The Thyroid Gland
The Thyroid gland is located in the lower part of your neck, in front of your windpipe, and just below your Adam’s apple. It is shaped somewhat like a butterfly or bow tie, being divided into two parts, the right and left lobes. These lobes are linked by a narrow band of tissue called the isthmus, which runs across the front of the windpipe. The thyroid gland is a softish tissue, and when healthy, it is smooth and not tender. It is seldom large enough to be visible in a person with normal thyroid function, although it may be discernible in a person with a thin neck when their chin is lifted up. The thyroid gland moves up and down with the Adam’s apple when swallowing and may be felt when a person’s head is tilted back.
The thyroid gland starts its life as a small piece of embryonic tissue that develops at the base of the growing baby’s tongue. Within the first trimester of pregnancy the thyroid tissue develops into the bi-lobed thyroid gland and slowly descends or migrates to its final destination at the front of the neck, just below the Adam’s apple. During the second trimester of pregnancy the baby’s thyroid gland continues to grow and begins to function on its own. During the third trimester, the complex mechanisms involving the brain and the thyroid gland gradually mature, so that by the time the baby reaches full term, the entire thyroid gland system is fine tuned and fully functional.
In some newborn babies the thyroid gland does not fully develop, or it does not descend properly and lodge in the correct place. A malformed or misplaced thyroid gland seldom functions normally and is often the cause of deficient thyroid hormone production. This condition is very rare but it can cause serious complications if not picked up early. An ultrasound scan of the thyroid gland will reveal the problem, and thyroid hormone therapy, and sometimes surgery, may be required.
The Endocrine System
The thyroid gland is part of the Endocrine System - Endo, (meaning, inward), Krinein (meaning, to secrete). The Endocrine system consists of ductless glands, which secrete their hormones directly into the bloodstream. These hormones have an effect on the function of specific tissues and target organs in other parts of the body. The Glands of the Endocrine system include: Pituitary gland, Pineal gland, Thyroid gland, Parathyroid glands, Adrenal glands, Pancreas, Gonads – ovaries/testes.
In relation to the thyroid gland, the Hypothalamus releases TRH (Thyrotrophin-releasing hormone), to instruct the pituitary on how much TSH to produce.
When the hypothalamus detects low levels of thyroid hormone it secretes TRH, stimulating the pituitary gland to release thyroid stimulating hormone (TSH). This in turn stimulates the thyroid to produce more thyroid hormones and keeps the amount of thyroid hormone in the blood at a normal level.
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When T4 and T3 levels fall, TRH and TSH increase.
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When T4 and T3 levels rise, TRH and TSH decrease.
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The Endocrine Glands
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Pituitary gland: master controller of the endocrine glands, releasing hormones to activate other glands.
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Pineal gland: although its role is not completely understood, it releases melatonin, which affects wakefulness.
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Thyroid gland: controls metabolism, and influences temperature regulation, growth and development.
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Parathyroid glands: control the level of calcium in the blood.
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Adrenal glands: control the fight or flight response to stress, and fluid, mineral and glucose balance.
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Pancreas: regulates blood sugar levels and digestive enzymes.
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Gonads: regulate fertility and secondary sexual characteristics.
The Endocrine glands are interrelated. They secrete into the bloodstream chemical substances called hormones. These hormones perform specific functions by acting on target tissues as well as interacting with other hormones via a complex feedback system.
Hormones affect every part of the body. They regulate body processes, such as, growth, development, reproduction, fluid and mineral balance, cardiac function and blood pressure, and the way the body uses oxygen and food to produce heat, energy and living tissue.
Thyroid Hormone Production and Control
The Thyroid gland makes two main hormones called T4 (thyroxine) and T3 (triiodothyronine). The thyroid gland, however, has no control over the amount of T4 and T3 it manufactures; this is decided by the ‘master’ gland, the Pituitary. The pituitary acts like head office and instructs the thyroid gland on how much T4 and T3 it produces and releases.
The pituitary gland is located at the base of the brain, and is linked to the brain by a narrow stalk of tissue. It produces hormones that control the activities of most of the other endocrine glands.
Thyrotrophin, commonly called Thyroid Stimulating Hormone (TSH) is the hormone produced by the pituitary to control the thyroid gland. The thyroid gland cells have specifically shaped places called receptor sites, which receive TSH. TSH links up with these receptors and the cells are stimulated to produce and release T4 and T3. If the body needs more T4 and T3, more TSH is produced by the pituitary gland. If less T4 and T3 are needed, less TSH is produced.
The pituitary gland, in turn, is told how much TSH to make by its ‘managing director’, the Hypothalamus. The Hypothalamus is located just above the pituitary gland at the base of the brain, and they are joined by the pituitary stalk. The hypothalamus controls many sophisticated functions, as well as coordinating the delicate balance between the nervous system and the endocrine system. It constantly monitors the levels of hormones in the bloodstream, to maintain the correct balance.
