Physiology Exam 3 Chapter 11 20 Thyroid Gland The thyroid gland is located just below the larynx its two lobes are positioned on either side of the trachea and it is connected anteriorly by the isthmus The thyroid is the largest pure endocrine gland It is composed of thyroid follicles and its follicular cells are simple cuboidal epithelium The follicular cells synthesize the principle hormone thyroxine T4 Thyroid follicles contain colloid protein rich solution Between follicles are parafollicular cells which produce a hormone called calcitonin Calcitonin is important in maintaining calcium homeostasis in blood it decreases amount of calcium and secrete more calcium in urine Calcitonin also inhibits osteoclast cells which dissolve bone bone is calcium reservoir Thyroid hormones are T3 triiodothyronine and T4 tetraiodothyronine thyroxine Thyroid Gland Under the influence of TSH thyroid stimulating hormone the thyroid follicles actively removes iodide from the blood and secretes it into the colloid in which it is oxidized into iodine Once iodine is in the colloid enzymes in follicles will combine iodine with tyrosine resides of a protein called thyroglobulin The attachment of one iodine to tyrosine produces monoiodotyrosine MIT and the attachment of two iodines produces diiodotyrosine DIT Within the colloid enzymes modify the structure of MIT and DIT and couple them together When two DIT molecules that are modified and coupled together a molecule of tetraiodothyronine T4 thyroxine is produced The combination of one MIT with one DIT forms triiodothyronine T3 At this point T3 and T4 are still attached to thyroglobulin Thyroid Gland Upon stimulation by TSH the cells of the follicle take up a small volume of colloid by pinocytosis enzymes cleave T3 and T4 from thyroglobulin hydrolyze T3 and T4 and secrete the free hormones into the bloodstream Effect of thyroid hormones on target cells trigger protein synthesis in cells promote maturation of the nervous system and increases BMR basal metabolic rate Thyroid Gland Goiter enlarged thyroid gland 2 types Endemic iodine deficient and Graves disease thyrotoxicosis In iodine deficiency endemic goiter the thyroid cannot produce adequate amounts of T4 and T3 because of an absence of sufficient dietary iodine The resulting lack of negative feedback inhibition causes abnormally high levels of TSH secretion which in turn stimulates the abnormal growth of the thyroid People who have inadequate secretion of thyroid hormones are said to be hyprothyroid People who are hypothyroid have an abnormally low BMR experience weight loss and adults can have swellings of the face hands and feet Hypothrodism not enough thyroid hormone cold all the time Thyroid Gland Graves disease is an autoimmune disease In Graves disease autoantibodies antibodies that bind to self antigens bind to TSH receptors on thyroid cells and overstimulate the thyroid gland Because the production of these autoantibodies is not inhibited by negative feedback the high secretion of thyroxine that results cannot turn off the excessive stimulation of the thyroid As a result the person is hyperthyroid and develops a goiter This condition is called toxic goiter or thyrotoxicosis Symptoms of hyperthyroidism include high BMR increased blood pressure and bulging of the eyes Hyperthyroidism too much thyroid hormone hot all the time Thyroid Gland Because of its stimulation of protein synthesis children need thyroxine for body growth and for the proper development of the nervous system The need for thyroxine if particularly important when the brain is undergoing its greatest rate of development from the end of the first trimester of prenatal life to six months after birth Hypothyroidism lack of iodine causes excessive growth of thyroid during this time may result in cretinism People with cretinism suffer severe mental retardation Parathyroid Glands These small flattened glands are embedded in the posterior surfaces of the lateral lobes of the thyroid gland There are four parathyroid glands a superior and an inferior pair Parathyroid hormone is the only hormone secreted by the parathyroid glands PTH is the most important hormone controlling the calcium concentrations in the blood and is antagonist to calcitonin which decreases calcium in blood It promotes a rise in blood calcium levels by acting on the bones kidneys and intestine It causes kidneys to reabsorb calcium ions and causes bones to release calcium A rise in blood calcium can then exert negative feedback inhibition on parathyroid hormone secreation Pancreas The pancreas is both an endocrine and exocrine gland Exocrine function in digestion in which pancreatic juice contains enzymes proteases breakdown of peptide bonds pancreatic lipase pancreatic amylase The endocrine portion of the pancreas consists of scattered clusters of cells called islets of Langerhans The cells in the islets are the alpha beta and delta cells Alpha cells produce hormone glucagon beta cells produce hormone insulin and delta cells produce hormone somatostatin inhibits activity of alimentary canal Pancreas There are more than twice as many insulin secreting beta cells as alpha cells in each islet Insulin is the primary hormone regulating the plasma glucose concentration A rise in plasma glucose stimulates the beta cells of the islets to secrete increased amounts of insulin Insulin then binds to its receptors in the plasma membrane of its target cells and through the action of signaling molecules causes intracellular vesicles containing GLUT4 carrier proteins to translocate the plasma membrane These carrier proteins promote the facilitated diffusion of glucose into the cells of insulin s target organs Once in cells insulin triggers lipogenesis formation of energy storage molecules and promote glycogenogenesis glucose to form glycogen Also insulin indirectly stimulate the activity of the enzyme glucagon synthetase in skeletal muscles and liver which promotes the conversion of intracellular glucose into glycogen for storage Insulin thereby causes glucose to leave the plasma and enter the target cells where it is converted into the energy storage molecules of glycogen and fat Through these effects insulin lowers the blood glucose concentration as it promotes anabolism Pancreas Glucagon secreted by the alpha cells of the pancreatic islets acts antagonistically to insulin it promotes effects that raise the plasma glucose concentration Glucagon secretion is stimulated by a fall