AUTHOR Dr. Carlos Orozco (BSc, MSc, ND, MD, PhD, FPAMS).
Most of the Earth’s iodine is found in its oceans. In general, the older an exposed soil surface, the more likely the iodine has been leached away by erosion.
Iodine, a non-metallic trace element, is required by humans for the synthesis of thyroid hormones. Iodine deficiency is an important health problem throughout much of the world. Most of the Earth’s iodine is found in its oceans. In general, the older an exposed soil surface, the more likely the iodine has been leached away by erosion. Mountainous regions, such as the Himalayas, the Andes, and the Alps, and flooded river valleys, such as the Ganges, are among the most severely iodine deficient areas in the world.
Iodine is an essential component of the thyroid hormones, triiodothyronine (T3) and thyroxine (T4) and is therefore, essential for normal thyroid function. To meet the body’s demand for thyroid hormones, the thyroid gland traps iodine from the blood and converts it into thyroid hormones that are stored and released into the circulation when needed. In target tissues, such as the liver and the brain, T3, the physiologically active thyroid hormone, can bind to thyroid receptors in the nuclei of cells and regulate gene expression. T4, the most abundant circulating thyroid hormone, can be converted to T3 by enzymes known as deiodinases in target tissues. In this manner, thyroid hormones regulate a number of physiologic processes, including growth, development, metabolism, and reproductive function.
The regulation of thyroid function is a complex process that involves the brain (hypothalamus) and pituitary gland. In response to thyrotropin-releasing hormone (TRH) secretion by the hypothalamus, the pituitary gland secretes thyroid-stimulating hormone (TSH), which stimulates iodine trapping, thyroid hormone synthesis, and release of T3 and T4 by the thyroid gland. The presence of adequate circulating T4 decreases the sensitivity of the pituitary gland to TRH, limiting its secretion of TSH (diagram). When circulating T4 levels decrease, the pituitary increases its secretion of TSH, resulting in increased iodine trapping, as well as increased production and release of T3 and T4. Iodine deficiency results in inadequate production of T4. In response to decreased blood levels of T4, the pituitary gland increases its output of TSH. Persistently elevated TSH levels may lead to hypertrophy (enlargement) of the thyroid gland, also known as goiter).