The endocrine system is a control system of ductless glands that secrete hormones within specific organs. Hormones act as "messengers," and are carried by the bloodstream to different cells in the body, which interpret these messages and act on them. The ability to maintain homeostasis and respond to stimuli is largely due to hormones secreted within the body. Without hormones, you could not grow, maintain a constant temperature, produce offspring, or perform the basic actions and functions that are essential for life.
The endocrine system provides an electrochemical connection from the hypothalamus of the brain to all the organs that control the body's metabolism, growth and development, and reproduction. The endocrine system regulates its hormones through negative feedback, except in very specific cases like childbirth. Increases in hormone activity decrease the production of that hormone. The immune system and other factors contribute to maintaining constant levels of hormones. The main endocrine glands are the hypothalamus (neuro-endocrine gland), pituitary (anterior and posterior lobes), thyroid, parathyroid, adrenal (cortex and medulla), pancreas, and gonads.
The thyroid gland is the primary endocrine gland involved in development. The thyroid gland is one of the largest endocrine glands in the body. It is positioned on the neck just below the Larynx and has two lobes with one on either side of the trachea. It is involved in the production of the hormones T3 (triiodothyronine) and T4 (thyroxine). These hormones increase the metabolic activity of the body's cells. A deficiency of iodine in the diet leads to the enlargement of the thyroid gland, known as a simple goiter. The thyroid also produces and releases the hormone calcitonin (thyrocalcitonin), which contributes to the regulation of blood calcium levels. Thyrocalcitonin decreases the concentration of calcium in the blood. Most of the calcium removed from the blood is stored in the bones.
Thyroid system
The thyroid gland controls how quickly the body uses energy, makes proteins, and how sensitive the body is to other hormones.
The thyroid gland is a two-lobed gland that manifests a remarkably powerful active transport mechanism for the up-take of iodide ions from the blood. As blood flows through the gland, iodide is converted to an active form of iodine. This iodine combines with an amino acid called tyrosine. Two molecules of iodinated tyrosine then combine to form thryroxine. The normal thyroid gland may store several weeks supply of thyroxine in this bound form. An enzymatic splitting of the thyroxine from the thyroglobulin occurs when a specific hormone is released into the blood. This hormone, produced by the pituitary gland, is known as thyroid-stimulating hormone (TSH). TSH stimulates certain major rate-limiting steps in thyroxine secretion, and thereby alters its rate of release. A variety of bodily defects, either dietary, hereditary, or disease induced, may decrease the amount of thyroxine released into the blood. The most popular of these defects is one that results from dietary iodine deficiency. The thyroid gland enlarges, in the continued presence of TSH from the pituitary, to form a goiter. This is a futile attempt to synthesize thyroid hormones, for iodine levels that are too low. Normally, thyroid hormones act via a negative feedback loop on the pituitary to decrease stimulation of the thyroid. In goiter, the feedback loop cannot be in operation - hence continual stimulation of the thyroid and the inevitable protuberance on the neck. The incidence of goiter has been drastically reduced by adding iodine to table salt.
Thyroxine serves to stimulate oxidative metabolism in cells; it increases the oxygen consumption and heat production of most body tissues, a notable exception being the brain. Thyroxine is also necessary for normal growth. The most likely explanation being that thyroxine promotes the effects of growth hormone on protein synthesis. The absence of thyroxine significantly reduces the ability of growth hormone to stimulate amino acid uptake and RNA synthesis. Thyroxine also plays a crucial role in the closely related area of organ development, particularly that of the central nervous system.
If there is an insufficient amount of thyroxine, a condition referred to as hypothyroidism results. Symptoms of hypothyroidism stem from the fact that there is a reduction in the rate of oxidative energy-releasing reactions within the body cells. Hypothyroidism in children, a condition known as cretinism, can result in mental retardation, dwarfism, and permanent sexual immaturity. Sometimes, the thyroid gland produces too much thyroxine, a condition known as hyperthyroidism. This condition produces symptoms such as an abnormally high body temperature, profuse sweating, high blood pressure, loss of weight, irritability, and muscular pain and weakness. Hyperthyroidism has been treated by partial removal or partial radiation destruction of the gland. More recently, several drugs that inhibit thyroid activity have been discovered, and their use is replacing the former surgical procedures. Unfortunately thyroid conditions require lifelong treatment and because of the body's need for a sensitive balance of thyroid hormone both supplementing and suppressing thyroid function can take months or even years to regulate.
The Endocrine System
Major endocrine glands. (Male left, female right. ) 1. Pineal gland 2. Pituitary gland 3. Thyroid gland 4. Thymus 5. Adrenal gland 6. Pancreas 7. Ovary 8. Testis