The main causes of thyroid cancer are not fully understood, but radiation exposure and certain genetic changes are the strongest known risk factors

Thyroid cancer develops when cells in the thyroid gland begin to grow abnormally. Unlike many cancers, thyroid cancer often has no single obvious cause. Most people who develop it have no family history and no known exposure to radiation. What researchers have found is that certain conditions and exposures make thyroid cancer more likely, though having a risk factor does not mean someone will definitely develop the disease.

The thyroid is a small gland in your neck that produces hormones controlling metabolism and growth. When cells in this gland accumulate genetic mutations—changes in their DNA—they can become cancerous. These mutations happen over time and are usually not inherited. Understanding what increases the risk helps explain why some people develop thyroid cancer while others do not.

Key Takeaways

  • Radiation exposure, especially in childhood, is the strongest established risk factor for thyroid cancer.
  • Most thyroid cancers develop from genetic mutations that occur during a person's lifetime, not mutations inherited from parents.
  • Certain inherited genetic syndromes, including familial adenomatous polyposis and Cowden syndrome, significantly increase thyroid cancer risk.
  • Hormonal factors like estrogen exposure and pre-existing thyroid disease may increase risk, though the evidence is less definitive than for radiation.
  • Having a risk factor does not mean thyroid cancer will develop; many people with risk factors never develop the disease.

Radiation exposure as the clearest risk factor

Radiation to the head, neck, or chest is the most well-established cause of thyroid cancer. This includes radiation from medical treatments, nuclear accidents, and atomic weapons testing. Children exposed to radiation are at higher risk than adults, because their thyroid cells divide more rapidly and are more vulnerable to damage. The risk increases with the dose of radiation received and decreases with time since exposure, though it can remain elevated for decades.

Medical sources of radiation include head and neck X-rays, CT scans of the chest or neck, and radiation therapy to treat other cancers. Atomic weapons testing in the 1950s and early 1960s exposed millions of people worldwide to radioactive iodine, which concentrates in the thyroid. The 1986 Chernobyl nuclear accident in Ukraine caused a sharp rise in thyroid cancer cases, particularly in children. Even small doses of radiation to the thyroid during childhood carry some increased risk.

Genetic mutations that develop over time

Most thyroid cancers result from genetic mutations that happen during a person's lifetime, not mutations inherited from parents. These mutations accumulate in thyroid cells as people age, which is why thyroid cancer becomes more common with age. The mutations that matter most are changes in genes called RET, BRAF, and TP53. When these genes are damaged, cells lose their normal controls on growth and division.

Researchers have identified specific mutation patterns in different types of thyroid cancer. Papillary thyroid cancer, the most common form, often has mutations in the BRAF gene or a rearrangement involving the RET gene. Follicular thyroid cancer frequently has mutations in genes controlling cell growth. These mutations are not present at birth; they develop in thyroid cells over time. Finding these mutations in a tumor helps doctors understand how aggressive it may be and guides treatment decisions.

Inherited genetic syndromes that increase risk

A small number of people inherit genetic changes that make thyroid cancer much more likely. Familial adenomatous polyposis (FAP) is an inherited condition where people develop hundreds of polyps in the colon and have a 10 to 20 percent lifetime risk of thyroid cancer. Cowden syndrome, caused by mutations in the PTEN gene, increases thyroid cancer risk and also increases risk for breast and other cancers. People with these syndromes often develop thyroid cancer at younger ages than those without them.

Familial medullary thyroid cancer is an inherited form where mutations in the RET gene are passed down through families. People who inherit this mutation have a very high risk of developing medullary thyroid cancer, a specific type that arises in cells producing calcitonin. Genetic testing can identify people at risk, and preventive surgery to remove the thyroid is often recommended before cancer develops. These inherited syndromes account for only a small percentage of all thyroid cancers, but they are important to identify because family members may also carry the mutation.

Hormonal factors and thyroid disease

Estrogen exposure appears to play a role in thyroid cancer risk, which may explain why women develop thyroid cancer more often than men. Pregnancy, hormone replacement therapy, and oral contraceptives all increase estrogen levels. Some studies show women who have been pregnant have higher thyroid cancer risk, and women taking hormone replacement therapy may have a slightly elevated risk. The mechanism is not fully clear, but estrogen may stimulate growth of thyroid cells or make existing mutations more likely to become cancerous.

People with pre-existing thyroid disease, including benign thyroid nodules and thyroid inflammation, have a modestly increased risk of thyroid cancer. However, having a thyroid nodule does not mean cancer will develop; most nodules remain benign. Thyroid disease itself does not cause cancer, but the conditions that lead to nodule formation may also increase the chance that a nodule becomes cancerous. Obesity has also been associated with higher thyroid cancer risk in some studies, though the reason is not yet understood.

Age and sex differences in risk

Thyroid cancer can occur at any age, but the risk increases with age. Most cases occur in people over 30, and the average age at diagnosis is around 50. However, when thyroid cancer does occur in children and young adults, it tends to be more aggressive and more likely to spread to lymph nodes. This may be because younger people have longer life expectancy, giving cancer more time to progress, or because their rapidly dividing cells are more vulnerable to radiation damage.

Women are diagnosed with thyroid cancer about three times more often than men. This difference is not fully explained by hormonal factors alone. Women may be more likely to have their thyroid examined for other reasons, leading to more cancers being found. Men who do develop thyroid cancer tend to have worse outcomes, possibly because their cancers are diagnosed at later stages. The reasons for these sex differences remain an active area of research.

Other factors under investigation

Researchers continue to study whether other exposures increase thyroid cancer risk. Iodine intake may influence which type of thyroid cancer develops—areas with low iodine intake tend to have more follicular cancers, while areas with adequate iodine have more papillary cancers. Occupational exposures to certain chemicals have been studied, but clear links have not been established. Some research suggests that previous benign thyroid surgery or thyroid biopsy may slightly increase risk, though this remains uncertain.

Viral infections have been investigated as a possible cause, but no virus has been definitively linked to thyroid cancer. Inflammation from autoimmune thyroid disease may increase risk slightly, but the association is weak. Obesity, diabetes, and metabolic syndrome have been associated with thyroid cancer in some studies, suggesting that metabolic factors may play a role. These associations are still being investigated, and it is not yet clear whether they represent direct causes or are related to other factors.

Frequently Asked Questions

If I have a family member with thyroid cancer, will I definitely get it too?

No. Most thyroid cancer is not inherited. Even if a family member had thyroid cancer, your risk is only higher if you share a specific inherited genetic mutation, which is rare. Genetic testing can determine whether you carry a mutation associated with familial thyroid cancer. If you do, your doctor can recommend screening or preventive measures.

Does having a thyroid nodule mean I will develop cancer?

No. Most thyroid nodules are benign and never become cancerous. Having a nodule does increase the chance that cancer will develop compared to people without nodules, but the absolute risk remains low. Your doctor can monitor nodules with ultrasound or recommend a biopsy if features suggest cancer is more likely.

Can thyroid cancer be prevented?

There is no proven way to prevent most thyroid cancers, since most result from mutations that happen by chance. If you have a family history of medullary thyroid cancer or inherited syndromes like FAP or Cowden syndrome, genetic testing and preventive surgery may be recommended. Limiting unnecessary radiation exposure, especially in childhood, is the only established prevention strategy.

Does radiation from cell phones or power lines cause thyroid cancer?

No. Cell phones and power lines emit non-ionizing radiation, which does not have enough energy to damage DNA or cause cancer. Only ionizing radiation—from X-rays, nuclear accidents, and radioactive materials—has been linked to thyroid cancer. The radiation from medical imaging and nuclear testing is the concern, not everyday sources like phones or power lines.

Why do women get thyroid cancer more often than men?

The reasons are not completely understood. Hormonal factors like estrogen likely play a role, since women's risk increases during reproductive years. Women may also be more likely to have their thyroid examined for other health reasons, leading to more cancers being detected. Men who develop thyroid cancer tend to have worse outcomes, possibly because their cancers are found at later stages.