Endocrinology and Metabolic Homeostasis

I. Introduction. (Overhead T19-1)
A. Endocrinology: the medical study of the endocrine system, its related structures, associated diseases and conditions, and treatments.
B. A physician who specializes in the diagnosis and treatment of endocrine disorders is called an endocrinologist.

II. Structure and Function of the Endocrine System. (Overhead T19-2)
A. The endocrine system is also called the hormonal system and regulates the metabolic functions of the body.
B. These functions include:
1. The rates of chemical reactions in cells.
2. The secretion of chemical substances called hormones.
3. Growth.
4. Homeostasis.
C. A gland is an organized collection of cells that secrete or excrete a substance.
D. The body contains two types of glands.
1. Endocrine glands secrete substances directly into the bloodstream.
(a) They secrete hormones that bind with specific receptors, which initiate specific biological activities.
2. Exocrine glands send their output through ducts.
(a) We have previously discussed many of these exocrine glands.
(b) They include the sweat, mucous, salivary, and mammary glands. This chapter will discuss the endocrine glands.
E. Hypothalamus. (Overhead T19-3)
1. The hypothalamus is part of the brain, but it is considered an endocrine gland because it releases hormones into the blood to act at sites outside the brain.
2. It controls the actions of the pituitary gland.
(a) It secretes hypothalamic-releasing hormones, which stimulate the pituitary to release hormones.
(b) It secretes hypothalamic-inhibiting hormones, which decrease the amount of hormones the pituitary releases.
F. Pituitary gland. (Overhead T19-4)
1. The pituitary gland is located in the sella turcica, a bony cavity at the base of the brain.
2. Hormones secreted by the posterior pituitary gland:
(a) Antidiuretic hormone (ADH) stimulates the kidneys to reabsorb water from the renal tubules and assists in controlling blood pressure.
(b) Oxytocin stimulates milk flow from the mammary glands and assists in childbirth.
3. Hormones secreted by the anterior pituitary gland:
(a) Growth hormone (GH) promotes growth of the entire body.
(b) Thyroid-stimulating hormone (TSH) controls the rate of secretion of triiodothyronine (T3) and thyroxin (T4). T3 and T4 control the rate of metabolism in cells.
(c) Adrenocorticotropic hormone (ACTH) stimulates the adrenal gland to secrete hormones that control metabolism.
(d) Gonadotropic hormones include follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which are concerned with the development ova and spermatozoa.
(e) Prolactin regulates breast growth and milk synthesis.
G. Adrenal glands. (Overhead T19-5)
1. The adrenal glands are located on top of the kidneys.
2. They function as two separate glands.
(a) The adrenal cortex synthesizes aldosterone and cortisol, as well as androgens.
(b) The adrenal medulla secretes epinephrine and norepinephrine (catecholamines), which act as neurotransmitters for sympathetic nerves.
H. Thyroid gland. (Overhead T19-6)
1. The thyroid gland is wrapped around the front of the trachea.
2. It secretes three hormones.
(a) Thyroxin (T4) and triiodothyronine (T3), which increase the rate of metabolism.
(b) Calcitonin, which promotes the uptake of calcium in the bones and decreases calcium concentration in the blood.
I. Parathyroid glands. (Overhead T19-7)
1. There are four parathyroid glands, two inferior and two superior, located on the posterior surface of the thyroid gland.
2. They secrete parathyroid hormone (PTH), which controls the calcium concentration in the blood.
J. Pancreas. (Overhead T19-8)
1. As discussed in Chapter 14, the pancreas performs digestive functions.
2. It also contains cellular masses called the Islets of Langerhans, which secrete insulin and glucagon and comprise the endocrine portion of the pancreas.
(a) Insulin lowers the amount of glucose in the blood and controls the rate of metabolism of most carbohydrates.
(b) Glucagon promotes conversion of glycogen to glucose in the muscles and liver and helps maintain a baseline blood glucose level.
K. Kidneys. (Overhead T19-9)
1. The kidneys regulate fluids, electrolytes, and plasma pH and are the primary organs that preserve homeostasis of body fluids.
2. They release three substances.
(a) Renin stimulates aldosterone production and increases blood pressure.
(b) Erythropoietin promotes the production of red blood cells.
(c) A metabolically active form of vitamin D increases calcium absorption by the intestines.
L. Pineal gland and thymus gland. (Overhead T19-10)
1. The pineal gland is located in the central portion of the brain.
(a) Little is known about its exact function.
(b) It secretes melatonin, which is believed to influence the onset of puberty and affect the wake-sleep cycle.
2. The thymus gland is located behind the sternum.
(a) It secretes thymosin, which promotes development and functioning of the immune system in newborns.
(b) It shrinks in size as one ages.

III. Metabolic Homeostasis. (Overhead T19-11)
A. Metabolism is the total of all chemical and physical changes that occur in body tissue.
1. Anabolism is the building of complex substances (proteins) from simple substances.
2. Anabolism requires energy and occurs in all cells as they maintain themselves, divide, and produce substances such as hormones.
3. Catabolism is the breaking down of larger molecules into smaller ones.
B. Homeostasis is the state of equilibrium in the body regarding bodily functions and chemical composition of tissues and fluids.
1. Vast feedback loops help regulate such important dynamic body states as body fluid volume, electrolyte and mineral concentrations, vascular and cellular hydration, and the pH of blood.
C. Hormones play a vital role in maintaining homeostasis.

IV. Examining the Endocrine System. (Overhead T19-12)
A. Assessment is largely through discovering patterns of symptoms that may signify an endocrine disorder.
B. Inquire about conditions such as excessive thirst, excessive hunger, weakness, and fatigue.
C. Screen for common endocrine diseases and conditions such as:
1. Diabetes mellitus (DM).
2. Thyroid dysfunction.
3. Adrenal insufficiency.

V. Diseases and Conditions. (Overhead T19-13)
A. Addison disease: also known as adrenocortical insufficiency because the adrenal cortex atrophies.
B. Cushing syndrome: hyperfunction of the adrenal cortex that leads to such symptoms as moon face, trunkal obesity, weakness, hypertension, etc.
C. Diabetes mellitus (DM): a disease in which plasma glucose control is defective because of insulin deficiency or decreased target-cell response to insulin.
D. Dwarfism: the condition of being abnormally undersized. There are many types and causes, one of which is lack of growth hormone.
E. Euthyroid goiter: an enlargement of the thyroid gland that is not due to neoplasm.
F. Graves disease: an autoimmune disorder characterized by an increase in metabolic rate, weakness, severe weight loss, goiter, etc.
G. Hyperglycemia: plasma glucose concentration increased above normal levels, a common feature of diabetes mellitus.
H. Hyperthyroidism: excessive activity of the thyroid gland with an increase in secretion of thyroid hormone, resulting in weight loss, weakness, etc.
I. Hypoparathyroidism: deficient secretion of parathyroid hormones, resulting in severe muscle spasms due to hypocalcemia.
J. Hypothyroidism: diminished activity of the thyroid gland with a decrease in production of thyroid hormones.
K. Metabolic acidosis: decreased pH and bicarbonate concentration in the body fluids caused by accumulation of acids or abnormal loss of bases.
L. Metabolic alkalosis: increased pH due to high bicarbonate concentration in body fluids from excessive intake of alkaline substances and loss of acid through urination or vomiting.
M. Obesity: excessive accumulation of fat in the body.
N. Tay-Sachs disease: an inherited fatal disorder characterized by the body's inability to properly process fat.
O. Thyroid cancer: carcinoma of the thyroid gland.

VI. Categories of Diabetes Mellitus. (Overhead T19-14)
A. Type 1 immune mediated diabetes: requires the use of insulin because the pancreas produces little to none.
B. Type 2: affects the way in which the body uses food. Typically diagnosed in patients over 30 years of age.
C. Type 3: diabetes mellitus with other conditions or syndromes.
D. Type 4 impaired glucose tolerance (IGT): glucose levels between normal and diabetic.
E. Type 5 gestational diabetes: a carbohydrate intolerance that develops in 2% to 5% of all pregnancies and disappears when a pregnancy is over.

VII. Common Diagnostic Tests and Procedures. (Overhead T19-15)
A. Adrenocorticotropic hormone (ACTH) stimulation test: used to evaluate adrenal hormone function. An initial blood sample is drawn. ACTH is then administered to the patient. After a specific time period (usually 1 hour), a second blood sample is taken. The plasma cortisol levels of the two samples are compared to evaluate adrenal functioning.
B. Ammonia test: measures the level of ammonia in the blood. Used to evaluate metabolism.
C. Biopsy of glandular tissue: a specimen is examined microscopically. Commonly used to diagnose malignant growths.
D. Calcitonin test: measures the level of calcitonin in the blood. Elevated levels may indicate thyroid cancer.
E. Fasting blood sugar: measures the level of circulating glucose after a 12-hour period of fasting.
F. Follicle-stimulating hormone (FSH) test: a 24-hour urine test that measures the levels of FSH and LH. Useful in determining whether the ovaries, testes, and pituitary are functioning properly.
G. Glucose tolerance test (GTT): measures serum glucose levels at specified intervals. Used to diagnose diabetes and other disorders of carbohydrate metabolism.
H. Radioactive iodine uptake (RAIU) test: measures the functioning of the thyroid. The uptake of radioactive iodine by the thyroid is measured following its oral administration.
I. Thyroid-stimulating hormone (TSH) test: a thyroid function test that measures the level of TSH in blood serum. Used to diagnose hyperthyroidism and hypothyroidism.
J. Thyroxine (T4), free: measures the level of available thyroxine in the serum. Used to diagnose hyperthyroidism and hypothyroidism.

VIII. Drug Classifications. (Overhead T19-16)
A. Antithyroid drug: inhibits the production of T3 and T4 in the thyroid gland to treat hyperthyroidism.
B. Corticosteroid: acts locally and systemically to produce an anti-inflammatory effect.
C. Hormone: produces effects on body systems, such as skeletal growth in children with dwarfism and water balance in diabetes insipidus.
D. Insulin: replaces pancreatic insulin, which is a major metabolic regulator and necessary for survival.
E. Oral antidiabetic drug: acts by stimulating the pancreas to produce more insulin, increasing the number of insulin receptors, or inhibiting digestion of glucose. Used to treat type 2 diabetes mellitus.
F. Thyroid replacement drug: replaces the thyroid hormones T3 and T4 in patients with hypothyroidism.

IX. Therapeutic Procedures. (Overhead T19-17)
A. Adenectomy: surgical excision of a gland.
B. Excision of a tumor: removal of a growth as a treatment for cancer.
C. Radioactive iodine ablation: administration of iodine 131, a radioactive form of iodine, that is preferentially taken up by thyroid cells, causing their cell deaths. This procedure is sometimes used to treat hyperthyroidism.
D. Thyroidectomy: surgical removal of the entire thyroid gland usually as a treatment for cancer. Thyroid replacement drugs are typically indicated following this procedure.
- posted by gopi @ 5:47 AM