Iodine is an essential micronutrient for the human body and an important raw material for the synthesis of thyroid hormones. Thyroid hormones maintain the basic activities of the body and have different effects on almost all systems of the body by promoting growth and development, participating in brain development, regulating metabolism, and impacting most organs and system functions. Thyroid hormones can promote the synthesis and catabolism of protein, fat, and sugar; enhance substance metabolism and energy metabolism by increasing oxygen consumption, generating energy, and influencing basal metabolic rate; and maintain body temperature.
Iodine is rapidly and completely absorbed in the upper segment of stomach and intestine, and after blood iodine is absorbed by the thyroid, iodine is concentrated and thyroid hormone is generated. Iodine in the human body is mainly excreted from urine but can also be excreted through breast milk to supply iodine to infants. Iodine is mainly provided to the human body from various foods and drinking water, and a lack of iodine will cause iodine deficiency disorders (IDD). During a critical period of brain development, the development of the nervous system depends on thyroid hormone, so an iodine deficiency in this period will lead to different degrees of brain development retardation (such as endemic cretinism, etc.), which is irreversible even if iodine or thyroid hormone is supplemented later.
Iodine excess is a state in which iodine intake significantly exceeds the human body’s requirement, and the main cause in China is iodine excesses in sources of water in addition to excesses in food and drugs. Iodine excess can lead to goiter, hypothyroidism, etc., and also can have adverse effects on the health of pregnant women and pregnancy outcomes. However, there is no definite evidence that excessive iodine intake is related to an increased risk of thyroid cancer.
IDD was once a global problem. The most popular iodine supplement measure is salt iodization, but other measures include eating iodine-rich foods and taking iodine-containing nutrient supplements, oral drugs, or iodine oil pills, etc. However, salt iodization is the safest and most effective measure recommended by the World Health Organization (WHO) to control IDD. Currently, more than 120 countries have implemented salt iodization policies, and at least 97 countries have enacted laws, regulations, or food safety standards to support salt iodization. Beyond salt iodization, some marine products have higher iodine contents, such as kelp, laver, hairtail, dried scallop, etc.; milk has varying levels of iodine depending on brand; and plants, especially fruits and vegetables have the lowest iodine content. When choosing an iodine supplement measure, iodized salt should be considered first, followed by iodine-rich foods, and then iodine-containing nutrient supplements. In areas with serious iodine deficiency issues, iodized oil pills can be given to women at childbearing age, pregnant women, and lactating women when iodized salt prevention measures cannot be effectively implemented (1).
China previously had a widespread and severe IDD problem. According to an investigation in the 1970s, all provincial-level administrative divisions (PLADs) in China, except Shanghai, have had an incidence of IDD to some degree. Since the 1950s, salt iodization was launched in some endemic areas, which has effectively curbed the IDD epidemic. In 1991, the Chinese Government signed the United Nation’s World Declaration on the Survival, Protection, and Development of Children, and thereafter made a commitment to eliminate IDD by the year 2000. In 1993, China’s State Council convened a “Mobilization Meeting to Achieve the Goal of Eliminating IDD by 2000” and adopted the “China’s Plan for Eliminating IDD by 2000”. The country adopted a series of prevention and control strategies focused on universal salt iodization (USI). Subsequently, it promoted the “Regulations on Eliminating the Harm of IDD by Salt Iodization” and “Regulation on Salt Monopoly”, etc., thus providing a reliable legal guarantee for the prevention and control of IDD. Subsequently, a surveillance system of IDD has been established step by step, which has key roles for making decision during the prevention and control progress (Figure 1). According to the surveillance results, the iodine content in salt was adjusted several times to reach adequate levels of iodine for the population (Figure 2).
The Iodine deficiency disorders surveillance system in China. Abbreviation: USI=universal salt iodization; IDD=iodine dificiency disorders.
The adjustment of iodine content in salt in China. Abbreviation: USI=universal salt iodization.