L1.INTRODUCTION to ENDOCRINOLOGY MEDICINE.pptx

Endocrinology concerns the synthesis, secretion and action of hormones. These are chemical messengers released from endocrine glands that coordinate the activities of many different cells. Endocrine diseases can therefore affect multiple organs and systems.

1. INTRODUCTION TO ENDOCRINOLOGY ASSISTANT PROFESSOR DR BILAL NATIQ NUAMAN CONSULTANT ENDOCRINOLOGIST Al-Iraqia Medical College 2024

2. Endocrinology concerns the synthesis, secretion and action of hormones. These are chemical messengers released from endocrine glands that coordinate the activities of many different cells. Endocrine diseases can therefore affect multiple organs and systems. The term endocrine was coined by Starling to contrast the actions of hormones secreted internally (endocrine) with those secreted externally (exocrine) or into a lumen, such as the gastrointestinal tract.

3. Unlike many other specialties in medicine, it is not possible to define endocrinology strictly along anatomic lines. The classic endocrine glands—pituitary, thyroid, parathyroid, pancreatic islets, adrenals, and gonads— communicate broadly with other organs through the nervous system, hormones, cytokines, and growth f actors. A hormone is a molecule— small or large, protein or lipid— secreted in a regulated fashion from one organ and acting on another.

4. Hormones can be divided into ve major types: (1) amino acid derivatives such as dopamine, catecholamine, and thyroid hormone; (2) small neuropeptides such as gonadotropin-releasing hormone (GnRH), thyrotropin-releasing hormone (TRH), somatostatin, and vasopressin; (3) large proteins such as insulin, luteinizing hormone (LH), and parathyroid hormone (PTH); (4) steroid hormones such as cortisol and estrogen that are synthesized from cholesterol-based precursors; and (5) vitamin derivatives such as retinoids (vitamin A) and vitamin D .

5. The physiologic functions of hormones can be divided into three general areas: (1) growth and differentiation, (2) maintenance of homeostasis, and (3) reproduction.

6. MAINTENANCE OF HOMEOSTASIS Although virtually all hormones affect homeostasis, the most important among them are the following: 1. Thyroid hormone—controls about 25% o basal metabolism in most tissues 2. Cortisol—exerts a permissive action or many hormones in addition to its own direct effects 3. PTH—regulates calcium and phosphorus levels 4. Vasopressin—regulates serum osmolality by controlling renal free-water clearance 5. Mineralocorticoids—control vascular volume and serum electrolyte (Na+, K+) concentrations 6. Insulin—maintains euglycemia in the fed and fasted states

7. There are several organs whose primary functions are non-endocrine but that also possess endocrine functions. These include the heart, kidneys, intestines, thymus, gonads, and adipose tissue. The heart possesses endocrine cells in the walls of the atria that are specialized cardiac muscle cells. These cells release the hormone atrial natriuretic peptide (ANP) in response to increased blood volume.

8. High blood volume causes the cells to be stretched, resulting in hormone release. ANP acts on the kidneys to reduce the reabsorption of Na + , causing Na +  and water to be excreted in the urine. ANP also reduces the amounts of renin released by the kidneys and aldosterone released by the adrenal cortex, further preventing the retention of water. In this way, ANP causes a reduction in blood volume and blood pressure, and reduces the concentration of Na +  in the blood.

9. The gastrointestinal tract produces several hormones that aid in digestion. The endocrine cells are located in the mucosa of the GI tract throughout the stomach and small intestine. Some of the hormones produced include gastrin, secretin, and cholecystokinin, which are secreted in the presence of food, and some of which act on other organs such as the pancreas, gallbladder, and liver. They trigger the release of gastric juices, which help to break down and digest food in the GI tract.

10. The kidneys themselves also possess endocrine function. Renin is released in response to decreased blood volume or pressure and is part of the renin-angiotensin-aldosterone system that leads to the release of aldosterone. Aldosterone then causes the retention of Na +  and water, raising blood volume. The kidneys also release calcitriol, which aids in the absorption of Ca 2+  and phosphate ions. Erythropoietin (EPO) is a protein hormone that triggers the formation of red blood cells in the bone marrow.

11. EPO is released in response to low oxygen levels. Because red blood cells are oxygen carriers, increased production results in greater oxygen delivery throughout the body. EPO has been used by athletes to improve performance, as greater oxygen delivery to muscle cells allows for greater endurance. Because red blood cells increase the viscosity of blood, artificially high levels of EPO can cause severe health risks.

12. The thymus is found behind the sternum; it is most prominent in infants, becoming smaller in size through adulthood. The thymus produces hormones referred to as thymosins, which contribute to the development of the immune response. Adipose tissue is a connective tissue found throughout the body. It produces the hormone Leptin in response to food intake. Leptin increases the activity of anorexigenic neurons and decreases that of orexigenic neurons, producing a feeling of satiety after eating, thus affecting appetite and reducing the urge for further eating. Leptin is also associated with reproduction. It must be present for GnRH and gonadotropin synthesis to occur. Extremely thin females may enter puberty late.

13. Feedback control, both negative and positive, is a fundamental feature of endocrine systems. Each o the major hypothalamic-pituitary hormone axes is governed by negative feedback, a process that maintains hormone levels within a relatively narrow range

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