Review article based on reporting by Jasmin Fox-Skelly, BBC Future (2025) © BBC.
Thyroid cancer is rising faster than many other cancers worldwide, especially in the United States, where incidence has more than tripled since the 1980s. Once considered an uncommon endocrine malignancy, thyroid cancer has become a global epidemiological puzzle — prompting researchers to ask whether we are detecting more cases or actually causing more disease.
A Silent but Rising Cancer
The thyroid gland controls metabolism, heart rate, blood pressure and body temperature. Although thyroid cancers are usually treatable, the speed of the rise — particularly in papillary thyroid cancers — has raised global concern.
US SEER data shows a sharp increase:
- Men: 2.39 → 7.54 per 100,000
- Women: 6.15 → 21.28 per 100,000
Childhood radiation exposure from disasters like Chernobyl is a known cause, but no equivalent events occurred in the US or many other countries. This led researchers to search for alternative explanations.
Theory 1: Overdiagnosis from Improved Imaging
From the 1980s, thyroid ultrasound became widely used, followed by fine needle aspiration biopsies in the 1990s. These tools enabled detection of extremely small thyroid nodules that would once have gone unnoticed.
This led to:
- Increased detection of small papillary cancers
- More diagnoses despite stable mortality
- Surges in thyroid cancer rates in countries with screening programmes
- Many unnecessary surgeries and radioactive iodine treatments
South Korea’s experience — a massive spike after screening began, then a sharp drop when screening ended — strongly supports the overdiagnosis theory.
In response, guidelines in the US now recommend:
- Watchful waiting for low-risk nodules
- Reducing unnecessary thyroid removal
- Limiting radioactive iodine treatment to aggressive cancers
Recent US data shows thyroid cancer incidence has begun to stabilise.
Theory 2: A Real Increase, Not Just Detection
However, many researchers argue that detection alone cannot explain global trends. Thyroid cancer is increasing even in countries with limited access to advanced imaging, and rates of larger and more aggressive tumours are also rising.
Multiple studies show:
- Increases in metastatic papillary cancers
- Increases in mortality in some regions
- Rising incidence in middle-income countries without extensive screening
This suggests a genuine biological increase in disease incidence.
Theory 3: Obesity as a Major Contributor
A growing body of evidence points to obesity as one of the key drivers. People with high BMI have:
- More than 50% greater lifetime risk of thyroid cancer
- Increased likelihood of aggressive tumours
- Higher thyroid cancer-related mortality
Possible mechanisms include:
- Chronic inflammation
- Insulin resistance
- Hormonal dysregulation
- Elevated thyroid-stimulating hormone (TSH), which promotes thyroid cell growth
Researchers believe obesity’s metabolic effects likely play an important role in cancer development.
Theory 4: Increased Exposure to Medical Radiation
Childhood exposure to ionising radiation is a strong established risk factor. Modern contributors may include:
- Increasing use of CT scans
- Higher cumulative radiation exposure in childhood
Modelling studies estimate that 3,500 future thyroid cancers per year in the US could be attributable to current CT scanning patterns.
Young thyroid tissue is especially vulnerable, making rising paediatric imaging a concern.
Theory 5: Endocrine-Disrupting Chemicals
Some scientists point to endocrine-disrupting chemicals (EDCs), such as PFOA and PFOS, found in:
- Cookware
- Packaging
- Personal care products
- Carpets
- Firefighting foams
These chemicals interfere with hormonal pathways. Evidence for a link with thyroid cancer is suggestive but inconsistent.
Theory 6: Trace Elements and Geographical Patterns
Certain island and volcanic regions show unusually high thyroid cancer rates. Hypotheses involve exposure to trace elements such as zinc, cadmium, and vanadium in volcanic soil and water.
However, evidence remains limited and requires further study.
A Multifactorial Global Trend
Consensus is shifting toward a multifactorial explanation. Rising thyroid cancer rates likely result from a combination of:
- Better detection
- Obesity
- Environmental exposures
- Ionising radiation from medical imaging
- Endocrine disruptors
- Possible genetic susceptibility
As endocrine surgeon Dr Sanziana Roman notes, current trends reflect a complex interplay of biological, environmental, and lifestyle factors.
A Mystery Still Unfolding
Although incidence is stabilising in some countries, the global rise remains unexplained in full. Continued research into environmental risks, metabolic health, radiation exposure, and trace elements will be essential to understanding and preventing thyroid cancer in the coming decades.
Source: Jasmin Fox-Skelly, BBC Future, “Why are thyroid cancer cases increasing across the world?”, 2025. © BBC.
