30 March 2015. Human brain development is critically dependent on the thyroid hormone. In the beginning of pregnancy, the fetus depends entirely on the mother’s hormone. Once the fetal thyroid begins to function, it relies on iodine from the mother’s thyroid hormone. If the mother is iodine deficient, the child may be mentally retarded. Even if maternal iodine intake is only slightly below what is needed, the child may suffer deficits in cognitive function, as indicated by lowered IQ.
However, providing an ample iodine supply is not enough to secure sufficient thyroid function. A variety of environmental chemicals can impair iodine uptake or thyroid hormone metabolism. These chemicals occur widely as pollutants in food and house dust or otherwise occur in the environment. A particular problem is perchlorate. This inorganic compound is an oxidizing agent used in rocket propellants, fireworks, and matches. In addition, perchlorate is used in the production of disinfectants, bleaching agents, and many other materials. It is even used as an antistatic agent for food packaging materials. Not surprisingly, perchlorate occurs as a common contaminant in both drinking water and food. Despite perchlorate being an important toxicant, its occurrence is usually not monitored.
In order to limit exposures, several U.S. environmental organizations have now petitioned the U.S. Food and Drug Administration (FDA) to put a stop to the use of perchlorate for food containers. This petition is worth supporting, and the FDA will accept comments until 15 May, 2015. In addition to limiting exposure sources, regulatory agencies should prioritize the monitoring of perchlorate in drinking water and food, because the effects at low-level exposures may be more serious than suggested by toxicology studies of perchlorate alone.
One concern is that susceptibility to thyroid interference is increased in women with low iodine status. The World Health Organization regards a total of 54 countries iodine insufficient, and many additional countries are regarded partially sufficient. In the U.S., for example, a sizable population has insufficient iodine intake.
An additional concern is that perchlorate is not the only substance that blocks uptake of iodine in the thyroid gland. So do thiocyanate and nitrate, both commonly occurring in food (and thiocyanate also in tobacco smoke). Further interference with the thyroid gland can occur from certain pesticides, such as mancozeb and pronamide. Other endocrine disruptors that interfere with thyroid functions include organophosphate pesticides, such as chlorpyrifos, as well as PCBs, brominated flame retardants, dioxins, and perfluorinated compounds.
There is a lot at stake. A recent European study identified women with decreased thyroid function during pregnancy. Children born to the women with the highest perchlorate excretion levels in the urine had a 3-fold increased risk of having an IQ in the lowest 10% at age 3 years. So perchlorate seems to act like or amplify iodine deficiency. As the first study on this topic, it deserves to be confirmed in further research. However, the results are highly plausible and highlight that we are not all equally sensitive to thyroid toxicants. To make matters worse, the combined effect of perchlorate, thiocyanate and nitrate will most likely be greater than the sum of the effects of each of the substances. Earlier this month, new estimates showed that current levels of exposures to organophosphates in the EU are associated with an annual loss of 13 million IQ points. The value of this loss was calculated to exceed €130 billion per year. Then add the effects of a host of other environmental chemicals, and the maternal thyroid may not be capable of providing the fetus with the necessary supplies.
When the thyroid gland is under chemical attack, so is the developing brain.