Hormones (from Greek hormao = to excite) are special chemical messenger produced in the endocrine gland. William M. Bayliss and his brother-in-law Ernest H. Starling, both of London University College were the first to use the term Hormone in 1904, they showed that the action of the pancreatic secretion could be stimulated by a chemical substance (secretin) from the intestine which was later called chemical messenger (Starling, 1905; Tata, 2005). In 1937, Went and Thimann defined a hormone as a substance which are produced in any part of an organism, which is then transferred to another part and it influences a specific physiological process. Hormones are secreted by cells into extracellular fluids, which regulates the metabolic function of other cells in the body from basic function such as hunger to complex function like emotions, reproduction and even moods (Mounika and Parveen, 2017). Hormones can either be peptides/proteins, lipids or amino acid derivatives as seen in Figure 1.1 which acts on receptors after their secretion act on receptors to cause reactions within the respective receptive cells. Hormones regulate each cell of the body by one way or the other regulated. Hence, hormones exert innumerable effects on the organism of animals and humans (Wuttke et al., 2010). The tissues or organs in which they are produced are called as effectors and those where they exert their influence as targets.
Endocrine gland are also called ductless gland which is a network of gland that synthesis and releases hormones into the blood streams which transports them to their required location. It’s in charge of maintaining homeostasis and regulation of development and reproduction. The collection of endocrine glands makes up endocrine system (Mounika and Parveen, 2017; Fallah 2017). In human system the major endocrine glands are pituitary gland (anterior and posterior
Figure 1.0: Location of the Major Endocrine Organs of the Body
lobes), gonad (testes and ovaries), adrenal glands (cortex and medulla), pineal gland, pancreas, thyroid glands, parathyroid glands aside these major endocrines gland there are other endocrine
glands secreted by some other part of the body system which performs secondary function such as erythropoietin and renin secreted by the kidney.
Hypothalamus: The hypothalamus is located in the brain which regulates anterior pituitary hormone, it is responsible for hunger, moods, body temperature, hunger and the release of hormones from other glands; and also controls thirst, sleep and sex drive.
Pituitary: It is attached to the hypothalamus of the lower forebrain. It is considered to be the master controller gland, because the pituitary gland is in control of other glands and makes the hormones that trigger growth.
Thymus: This gland plays a role in the function of the adaptive immune system and the maturity of the thymus, and produces T-cells.
Pancreas: it is located in the stomach along the lower curvature close to meeting of the first region of the small intestine, the duodenum.it is responsible for the production of insulin that helps control blood sugar levels.
Thyroid: It is located on anterior trachea just below the larynx. It produces hormones associated with calorie burning and heart rate.
Adrenal: It is located above kidneys and it produces the hormones that control sex drive and cortisol, the stress hormone.
Parathyroid: It consist of four masses of tissues, in which two are embedded posterior in each lateral mass of the thyroid gland. It is responsible for the regulation the level of calcium in the body.
Figure 1.1: Structural Classification of Hormones.
Pineal: It is also called the thalamus and located in dienchephalon which produces serotonin a derivatives of melatonin, which affects sleep.
Gonads: it consist of the ovaries (in women) and testes (in men) they are found in the pelvic cavity. The ovaries secrete estrogen, testosterone and progesterone, the female sex hormones while the
testes produce the male sex hormone, testosterone, and produce sperm (www. health guidance.org/medicine).
1.1 THYROID HORMONES
One of the largest endocrine glands in the body is the thyroid gland (Greek thyros,”shield”) is shaped like a shield and lies just below the Adam’s apple in the front of the neck . Whartonin firstly described it in 1659 who gave it the descriptive |name, thyroid. Thyroid gland produces thyroid hormones which consists of follicles which it’s synthesized through iodination of tyrosine residues in the glycoprotein thyroglobulin (Rubio 2009; Zimmermann 2009; Brent 2012). Thyroid hormones are unique type of hormone due to the content of the trace element iodine for biological activity
There are mainly two iodinated hormone that thyroid gland secretes which are T3 (3, 5, 3?triiodothyronine) and T4 (3, 5, 3, 5? tetraiodothyronine; also known as thyroxine). These molecules can both generate biological activity in responsive tissues by binding to the thyroid hormone receptors; however, T4 is considered as the inactive hormone while T3 is considered as the biologically active hormone. The affinity of the thyroid hormone receptors is approximately tenfold higher for T3 than for T4 (Samuel et al., 1974); therefore, T4 must be converted to T3 to produce potent thyroid hormone receptor-mediated effects.
Figure 1.2: Thyriodal Hormone
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Wuttke, W., Jarry, H. and Seidlov?-Wuttke, D. (2010). Definition, classification and mechanism of action of endocrine disrupting chemicals. Hormones 9(1): 1-15
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