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THYROID HORMONES
The thyroid gland is responsible for the creation, storage, and secretion of thyroid hormones. Iodine's primary role within the thyroid cells is to provide raw material for hormone synthesis.
Thyroxine (T4) is the primary hormone created by the thyroid gland. It consists of four atoms of iodine attached to a molecule of tyrosine. Thyroxine (T4) is 65% iodine by weight. T4 accounts for up to 90% of the protein-bound iodine in the blood. About 70% of the T4 secreted daily is deiodinated to yield equal amounts of T3 and rT3.
Triiodothyronine (T3) is the second hormone created by the thyroid and is created and secreted in much smaller amounts. It consists of three atoms of iodine attached to a molecule of tyrosine. For T3, 70-90% of the daily production is created by extrathyroidal deiodination from T4.
The thyroid hormones are secreted into the blood stream to be transported to the various cells in the body. More than 99% of both T4 and T3 combine with transport proteins in the blood. Less than 1% remains as free T3 or T4.
T4 seems to function primarily as a storage form, being transformed into T3 (or rT3) as needed to maintain proper body function. T3 is the primary hormone used in cell processes. However it is possible that T4 also has its own functions as an active hormone. The function of rT3 is still uncertain.
Deiodination is the process by which an iodide atom is removed -- e.g., in transforming T4 to T3 (or rT3). It occurs throughout the body, including the thyroid, liver, pituitary, brain, and other organs. Deiodination is largely responsible for creating the active T3. It is performed by enzymes containing selenium, which is one of the reasons adequate dietary selenium is so important.
When the blood passes through the liver, about 98% of the T4 is taken up and then returned to the circulation unmetabolized. If the residual 2% were metabolized to T3, the liver could be responsible for roughly 40% of the whole body deiodination of T4. The liver itself contains about 30% of the extra-thyroidal pool.
In addition to T4 and T3, there are other thyroid hormones, such as rT3, T2, and T1. Traditionally, these hormones have been considered inert, but recent research indicates that they have numerous functions of their own.
Most body cells have receptors for thyroid hormones on the cell membrane. Once inside the cell, the hormones bind to specific nuclear receptors, where they then regulate gene expression. By means of increasing or decreasing the expression of various genes, the hormones affect an incredible number of cell processes.
For example, the thyroid hormones regulate a variety of functions including oxygen use and basal metabolism rate, cellular metabolism, and growth and development.
The thyroid hormones stimulate the use of cellular oxygen to produce ATP (Adenosine Triphosphate, the energy source for the cell). They also stimulate the synthesis of sodium-potassium pumps, which use the ATP for energy to eject excess sodium ions (Na+) from the cell.
As the cells produce and use more ATP, more heat is given off, and the body temperature rises. Thus, people with low levels of thyroid hormone have lower body temperature than people with higher levels.
Thyroid hormones stimulate protein synthesis and increase the use of glucose and fatty acids for ATP production. They also increase the decomposition of fats and enhance cholesterol excretion, thus reducing blood cholesterol levels. Thus, people with low levels of thyroid hormones tend to have higher levels of blood cholesterol.
Thyroid hormones affect the regulation of body fluids, vascular resistance and, ultimately, blood pressure. People with low levels of thyroid hormones tend to have low cardiac output, a slow heart rate, low blood pressure, and elevated vascular peripheral resistance. In contrast, people with high levels of thyroid hormones tend to have high cardiac output, a rapid heart rate, high blood pressure, and decreased vascular peripheral resistance.
Thyroid hormones also accelerate body growth, particularly the growth of nervous tissue. Therefore, a deficiency of thyroid hormones during fetal development, infancy, or childhood results in stunted growth and mental retardation.
Wikipedia has a good discussion of thyroid hormones.
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