Endocrine System. Mechanisms of Hormone Action The endocrine system acts by releasing hormones that in turn trigger actions in specific target cells.

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1 Endocrine System The endocrine system is a collection of glands that secretes chemical messages called hormones. These signals are passed through the blood to arrive at a target organ, which has cells possessing the appropriate receptor. by Mrs. Bailey The endocrine system uses cycles and negative feedback to regulate physiological functions. egative feedback regulates the secretion of almost every hormone. Cycles of secretion maintain physiological and homeostatic control. Mechanisms of Hormone Action The endocrine system acts by releasing hormones that in turn trigger actions in specific target cells. Receptors on target cell membranes bind only to one type of hormone. More than fifty human hormones have been identified; all act by binding to receptor molecules. The binding hormone changes the shape of the receptor causing the response to the hormone. There are two mechanisms of hormone action on all target cells: onsteroid hormone action binds to a fixed receptor on plasma membrane - use of 2 nd messenger. Steroid hormone action diffuses into target cell into nucleus binds to a mobile receptor transcription. 1

2 onsteroid hormones (water soluble) do not enter the cell but bind to plasma membrane receptors, generating a chemical signal (second messenger) inside the target cell. Five different second messenger chemicals, including cyclic AMP have been identified. Second messengers activate other intracellular chemicals to produce the target cell response. G protein Kinases are enzymes that catalyze the phosphorylation of proteins! Refer to page 491 onsteroid G protein PIP 2 Refer to page 492 2

3 Refer to p. 490 Diseases of the Endocrine System Goiter - Enlargement of the thyroid OT associated with overproduction of thyroid hormone or malignancy. Causes: Diet deficient in Iodine (Rare in the US) or an increase in TSH due to a defect in normal hormone synthesis within the thyroid gland. Treatment: Only needs to be treated if it is causing symptoms. The enlarged thyroid can be treated with radioactive iodine to shrink the gland or with surgical removal of part or all of the gland (thyroidectomy). Small doses of iodine may help when the goiter is due to iodine deficiency. Refer to page 504 3

4 Hypothyroidism Most common cause is Hashimoto's thyroiditis - body's immune system mistakenly attacks the thyroid gland (autoimmune). Common symptoms: Fatigue Weight gain Feeling cold Dry skin and hair Heavy menstrual periods Constipation Slowed thinking Cretinism Congenital hypothyroidism Cretinism is a congenital deficiency in the secretion of the hormone thyroxine from the thyroid gland. The brain and skeleton fail to develop properly, resulting in mental retardation and dwarfism. Symptoms: gradual development of a characteristic coarse, dry skin, a slightly swollen face and tongue, umbilical hernia, and an open mouth that drools. The baby is usually listless, slow-moving, constipated, and a slow feeder. Treatment: Thyroid hormone promotes normal physical and mental development. It is essential that treatment be started during the first six weeks of life or irreversible changes may take place! All states and U.S. territories test newborns for hypothyroidism. Hyperthyroidism Most common cause is Graves' disease. This occurs when the body's immune system overstimulates the thyroid. Common symptoms: Shaking, increased nervousness, irritability Rapid heart beat or palpitations Feeling hot Weight loss Fatigue More frequent bowel movements Shorter or lighter menstrual periods Refer to page 504 Diabetes Mellitus The body does not produce or properly use insulin. The cause of diabetes continues to be a mystery, although both genetics and environmental factors such as obesity and lack of exercise appear to play roles. Major Types of Diabetes Type 1: body's failure to produce insulin. Type 2: insulin resistance (body fails to properly use insulin), as well as relative insulin deficiency. Most Americans who are diagnosed with diabetes have type 2 diabetes. Gestational diabetes: Pregnant women who have never had diabetes before but who have high blood sugar (glucose) levels during pregnancy are said to have gestational diabetes. 4

5 Addison Disease Most cases are caused by the gradual destruction of the adrenal cortex by the body's own immune system. Often both cortisol and aldosterone are lacking. Rare. Common symptoms: Drop in blood sodium and glucose Increase in blood potassium Dehydration Weight loss Treatment: Replace missing hormones with similar steroids. Since Addison's disease is a chronic condition, daily replacement medication can never be stopped. Routine care includes regular physician visits, avoidance of dehydration, and the use of extra medication during illness. Cushing Syndrome The body's tissues are exposed to excessive levels of cortisol for long periods of time. Refer to p 508 Many people suffer the symptoms of Cushing s because they take prednisone for asthma, rheumatoid arthritis, lupus or other inflammatory diseases. Others develop Cushing syndrome because of overproduction of cortisol by the body. Benign tumors of the pituitary gland cause most cases of Cushing syndrome. The tumor causes increased secretion of ACTH. This form of the syndrome, known as "Cushing's disease," affects women five times more frequently than men. Cushing Syndrome Common symptoms: Redistribution of body fat fatty moon face Thin reddened skin 5

6 Gigantism Caused by hypersecretion of GH during the growth years (before ossification of the epiphyseal plates). Robert Wadlow was one of history's largest humans standing 8' 11.1". Born and educated in Alton, Illinois, he was later buried there at age 22, weighing 490 pounds at the time of his passing. Acromegaly Caused by hypersecretion of GH after skeletal fusion has occurred. The cartilage still left in the skeleton continues to form new bone. May result in a distorted appearance due to enlargement of hands, feet, face, jaw (causing separation of the teeth), and other body parts. The patient s skin usually thickens and the pores are more pronounced. Pituitary Dwarfism Involves abnormally short stature with normal body proportions caused by hyposecretion of GH during the growth years. 6