma (a tumor of the adrenal gland that can cause high blood pressure, headaches, and a racing heartbeat) and parathyroid gland tumors, which can cause the parathyroid glands to make too much parathyroid hormone and raise blood calcium levels. In practical terms, this means that if you or a family member is diagnosed with MEN2A, your care team will want to screen regularly for these other tumors as well, not just monitor the thyroid.
For people with MEN2B, pheochromocytoma is also a risk, along with a distinctive set of physical features sometimes called a Marfanoid body type, including long arms and a high arched foot, as well as small growths called mucosal neuromas that can appear on the lips, tongue, and lining of the eyes and intestines. MEN2B tends to cause medullary thyroid carcinoma earlier in life and more aggressively than MEN2A, which is why early genetic testing and, when appropriate, preventive thyroid surgery are discussed so early for children in these families.
For people with Cowden syndrome, the related cancer risks extend beyond the thyroid to include breast cancer, endometrial cancer, and kidney cancer, along with a higher chance of developing more than one cancer over a lifetime. For people with familial adenomatous polyposis, the primary related cancer risk is colorectal cancer, which is typically the more urgent concern requiring its own screening and management alongside thyroid monitoring.
What this means practically is that a thyroid cancer diagnosis linked to one of these inherited syndromes is rarely just about the thyroid. If your care team suspects an inherited syndrome based on your personal or family history, genetic counseling and testing can identify the syndrome so that you and your blood relatives can be screened appropriately for these related tumors and cancers, often starting at a younger age than average risk guidelines would suggest. This is exactly the kind of decision that should be made with a specialist team, including a genetic counselor, rather than navigated alone. Always talk with your own care team about what your specific test results and family history mean for you.
(Source: American Thyroid Association)