Is the surge in the incidence of thyroid cancer caused by iodized salt?

The incidence of thyroid cancer has risen sharply. Is iodized salt to blame?

Various rumors and rumors about the health hazards caused by iodized salt have never stopped, among which the statement that iodized salt causes thyroid cancer attracts the most attention.

groundless talk is not without cause. There is some basis for this statement, that is, the incidence of thyroid cancer in China has risen sharply in recent decades, 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 caused an increase in thyroid cancer. As shown in the following figure, the change trend of thyroid cancer incidence and iodine nutrition status in China after the implementation of universal salt iodization, the solid line is the trend of thyroid cancer incidence in the population, and the dotted line is the change of median urine iodine concentration (reflecting iodine nutrition status, that is, the size of intake).

However, in the same period, the incidence of thyroid cancer in other countries in the world also showed a trend of increasing significantly. For example, after the United States adjusted the excessive iodine intake in the early 197s, the iodine nutritional status remained stable for nearly 3 years, but the incidence of thyroid cancer also rose sharply, with an annual increase of more than 5% from 1973 to 213.

or some people will say that both China and the United States had excessive iodine supplementation in the initial stage, and the increase in the incidence of thyroid cancer after 1-2 years of incubation period can just explain that it is the result of excessive iodine.

However, as the first country in the world to promote salt iodization, the concentration of salt iodine is 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 has increased significantly in almost all countries and regions in the world in recent decades, regardless of iodine nutritional status.

It seems that things are a bit complicated. At least, it is not enough to form a reasonable explanation for this increase in incidence by using salt to supplement iodine.

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 made the greatest contribution to this increase, which has led to the improvement of detection rate. That is to say, because most thyroid cancers (95%) have a high degree of differentiation, it is difficult to find and make a diagnosis at an early stage with previous medical technology unless there are obvious symptoms in the advanced stage (late stage). Nowadays, with the popularization of high-resolution ultrasound and fine needle aspiration 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 causes and risk factors in the occurrence of diseases. Is there a real increase in the incidence of thyroid cancer in the population? If so, will the iodine intake level affect it?

At least, a lot of evidence has proved that even a slight 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, the iodine intake level will directly or indirectly affect the occurrence of thyroid cancer, and it is not impossible.

Let science clear the fog of rumors

Scientists' research should only be carried out from the following aspects:

Animal experiments

Female rats undergo 6? After 2 months of iodine deficiency diet feeding, it was found that serum thyroid stimulating hormone (TSH) increased, while follicular adenoma and follicular carcinoma increased by 54-1%. 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 cause thyroid cancer, while feeding with high iodine diet has only a slight promotion effect.

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

Possible mechanisms between 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 follicular epithelial proliferation, thus increasing the chance of cell DNA variation and carcinogenesis; 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 iodine excess is a weak risk factor on the one hand and a protective factor on the other.

Epidemiological investigation of iodine intake level and incidence of thyroid cancer

Besides China, the United States and Switzerland, the correlation between iodine intake level and incidence of thyroid cancer has been widely studied in the world. However, due to many mixed and biased factors, the results need to be carefully interpreted, and 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 the total incidence of thyroid, but the specific pathological types of thyroid cancer changed before and after iodine supplementation. Specifically, papillary cell carcinoma with low malignancy was increased, and the percentage of follicular carcinoma and undifferentiated carcinoma with relatively high malignancy was decreased.

the 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 promotion of universal salt iodization in the world may have made an indirect contribution to the decline of global mortality of thyroid cancer in recent decades.

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

correlation between iodine intake level and autopsy occult thyroid cancer

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

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

after excluding the autopsy research in Japan, the comprehensive weighted analysis of dozens of studies found that the prevalence of thyroid occult cancer in areas with excessive iodine intake was significantly lower than that in areas with adequate iodine intake and iodine deficiency, while the difference between areas with insufficient iodine intake and areas with adequate iodine intake was significant.

That is to say, autopsy data shows that the increase of iodine intake reduces the incidence of occult cancer and is a protective factor.

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

Case-control study on the relationship between total iodine intake calculated by seafood and iodized salt and incidence of thyroid cancer was conducted in Hawaii, California and two Pacific islands in the United States. 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 lowest intake. Although the difference was not significant, it showed that high iodine intake was a trend protective factor for the occurrence of thyroid cancer.

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

The Chernobyl nuclear accident in p>1986 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 nutritional status of the population at the time of the accident, so it is difficult to draw definitive conclusions. 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 is a protective factor for thyroid radiation damage.

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.