Term
| Where is the thyroid gland found? |
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Definition
| on both sides of trachea, below larynx |
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Term
| Describe the structure of the thyroid gland (3 points) |
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Definition
-heavily vascularised
-single layer of epithelial cells secrete thyroid hormones
-lumen filled with colloid (protein rich fluid)
-parafollicular cells secret calcitonin |
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Term
| Describe the steps involved in the synthesis of thyroid hormones |
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Definition
1. Active uptake of iodine
-iodide actively transported through follicular cells
-accumulates in colloid as iodine (I2)
2. Iodination of thyroglobulin
-Tgb released into lumen
-Tyrosine peroxidase binds I2 to tyorosine residues
3. Tgb-I2 uptake
-endocytosis brings the complex from colloid into cells, where it fuses to lysosomes and is split into thyroid hormones |
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Term
| Describe the structure and release of Thyroid Hormone |
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Definition
-each tyrosine residue has 2 possibly sites for I2 binding
(monoiodotyrosine vs diiodotyrosine)
T4 = 2 cleaved diiodotyrosines = Thyroxine
T3 = 1 monoiodotyrosone + 1 diiodotyrosone = Triiodothyronine
-T3 & T4 are lipophilic, can diffuse across basal membrane into interstitial space then into blood capillaries
-will then bind to carrier proteins in the blood |
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Term
| What proportion of thyroid hormones are T3, and what proportion are T4? |
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Definition
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Term
| Describe the transport & metabolism of Thyroid Hormones |
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Definition
-TBG carrier protien binds majority of circulating hormones, some are free, remainder are bound to albumins
-only the free portion is active and can diffuse into tissues
-T4 is transformed into T3 into tissues cells, which is degraded after acting |
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Term
| What happens to I2 after T3 has done its job in the target cell? |
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Definition
| it is deiodinated, diffuses out of cell, degraded |
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Term
| Which is the active form of TH in tissues T3 or T4? Why? |
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Definition
T3
the nuclear TH receptor in target cells has a much higher affinity for T3 |
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Term
| Describe the action of Thyroid Hormones in their target cells (6 points) |
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Definition
-T4 is converted to T3, binds to nuclear receptor, activates gene transcription
-increase basal metabolic rate of cells, increase heat production & O2 consumption
-also stimulate secretion of GH during development (important for bone & tissue growth)
-increase # of (nor)adrenalin receptors
-essential for normal function of gonads
-increase velocity of nerve conduction |
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Term
| What happens if TH is deficient during late gestation and early after birth? |
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Definition
| -improper brain function (cretinism) |
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Term
| How is Thyroid Hormone regulated? |
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Definition
1. TRH from hypothlamus increases TSH from pituitary
2. TSH binds to receptor on follicular cells, stimulate cAMP pathway, stimulates endocytosis of Tgb-I2 & TH synthesis
3. TH creates negative feedback on pituitary & hypothalamus |
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Term
| What happens if an animal is exposed to cold for a long period of time? |
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Definition
| TRH release will increase, stimulating pituitary to release TSH which will in turn upregulate the release of Tgb-I2 & TH to increase metabolism and produce more heat |
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Term
| What is primary hypothyroidism? Secondary hypothyroidism? How can you treat it? |
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Definition
Primary: low T4 & T3 concentrations, yet high TSH, usually due to damage to follicular cells of thyroid
Secondary: low TSH secretion, usually due to pituitary/hypothalamus damage
-use the drug thyroxine
-make sure adequate iodine in diet |
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Term
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Definition
enlarged thyroid gland
-due to iodine deficiency, tumors etc
-Low T4/T3 = no feedback, so TRS & TSH are released causing hyperplasia & hypertrophy of follicular cells of thyroid to try and increase the T3/T4 production |
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Term
| What is hyperthyroidism? How can you treat it? |
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Definition
Overproduction of T3/T4
-increases metabolism, excitability/nervousness, heart rate, respiration, digestive tract activity all increase
Treatment: surgery or radioactive I2 to destroy follicular cells |
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Term
| Describe the location & structure of the adrenal glands, and the types of hormones secreted |
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Definition
-cap the kidneys
-consist of outer cortex layer & inner medulla layer
-produces steroid hormones
Medulla: produces catecholamines (nor)epinephrine
3 zones of outer cortex
glomerulosa - mineralcorticoids
fasiculata - glucocorticoids
reticularis - androgens |
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Term
| What type of nerve structure is the adrenal medulla equivalent to? |
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Definition
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Term
| How are the hormones of the adrenal cortex produced? |
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Definition
| -cholesterol is converted into a variety of hormones via enzyme action |
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Term
| What major enzyme is involved with the conversion of cholesterol to various steroid hormones? |
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Definition
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Term
| Describe the synthesis, secretion & circulation of mineralcorticoids |
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Definition
Synthesis: stimulated by angiotensin & [K+] in cortical cells
Secretion: stimulated by above, plus ACTH (minor)
Circulation: loosely bound to cortisol binding globulin (CBG) |
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Term
| What is the most potent mineralcortioid |
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Definition
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Term
| Describe the action of mineralcorticoids such as aldosterone |
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Definition
-binds to receptor in target cell
-increases gene transcription for Na/K pumps in basolateral mmb and Na channels in apical membrane
-stimulates the reabsorption of Na & secretion of K in the distal kidney tubules & collecting ducts |
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Term
| What is the most potent glucocorticoid |
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Definition
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Term
| What do glucocorticoids do? |
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Definition
| regulate glucose metabolism by entering target cells, binding to receptors & stimulating/inhibiting gene transcription |
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Term
| Describe the permissive action of glucocorticoids |
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Definition
| -activates transcription of enzymes that facilitate the action of other hormones |
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Term
| Describe the effect of stress on cortisol production and what results from this |
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Definition
-increases cortisol production
-enhances the effect of norepinephrine on blood pressure
-stimulates gluconeogenesis & inhibits tissue use of glucose to increase blood [glucose]
-stimulates fat/protein degradation to increase levels in blood
Overall effect: inhibit growth, energy conservation during periods of stress |
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Term
| Describe the anti-inflammatory effects of glucocorticoids |
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Definition
-inhibits formation of prostaglandins & cytokines
-reduces lymphocyte migration |
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Term
| How are glucocorticoids regulated? |
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Definition
1. hypothalamus releases ACTH-RH
2. stimulates ant. pit. to release ACTH
3. ACTH binds to receptor on cell surface of adrenal cortex, increases cAMP
4. Cholesterol is mobilized for action by P-450
5. Glucocortoids produced
6. ACTH has negative feedback on ACTH-RH
in hypothalamus
7. Glucocorticoids have negative feedback on hypothalamus AND pituitary |
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Term
| What is Addison's disease? |
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Definition
Glucocorticoid deficiency
-destruction of adrenal cortex cells = no negative feedback = excess ACTH secretion |
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Term
| What is Cushing's disease? |
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Definition
Overproduction of glucocortoids (cortisol)
-often due to tumours on pituitary or adrenal cortex |
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Term
| What are androgens similar to? |
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Definition
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Term
| What role do androgens play in females? |
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Definition
transformed to estrogen in adipose tissue
-are the sole source of estrogen after menopause |
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Term
| What controls the secretion of catecholamines from the adrenal medulla? |
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Definition
-Ach released from preganglionic sympathetic nerves
-Ach depolarizes medulla cells, induce Ca2+ entry & exocytosis of hormones |
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Term
Describe the action of catecholamines
-type receptor they bind to
-alpha receptor effects
-beta receptor effects
-effects on skeletal muscle |
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Definition
Bind to adrenergic receptors (alpha & beta)
Alpha cells associated with SNS
-constrict smooth muscle in arterioles & sphincter of GI tract
Beta cells associated with cardiac muscles (increase contraction) and bronchioles (inhibit contraction)
Skeletal muscle
-low epinephrine levels activate B receptors (relaxation)
-high levels activate alpha receptors (contraction) |
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Term
| What direct & indirect effects does stress have on the body? |
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Definition
Direct: sympathetic system acts on innervated organs
Indirect: increase catecholamine release
-increase cortisol secretion via ACTH-RH & ACTH
-increased alertness, higher heart rate & BP, higher blood [glucose], increase thyroid hormone secretion |
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