The thyroid is one of the body’s hormone producing glands that form part of the endocrine system. It is about 5 cm long and has two lobes on either side of the windpipe, which are connected in the front of the windpipe by a strip of thyroid tissue – known as an isthmus – which give the thyroid a butterfly shape. This small gland produces hormones that play a vital role in the functioning of the body.
Hormones produced in the thyroid:
The cells in the thyroid gland absorbs iodine from the blood stream, stores it, and converts the iodine in combination with the amino acid, tyrosine, into the thyroid hormones thyroxine (T4) and triiodothyronine (T3).
Dietary sources of iodine are seaweed, seafood, dairy products, grain products, eggs, some fruit and vegetables, and iodized salt.
The thyroid gland produces three types of hormones:
• Thyroxine (T4) is a relatively inactive prohormone that contains four iodine atoms. (A prohormone needs to be converted to the active form of a hormone before it has an effect in the body.) This is the main hormone secreted into the bloodstream by the thyroid gland.
• Triiodothyronine (T3) is the active hormone that is secreted in low amounts and contains three iodine atoms. The thyroid gland produces about 20% of the body’s triiodothyronine needs. Most of the T3 in the body (about 80%) is produced from the conversion of T4 to T3 (where one of the four iodine atoms are cleaved off) that takes place outside the thyroid gland in the body’s tissues, particularly in organs such as the liver, brain and kidneys, as well as in muscle and skin. Triiodothyronine is the hormone that plays a vital role in the body’s functions such as metabolic rate, heart and digestive functions, brain development and function, the maintenance of bones, and muscle control. Just the right amount of T4 is converted to T3 at the right time to meet the body’s immediate needs.
• Calcitonin is a hormone produced by the parafollicular cells, situated in the spaces between the follicular cells of the thyroid gland, and plays a relative minor role in regulating calcium and phosphate levels in the blood. Secretion of calcitonin is determined by the level of calcium in the blood.
Functions of the thyroid hormones:
The main functions of the thyroid are to produce and secrete hormones that control metabolism, growth and development.
The most important function of the thyroid hormones is to regulate the body’s metabolism – which refers to the rate by which calories and oxygen are converted into energy. The thyroid produces the master metabolism hormones which play a vital role in regulating the metabolic rate (the rate at which the body burns calories) of every cell in the body in order to perform its function. For example, thyroid hormones regulate metabolism and body weight by controlling the burning of fat for heat and energy. High levels of thyroid hormone cause metabolic processes to occur faster or more frequently.
Thyroid hormones have an impact on every cell and every organ in the body. The thyroid hormones bind to proteins in the blood and circulate freely in the bloodstream in order to be available immediately to enter a cell – by crossing the cell membrane from the blood to the interior of the cell – whenever needed to “instruct” the DNA to stimulate or inhibit the production of specific proteins.
The hormones that are secreted by the thyroid interact with the other hormones in the body, for example insulin, cortisol, estrogen and testosterone. These hormones are all interrelated and in constant communication.
As the thyroid hormones are critical for the body’s metabolism, growth and development, they play an important role in the normal development of the fetal and neo-natal brain, the brain and skeletal system in infants and the normal development of children. They are equally important for the normal functioning of many chemical processes in the adult body. They signal the production of virtually all growth factors in the body, such as somatomedins needed for skeletal tissue growth; Erythropoietin for red blood cell production; nerve growth factor and epidermal growth factor. Thyroid deficiency can result in growth retardation in children.
The thyroid hormones are partly responsible for the healthy functioning of the organs in the body, for example, they play a vital role in regulating the heart rate and blood volume.
The thyroid hormones are involved with fat mobilization, and also enhance the oxidation of fatty acids in many tissues of the human body. Thyroid hormones stimulate almost all aspects of carbohydrate metabolism. In pregnant women thyroid hormones are involved in the production of the hormone prolactin, which is responsible for milk production in pregnant women.
The physiologically active thyroid hormone T3 (triidiothyronine) plays a vital role in the control of body temperature and body heat production. The T3 initiates a cascade of chemical reactions within each cell, which affects each cell’s metabolic rate (”processing speed”). The metabolic rate of the cells determines the metabolic rate of the body, which, together with the surface area, activity level, sweating, environmental conditions, and other factors, determine the body’s temperature. The body’s temperature affects the function of enzymes which are largely responsible for the most important chemical reactions in the body. These chemical reactions, in turn, are key to the body’s functions.
The thyroid hormones are associated with nerve stability and the irritability of the nervous system.
The hormone calcitonin plays a very minor role in the regulation of calcium levels in the body, not only for bone growth and development, but also by slowing down the rate at which bone is broken down.
Control of the thyroid system:
The thyroid is part of a feedback loop system called the hypothalamic-pituitary-thyroid axis. The thyroid system gets triggered when the hypothalamus detects that insufficient thyroid hormone is available for the brain. Thyrotropin-releasing hormone (TRH) is then released along nerve fibers running from the hypothalamus into the blood supply of the pituitary gland situated just below, where it triggers the release of the thyroid stimulating hormone (TSH), which in turn stimulates the thyroid to make more thyroid hormone for release into the blood stream. When these hormone levels are sufficiently high, the production of thyrotropin-releasing hormone (TRH) in the hypothalamus is switched off and the production of the thyroid stimulating hormone (TSH) in the pituitary gland is stopped.
The hypothalamus is a pearl sized gland located at the base of the brain, which produces a number of hormones that help to stimulate many important processes in the body. In the case of the thyroid system, the hypothalamus monitors the level of thyroid hormones in the body and secretes thyrotropin-releasing hormone (TRH) when levels are low.
The pituitary gland is only pea sized, but is regarded as the “master gland”, as it produces hormones that control many different processes in the body. It secretes a number of hormones into the blood stream to regulate the function of different glands and organs. In the thyroid system, it regulates the formation and secretion of thyroid stimulating hormone (TSH), which enters the bloodstream and travels to the thyroid gland where it stimulates the thyroid gland to produce the hormones thyroxine (T4), triiodothyronine (T3) and calcitonin in the quantities that the body requires.
When released into the blood stream, the thyroid hormones travel to every cell in the body to regulate the speed at which the cells run their metabolism. Inside the nucleus of the cell, hormones interact with the DNA and plays a role in modulating gene expression. In fact, T3 engages thyroid hormone receptors in the nucleus of cells, which together bind to DNA, leading to transcription of thyroid responsive genes that have multiple actions in different cells, affecting cell metabolism.
Maintaining the level of thyroid hormones in the blood stream is a vital function, as both under-secretion and overproduction of thyroid hormones are associated with disease. As the liver does most of the conversion of T4 to T3, liver dysfunction can also affect thyroid hormone levels in the body.
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HEALTH INSIGHT
January 2019