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37 Cards in this Set
- Front
- Back
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What are the extreme importance of the thyroid gland? |
- regulates metabolism body-wide - crucial determinant of normal growth and development - optimizes sensitivity of particular tissues to other hormones |
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Why is fetal thyroid tissue particularly important? |
- maternal thyroid hormones cannot cross the placenta - fetal thyroid hormone necessary for MANY systems (CNS, bone, respiratory...) |
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What are the 5 systemic effects of thyroid hormones? |
1. increase basal metabolic rate 2. promote growth and maturation 3. central nervous system effects 4. autonomic nervous system 5. temperature regulation |
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How does the TH increase basal metabolic rate? |
- increases cardiac output - blood oxygenation - glucose uptake - glycogen formation - lipid mobilization - protein synthesis |
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How does TH promote growth and maturation? |
- required for normal skeletal growth - promotes bone growth - stimulates both GH secretion and somatomedin production |
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How does TH effect the CNS? |
- necessary for perinatal maturation - promotes neural branching and myelinization of nerves |
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How does TH effect the autonomic nervous system? |
- increases sympathetic activity |
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How does TH effect temperature regulation? |
- increases thermogenesis |
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Is the thyroid well or poorly perfused? |
- second best perfused gland in the body |
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What are the function components of the thyroid gland? |
- follicular cells: collection of cells surrounding a cavity that stores material secreted by these cells - colliod: the material inside the follicular lumen - parafollicular cells: single or small groups fo cells tucked among the bases of the follicular cells; participate in Ca regulation - capillaries: abundant; rich supply to follicular cells |
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What amino acid is thyroid hormone derived form? |
- tyrosine is the foundation of thyroid hormone molecules |
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What are the two active and one inactive forms of thyroid hormone? |
- T4= thyroxine: main secretory product, but less potent - T3: mostly formed from T4 in tissues- most biologically potent - reverst T3- an accident! essentially ZERO potency |
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When the term thyroid hormone is used what does it refer to? |
- T3 - T4 |
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What is another important substance required for thyroid hormones? |
- iodine |
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Is iodine strictly regulated? |
- iodine is so important that its reverses are strictly regulated - very little eliminated - several systems for reabsorption and recycling |
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What is the source of iodine? |
- DIET! - absorbed into the blood from small intestine |
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What does the formation of thyroid hormone depend on? |
adequate dietary supply of iodine |
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What is the iodide pump? |
- follicular cells have iodide pump that takes iodide from blood and pumps it into follicular cells |
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Does the thyroid adapt to the iodine supply? |
- yes - animals living in areas with iodine-poor soils up-regulate the iodine active transport systems - iodine trapping thus facilitated - this upregulation takes time, though: sudden deprivation increases risk of hypothyriodism or even death |
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Is thyroglobulin a thyroid hormone? |
No |
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What is thyrogobulin? |
- essential for thyroid hormone production - large protein molecules synthesized by thyroid follicular cells that contains thyrsines in its amino acid sequences - made inside the follicular cells and then transported into the follicular lumen where it is stored pending iodide arrival |
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Describe the acquisition and processing of iodine. |
- enterocytes pick up iodine - iodide transported through systemic blood to thyroid follicular cells - thyroid follicular cells actively pump iodide from plasma into their cytosol - iodide is oxidized intracellulary and then transported into follicular lumen - the oxidized iodine is highly reactive and quickly binds to a tyrosyl residue on thyroglobulin in the lumen |
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What is MIT? DIT? |
MIT: results when one carbon on tyrosine ring is iodinated
DIT: results when tow carbons on tyrosine ring are iodinated |
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Where does the combining of MIT/DIT occur? |
- must take place on thyroglobulin molecules, in follicular lumen |
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MIT + DIT=? |
T3- triiododthyronine |
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DIT + DIT=? |
- T4- tetraiododthyronine |
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How is thyrogobulin stored? |
- iodonated thyrogobuline molecule is endocytoses from the follicular lumen and back into the follicular cell - thyroglobulin has a multiple substances on it, including MIT, DIT, T3, T4 |
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How is thyroid hormone released into the blood? |
- upon "cue" thyroglobulin molecules is endocytosed from the follicular lumen and back into the follicular cell - proteolysis of thyroglobulin causes release various things: MIT and DIT (deiodonated both the iodines and tyrosines are re-used) AND T3 and T4 (thyronines) - the lipid-soluble thyronines freely diffuse through basal cell membrane, into interstitium, and into the systemic blood - thyronines picked up by binding proteins and transported throughout body |
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Where does T3 formation take place? |
- some takes place in follicular lumen, but.. - MOST T3 formed outside the thyroid gland - tissues tha makes lots because has lots of enzyme: LIVER and KIDNEY - tissues that does lots by virtue of mass: SKELETAL MUSCLE |
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What is reverse T3? |
- another triple-idodnated tyrosine, but 4th iodine was removed at wrong site - mistake with little if any biological activity |
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Are thyroid hormones water or lipid soluble? |
Lipid soluble so requires carrier proteins to move through plasma |
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Which one, T4 or T3, binds tighter to the carrier protein? |
- T4 carriers bind very tightly - T3 carries bind less tightly - partly why T3 has much greater activity than T4 (remember only unbound hormone is active) |
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What is the hypothalamic-pituitary- thyroid axis? |
- main feedback is the T3/T4 inhibition of TSH - smaller effect on thyroid-releasing hormone |
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What is the typical day-to day stimulation of thyroid hormone synthesis? |
- plasma levels of T3/T4 act directly on anterior pituitary to influence thyroid stimulating hormone release - lower T3/T4 levels cause TSH release and vice-versa |
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What is the stimulation of thyroid hormone synthesis and release during short periods of stress or high metabolic demand? |
- hypothalamus responds by secreting thyroid releasing hormone - TRH travels to anterior pituitary gland - TRH there supersedes the "day-to-day" control of T3/T4 and causes increased release of thyroid stimulating hormone - thyroid gland responds by making/releasing more thyronines |
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How is thyroid hormone synthesis/release inhibited day-to-day? (Prolonged high stress? ) |
- day-to-day: adequate plasma T3/T4 levels feed back to the anterior pituitary and inhibit release of further TSH (- prolonged high stress: extra-high levels of T3/T4 cause TRH shutdown ) |
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What are the general disorders of thyroid function? What species are the typically found in? |
hypothyroidism: dogs Hyperthyroidism: cats |
