Is the surge in thyroid cancer caused by iodized salt?

The incidence of thyroid cancer has risen sharply. Is iodized salt the culprit?

Various rumors and rumors about the harm of iodized salt to health have never stopped, among which the statement that iodized salt causes thyroid cancer is the most concerned.

A hole in the wind is not without cause. This statement is based on the fact that in recent decades, the incidence of thyroid cancer in China has risen sharply, especially among women. However, this increase happened just after universal salt iodization, so many people attributed the two to causality and concluded that iodized salt led to an increase in thyroid cancer. The following figure shows the changing trend of thyroid cancer incidence and iodine nutritional status in China after the implementation of universal salt iodization. The solid line shows the changing trend of thyroid cancer incidence in the population, and the dotted line shows the change of median urinary iodine concentration (reflecting iodine nutritional status, that is, intake).

However, during the same period, the incidence of thyroid cancer in other countries in the world also increased significantly. For example, after the United States adjusted its excessive iodine intake in the early 1970s, the iodine nutritional status remained stable for nearly 30 years, but the incidence of thyroid cancer also rose sharply, with the annual incidence increasing from 1973 to 20 13, with an increase of over 500%.

Or some people may say that both China and the United States had an initial iodine overdose, and the increase in the incidence of thyroid cancer after the incubation period of 10~20 years can just explain that it is the result of iodine overdose.

However, as the first country in the world to promote salt iodization, the concentration of salt iodine has been increasing, and the iodine nutritional status has been stable at an adequate level for decades. However, the incidence of thyroid cancer has also increased significantly.

More epidemiological surveys show that the incidence of thyroid cancer in almost all countries and regions in the world has increased significantly in recent decades, regardless of iodine nutritional status.

It seems that things are a bit complicated, and at least using salt to supplement iodine is not enough to form a reasonable explanation for the rise of this incidence.

What the hell is going on here?

Scientists have been trying to find the mystery. At present, it is believed that the development of medicine and the progress of detection technology have contributed the most to this increase, which has led to the improvement of detection rate. That is to say, because most thyroid cancers (95%) are highly differentiated, it is difficult to find and make a diagnosis at an early stage with previous medical technology unless there are obvious symptoms in the late stage. Nowadays, with the popularization of high-resolution ultrasound and fine needle biopsy, more thyroid cancers are diagnosed at an early stage.

Epidemiological data also show that the mortality rate of thyroid cancer has decreased significantly while the incidence rate has increased sharply, which also confirms this statement from one side.

However, we know that there are many reasons and risk factors for the occurrence of diseases. Has the incidence of thyroid cancer really increased in the population? If so, will the iodine intake level affect it?

At least, a lot of evidence has proved that even a small change in iodine intake level will have an impact on benign thyroid diseases in the population; However, diseases such as goiter and thyroid nodules are related to the occurrence of cancer. Therefore, it is not impossible that the iodine intake level will directly or indirectly affect the occurrence of thyroid cancer.

Let science dispel the fog of rumors

Scientists' research can only be carried out in the following aspects:

animal testing

Is the female mouse over 6? After 20 months of iodine deficiency diet feeding, it was found that serum TSH increased, and follicular adenoma and follicular carcinoma increased by 54- 100%. Similar findings have been made in other animal experiments.

In addition, exposure to chemical carcinogens and ionizing radiation in rats fed with iodine deficiency is more likely to lead to thyroid cancer, while feeding with high iodine diet has only a slight promotion effect.

Overall, consistent data show that iodine-deficient animals increase the risk of thyroid cancer (mainly follicular cell carcinoma).

The possible mechanism of iodine intake and thyroid cancer

At present, it is generally believed that the possible mechanisms include: 1) iodine deficiency leads to the increase of blood TSH, which in turn stimulates the proliferation of follicular epithelium, thus increasing the chance of cell DNA variation and canceration; 2) iodine deficiency increases the production of reactive oxygen species mediated by H 2 O 2, destroys DNA and increases the chance of mutation. Exogenous iodine can reduce the production of H2O2 in thyroid cell line and induce the apoptosis of thyroid cells and thyroid cancer cells. 3) Iodine deficiency increases the probability of BRAF mutation or RET rearrangement in thyroid cancer cells, while high iodine shows protective effect.

That is to say, among several possible mechanisms related to iodine, iodine deficiency is a risk factor, while high iodine is a weak risk factor on the one hand and a protective factor on the other.

Epidemiological investigation of iodine intake and incidence of thyroid cancer

In addition to China, the United States and Switzerland, the relationship between iodine intake level and thyroid cancer has been widely studied all over the world. However, due to many mixed and biased factors, the results need to be carefully interpreted. Many rumors and rumors are based on intentional or unintentional misinterpretation and misreading of a certain research.

Overall, the study found that there was no significant correlation between iodine intake level and total incidence of thyroid, but the specific pathological types of thyroid cancer changed before and after iodine supplementation. Specifically, papillary cell carcinoma with low degree of malignancy increased, and the percentage of follicular carcinoma and undifferentiated carcinoma with relatively high degree of malignancy decreased.

Relationship between iodine intake and mortality of thyroid cancer

Because the above iodine intake may reduce the percentage of follicular cancer and undifferentiated cancer with relatively high malignancy, the popularization of salt iodization around the world may have indirectly contributed to the decline of global thyroid cancer mortality in recent decades.

It is found that this contribution is mainly reflected in women, showing a weak but significant indirect correlation; There is no correlation between men.

Correlation between iodine intake and occult thyroid cancer in autopsy

Autopsy occult thyroid cancer is not interfered by biased factors such as screening and diagnosis technology and intensity, so it can truly reflect the actual incidence and the influence of genetic and/or environmental factors, and has high value in evaluating the correlation between iodine intake and thyroid cancer incidence.

Japan is a country with the highest incidence of thyroid diseases and a region with high iodine intake. Therefore, studies in Japan, including autopsies, are often strong evidence for those who hold the view that high iodine intake increases the incidence of thyroid cancer. Then, in the immigration research, it shows that the genetic factors of Japanese race are an important determinant of the occurrence of dangerous thyroid cancer.

After excluding Japanese autopsy studies, a comprehensive weighted analysis of dozens of studies shows that compared with iodine-deficient areas and iodine-deficient areas, the prevalence of thyroid occult cancer in iodine-deficient areas is significantly lower, while the difference between iodine-deficient areas and iodine-deficient areas is significant.

In other words, autopsy data show that the increase of iodine intake reduces the incidence of occult cancer, which is a protective factor.

A case-control study on the relationship between total iodine intake and incidence of thyroid cancer

A case-control study on the relationship between total iodine intake calculated by seafood and iodized salt and the incidence of thyroid cancer was conducted in Hawaii, California and two Pacific islands. After meta-analysis, it was found that the incidence of thyroid cancer in the quintile with the highest iodine intake was 23% less than that in the quintile with the lowest iodine intake. Although the difference is not significant, it shows that high iodine intake is a trend protective factor for thyroid cancer.

Study on iodine nutritional status and risk of thyroid cancer after exposure to ionizing radiation

1986 The Chernobyl nuclear accident provided a huge target population for the study of ionizing radiation and thyroid cancer. Although in these studies, it is difficult to accurately evaluate the actual iodine nutrition status of the population at the time of the accident, so it is difficult to draw a clear conclusion. However, according to some indirect information, most studies have found that the worse the iodine nutritional status of children at the time of the accident, the risk of thyroid cancer will increase by about 2-3 times.

It is suggested that iodine has protective effect on thyroid radiation injury.

To sum up, the existing evidence shows that iodine deficiency is a risk factor for thyroid cancer, and adequate and appropriate excessive iodine intake is a trend protective factor for thyroid cancer.