2. Thyroid Gland located anteriorly in cervical region, just inferior to thyroid cartilage; two...
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Transcript of 2. Thyroid Gland located anteriorly in cervical region, just inferior to thyroid cartilage; two...
2 .Thyroid Gland• located anteriorly in cervical region, just inferior to
thyroid cartilage; two lobes connected by thin isthmus
• largest purely endocrine gland in body
• consists of follicles (cuboidal or simple squamous epithelium) filled with colloid (combination of protein [thyroglobulin] containing amino acid tyrosine [building block of thyroid hormones])
• parafollicular cells produce calcitonin
http://www.usc.edu/hsc/dental/ghisto/end/c_26.html
2. Thyroid Gland: T4 and T3
• hormones based on amino acid tyrosine (differ in number of iodine ions)
– thyroxine (tetraiodothyronine [T4]) and
– triiodothyronine (T3)
• T3 is 10x more active, but less common (T4 accounts for about 90% of all thyroid hormone)
• much T4 converted to T3 by liver, kidneys, some other tissues
2 .Thyroid Gland: T4 and T3• affect metabolic rate of every cell in the body, except
brain, spleen, testes, uterus and thyroid gland
– affect other activities within these organs and glands
• readily cross membranes (diffuse through plasma membrane to bind to mitochondrial receptors and receptors in nucleus)
2 .Thyroid Gland
T4 and T3: Actions
• increase synthesis of enzymes involved in cellular respiration --> increase basal metabolic rate
– increases glucose oxidation --> ATP synthesis
• increases ATP synthesis in cytoplasm and by mitochondria
– results in increased heat production (calorigenic effect)
• work with GH to promote normal tissue growth and development, especially important to growth/development of CNS, skeletal and reproductive systems
T4 and T3: Control
• release stimulated by TSH (thyroid-stimulating hormone from adenohypophysis)
– release of TSH stimulated by TRH from hypothalamus
• release of TRH is stimulated by cold, pregnancy, low thyroxine
• release inhibited by GHIH, high glucocorticoid levels, high sex hormone levels, high iodine
Hypothyroidism• too little thyroid hormone (thyroid gland defect, inadequate TSH, TRH,
or iodine)
– Hashimoto’s thyroid – autoimmune disorder in which thyroid is attacked and function decreases
– myxedema - low BMR, constipation, puffy eyes, edema, lethargy, mental sluggishness
– endemic goiter - enlargement of thyroid gland usually due to lack of sufficient iodine
– cretinism - genetic deficiency of thyroid gland or lack of dietary iodine during development resulting in mental retardation, disproportionate growth, short body with thick tongue and neck
• treatment - reversed by iodine supplements or hormone replacement therapy
Goiter
swelling in neck due to thyroid hypertrophy both hypo- and hyperthyroidism
Hyperthyroidism• too much thyroid hormone (thyrotoxicosis)
– Grave’s disease - autoimmune disease in which abnormal antibodies similar to TSH mimic its function and continuously stimulate release of thyroid hormones; results in high BMR, sweating, rapid heart rate, weight loss, restlessness, mood shifts, fatigues easily, limited energy; also toxic goiter
– exophthalmos - protrusion of eyeballs, fibrous tissue become edematous (swollen)
• treatments - removal of thyroid gland or irradiation– patient must be on synthetic thyroid hormone the rest of
his/her life
2 .Thyroid Gland: Calcitonin (CT)• polypeptide produced by parafollicular cells• actions: decreases blood calcium levels by:
– stimulating osteoblasts (Ca2+ uptake and incorporation into bone)
– inhibiting osteoclast activities (osteoclasts break down bone matrix releasing calcium)
• control: responds directly to blood calcium levels• very rapid effect• probably more important during childhood when it
stimulates bone growth• important because at high blood Ca2+, membranes become
less permeable to Na+
3 .Parathyroid Glands• 2 paired structures on posterior of thyroid gland
• oxyphyil cells - function unknown
• chief cells secrete parathyroid hormone (PTH; protein)
• actions: increases blood Ca2+ by:
– stimulating osteoclast activity (which break down bone matrix) while inhibiting osteoblasts (which form bone matrix)
– stimulating increased reabsorption of Ca2+ by kidney
– indirectly stimulating increased absorption of Ca2+ by small intestine by causing secretion of calcitrol form kidneys
3 .Parathyroid Glands
http://www.usc.edu/hsc/dental/ghisto/end/
c_32.html
Hyperparathyroidism
• rare; caused by parathyroid gland tumor
• results in hypercalcemia (excess Ca2+ levels in blood) --> depression of nervous system (because of effect on sodium permeability), abnormal reflexes, skeletal muscle weakness, nausea, vomiting, kidney stones, calcium deposits in soft tissues; bones become soft
Hypoparathyroidism
• trauma to or removal of parathyroid gland
• results in hypocalcemia (low blood Ca2+) --> neurons become
too excitable --> muscle tetany --> spasms/cramps -->
respiratory paralysis --> death
4 .Adrenal Glands• located in abdominal cavity against back wall
(retroperitoneal), superior to kidney• surrounded by connective tissue capsule• two regions:
– cortex - outer region, “glandular”, three zones• zona glomerulosa - outer zone• zona fasciculata - middle zone• zona reticularis - inner zone
– medulla - inner region, modified neural tissue (develops from same tissue in embryo as ganglionic [postganglionic] neurons of sympathetic division)
4 .Adrenal Gland: Regions and Zones
Adrenal Cortex: Zona Glomerulosa• produces steroid hormones based on cholesterol
• mineralocorticoids - ion (and water) balance
– main hormone is aldosterone
• action:
* stimulates reabsorption of Na+ and secretion of K+ from kidney, sweat glands, salivary glands, pancreas
– secondarily, increases water reabsorption in kidney (water follows Na+)
Adrenal Cortex: Zona Glomerulosacontrol:
• aldosterone release stimulated by:
– high K+, low Na+
– angiotensin II (result of renin-angiotensin pathway stimulated by low blood pressure),
– ACTH (when under severe stress)
• inhibited by low K+, high Na+
Adrenal Cortex: Zona Glomerulosa
Disorders:
• aldosteronism = hypersecretion (adrenal tumor)
→ increased water and Na+ reabsorption --> hypertension, edema;
→ loss of K+ --> disruption of neural and muscle function
Adrenal Cortex: Zona GlomerulosaDisorders:• Addison’s Disease = hyposecretion glucocorticoids and
mineralocorticoids– results in decreased Na+ and water reabsorption,
increased blood K+ --> low blood volume --> hypotension, dehydration;
– changes in membrane potentials --> disruption in neural and muscular function
– also decreased cortisol secretion by zona fasciculata --> decreased blood glucose levels (especially during prolonged stress)
Adrenal Cortex: Zona Fasciculata• glucocorticoids - effects on glucose metabolism
• main hormone is cortisol (hydrocortisone)
• actions:
– maintains blood glucose levels, especially in times of stress, by:
• promoting gluconeogenesis (making new glucose in liver) and use of alternative fuels by other cells (saves glucose for the brain)
– anti-inflammatory decrease immune response
* can be used clinically to treat allergic reactions (e.g., poison ivy), rheumatoid arthritis
Adrenal Cortex: Zona Fasciculata• Control
– stimulated by ACTH
– inhibited by cortisol (inhibits secretion of CRH from hypothalamus)
– blood levels peak in the morning
Disorders:
• Addison’s Disease
- hyposecretion of glucocorticoids and mineralocorticoids
Zona Fasciculata: Cushing’s Disease• hypersecretion of glucocorticoids • caused by hypersecretion of ACTH due to tumor in ZF,
pituitary, lungs, kidneys, or pancreas• suppresses glucose metabolism resulting in
– hyperglycemia (elevated glucose= steroid diabetes), – stimulates lipid metabolism (weight loss), – loss of muscle and bone mass,– “buffalo neck” and “moon face” (fat redistribution),– anti-inflammatory effects (mask infection)– water and salt retention (effect of aldosterone
hypersecretion --> water retention --> hypertension)
Adrenal Cortex: Zona Reticularis• gonadocorticoids
• most are androgens (“male” sex hormones) - converted to testosterone; small amounts of estrogens
• actions: may contribute to onset of puberty (levels rise between 7 and 13 years of age; exact function compared to hormones from ovaries or testes unclear)
• control: stimulated by ACTH
Adrenal Cortex: Zona Reticularis
• hypersecretion results in:
– masculinization and masculine pattern of hair distribution in females
– in males - rapid maturation of reproductive organs, secondary sex characteristics; hypersecretion of estrogens causes feminization and gynecomastia (enlarged breasts)
Adrenal Medulla• chromaffin cells (modified neurons - arise from
same embryonic tissue as postganglionic neurons of sympathetic division)
• catecholamines - epinephrine (~80%), norepi (NE)
• control: secretion stimulated by preganglionic fibers of sympathetic nerves during flight-or-fight response
Adrenal Medulla• actions:
– epinephrine (more potent) - increases HR (beta receptors), bronchodilation (in lungs), increased blood glucose (breakdown of glycogen in liver and skeletal muscle, and breakdown of adipose tissue)
– NE - peripheral vasoconstriction --> increased BP