Creswell J Eastman, M.D. and Michael B Zimmermann, M.D.
ABSTRACT
This chapter provides an overview of the disorders caused by iodine deficiency. Extensively referenced, it includes data on dietary sources of iodine, goitrogens, the effects of iodine deficiency throughout the lifecycle, the pathophysiology of iodine deficiency, as well as strategies for control and monitoring of the iodine deficiency disorders, such as iodized salt and iodized oil. It emphasizes the role of iodine deficiency in the development of brain damage and neurocognitive impairment, assessment of the iodine status of a population, the potential side effects of excessive iodine intake and current worldwide epidemiological data.
INTRODUCTION
This chapter provides a global overview of the disorders caused by iodine deficiency. Special emphasis will be put on recent developments such as the role of iodine deficiency in the development of brain damage and neurocognitive impairment, assessment of the iodine status of a population, strategies for control and monitoring of the iodine deficiency disorders (IDD), as well as side effects of iodine. Up to date information on IDD can be obtained by visiting the website of the Iodine Global Network (IGN) http://www.ign.org.
ETIOLOGY
Iodine (atomic weight 126.9 g/atom) is an essential component of the hormones produced by the thyroid gland. Thyroid hormones, and therefore iodine, are essential for mammalian life. Iodine (as iodide) is widely but unevenly distributed in the earth’s environment. Most iodide is found in the oceans (≈50 μg/L), and iodide ions in seawater are oxidized to elemental iodine, which volatilizes into the atmosphere and is returned to the soil by rain, completing the cycle. However, iodine cycling in many regions is slow and incomplete, and soils and ground water become deficient in iodine. Crops grown in these soils will be low in iodine, and humans and animals consuming food grown in these soils become iodine deficient (1). In plant foods grown in deficient soils, iodine concentration may be as low as 10 μg/kg dry weight, compared to ≈1 mg/kg in plants from iodine-sufficient soils. Iodine deficient soils are most common in inland regions, mountainous areas and areas of frequent flooding, but can also occur in coastal regions (2). This arises from the distant past through glaciation, compounded by the leaching effects of snow, water and heavy rainfall, which removes iodine from the soil. The mountainous regions of Europe, the Northern Indian Subcontinent, the extensive mountain ranges of China, the Andean region in South America and the lesser ranges of Africa are all iodine deficient. Also, the Ganges Valley in India, the Irawaddy Valley in Burma, and the Songkala valley in Northern China are also areas of endemic iodine deficiency. Iodine deficiency in populations residing in these areas will persist until iodine enters the food chain through addition of iodine to foods (e.g. iodization of salt) or dietary diversification introduces foods produced in iodine-sufficient areas.