Okay, let's talk about this issue that many people are concerned about.

Is Iodine Intake (Too Much or Too Little) Related to Thyroid Cancer?

Regarding the relationship between iodine and thyroid cancer, it's indeed quite complex. Simply put, there is a connection, but it's not the direct cause-and-effect relationship of "eating it causes cancer" that you might think. Iodine is to the thyroid what fuel is to a car engine – too little is bad, and too much can also cause problems.

Let's look separately at the two situations: "too little iodine" and "too much iodine."

1. Too Little Iodine: May Increase the Risk of Certain Types of Thyroid Cancer

You can think of iodine as the raw material for the thyroid's work. The thyroid needs iodine to produce thyroid hormones, which are crucial for regulating our body's metabolism.

If the raw material is insufficient (iodine deficiency): To meet production demands, the thyroid has to work extremely hard. A "boss" in our brain called the pituitary gland secretes more Thyroid-Stimulating Hormone (TSH) to constantly push the thyroid.
Consequences of long-term overwork: Under this prolonged stimulation and prodding, thyroid cells may overproliferate and become enlarged, leading to what we commonly call "goiter." In this state of constant stimulation, cells are more prone to "errors" during division and proliferation, generating abnormal cells. This increases the risk of developing certain types of thyroid cancer, particularly follicular carcinoma and anaplastic carcinoma. These two types are relatively more aggressive than the most common one.

Therefore, long-term severe iodine deficiency is indeed a risk factor for thyroid cancer. This is why countries implement iodized salt programs – to prevent illnesses caused by iodine deficiency.

2. Too Much Iodine: May Be Associated with an Increase in Papillary Thyroid Carcinoma (PTC)

Nowadays, people are more worried about the issue of "too much iodine."

An interesting phenomenon: Many studies have found that since the global promotion of iodized salt, the incidence of the aforementioned "follicular carcinoma" has decreased, but the incidence of another very common thyroid cancer – Papillary Thyroid Carcinoma (PTC) – has increased.
Possible explanation: The current scientific consensus is that a high-iodine environment may induce or exacerbate an autoimmune disease called "autoimmune thyroiditis" (such as the commonly heard "Hashimoto's thyroiditis"). This disease involves our own immune system attacking the thyroid, causing it to be in a state of chronic inflammation.
The soil of chronic inflammation: This persistent inflammatory environment, like unhealthy soil, may provide a "breeding ground" for the development and progression of Papillary Thyroid Carcinoma (PTC).

However, there are several key points you must understand:

Correlation does not equal causation: While high iodine intake is associated with the increase in PTC, there is currently no evidence proving that high iodine causes the cancer. The rise in PTC incidence is also significantly related to the increased availability of health check-ups and advancements in ultrasound technology, which allow the detection of more tiny lesions.
PTC is a "lazy cancer": Papillary Thyroid Carcinoma (PTC) is the most common type of thyroid cancer (accounting for over 80% of cases), but it is very low-grade, progresses very slowly, and responds extremely well to treatment. Hence, many people call it a "lazy cancer" or "good cancer."

Core Viewpoint: The Key is "Moderation" and "Balance"

After all this, what should we actually do? It's actually quite simple: remember two words: moderation and balance.

For the vast majority of people: Living in non-high-iodine areas (most parts of China are not), consuming iodized salt normally is completely safe. This is the simplest and most effective way to prevent iodine deficiency. There's absolutely no need to feel anxious about it, and definitely don't arbitrarily switch to non-iodized salt on your own.
For specific groups:
- If you live in a water-source high-iodine area (e.g., certain specific coastal or inland regions), or if you consume very large amounts of seaweed/kelp in your daily diet.
- Or if you have already been diagnosed with a thyroid condition, such as Hashimoto's thyroiditis or hyperthyroidism (Graves' disease).
- Please follow your doctor's advice! Your doctor will tell you based on your specific situation whether you need to restrict iodine intake, such as choosing non-iodized salt.
Avoid both extremes: Don't buy high-concentration iodine supplements and take them indiscriminately out of fear of deficiency, nor avoid iodine altogether out of fear of excess. Both extremes are inadvisable.

Simple Summary

Too little iodine: A clear risk factor that may increase the chance of developing more aggressive thyroid cancers (like follicular carcinoma).
Too much iodine: May be associated with an increase in the most common but mildest thyroid cancer (papillary carcinoma), but the relationship is unclear, and this cancer has an excellent prognosis.
The real "culprit": Is not iodine itself, but rather a long-term state of iodine intake imbalance (severe deficiency or chronic excess).
What we should do: For the average person, maintain a balanced diet and use iodized salt normally. Friends with thyroid issues, listen to your doctor. More important than obsessing over daily salt intake is regular health check-ups. Early detection and early management are key.