Over few decades, there has been growing concern about human and wildlife exposure to endocrine disruptors — commonly encountered chemicals that could disrupt endogenous hormone function. The thyroid gland (a hormonal gland in the neck; also called Adams apple in male) is largely affected by some of these environmental toxins. Over 100 naturally occurring and synthetic substances have been reported to have effects on thyroid function or thyroid hormone metabolism.
Environmentally mediated alterations in thyroid function are of particular concern for pregnant women and infants. Thyroid hormone is essential for development of baby in uterus and in infancy and childhood. Common substances that inhibit thyroidal iodine uptake include Perchlorate, thiocyanate, Poly-chlorinated biphenyls, Poly-brominated diphenylethers and nitrate etc. When present in sufficiently high concentrations, these substances decrease the active transport of iodine, an essential element to synthesis thyroid hormone into the thyroid gland and thereby decrease the hormone level.
Perchlorate: Perchlorate salts are used as oxidisers in solid propellants for rockets and missiles, fireworks, road flares, matches and air bag inflation systems. Perchlorate is also present in large concentrations in Chilean nitrate fertilisers. Low levels of perchlorate may also be found in the environment due to natural processes. Following the development of sensitive detection methods, perchlorate has been detected in the drinking water also. Since perchlorate is essentially not biodegradable, industrial contamination of water supplies is difficult and expensive to treat. The developing fetus is likely to be most vulnerable to the adverse effects of perchlorate exposure on thyroid function since thyroidal iodine turnover is highest in fetal life and the fetus requires adequate thyroid hormone for normal neurodevelopment. Iodine, required for infant nutrition, is secreted into breast milk and this process may be inhibited by perchlorate.
Thiocyanate: Cigarette smoke contains cyanide that is metabolised to Thiocyanate. It has been reported that women who smoke during pregnancy are more likely to give birth to neonates with decreased serum T4 levels (thyroid hormone), increased TSH (another hormone) levels and thyroid enlargement. It has also been shown that diets high in thiocyanate contribute to the development of goitre in iodine-deficient regions. Similar to perchlorate, thiocyanate may decrease iodine secretion into breast milk.
Nitrate: Nitrates occur naturally in soil and groundwater due to the decomposition of organic materials and are present in plants. Sodium nitrite is also used as a preservative in cured meats and other foods. They are found in vegetables such as beets, celery, lettuce and spinach. In addition, inorganic nitrates are used as fertilisers. High nitrate ingestion may occur due to contamination of the water supply by human sewage or livestock manure, especially from feedlots or by runoff from farmland.
Polychlorinated biphenyls (PCBs): It has been estimated that a total of about 1.5 million metric tons of PCBs were produced worldwide before bans were imposed. Although the levels have decreased, PCBs remain widespread in the environment and the food chain. Most human PCB exposure is through food sources.
Dr Shahjada Selim
Shaheed Suhrawardi Hospital,
Dhaka.
Source: The Daily Star, February 06, 2010
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