Biomed mod.14 obj.17-18

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  1. Thyroid follicle
    • The main functional unit of the thyroid gland
    • Each consists of a central, internal space called the lumen, surrounded by a wall of cells called follicular cells.
    • These cells change shape slightly from squamo-cuboidal to cuboido-columnar when stimulated to produce thyroid hormone
  2. Parafollicular cell or C cells
    a scattered group of cells surrounding each follicle.
  3. What two hormones do thyroid follicles secrete?
    • Thyroxin (tetraiodothyronine, T4)
    • Triiodothyronine, T3 
    • The prefix for each term describes the number of iodine molecules they contain, tetra- meaning four(T4) and tri-meaing three (T3)
  4. Thyroid Function (part 1)
    • Thyroglobulin (TGB)is a protein synthesized inside the thyroid follicular cells.
    • TGB consists of 5000 amino acids, >100 of which are the amino acid tyrosine.
    • To synthesize the thyroid hormones, TGB is released into the lumen of the thyroid follicles.
    • Within the TGBmolecules, one or two iodine atoms attach to each tyrosine molecule.
  5. Thyroid Function (part 2)
    • Tyrosin molecules link to form thyroid hormones containing three or our atoms of iodine.
    • When needed, TGB moves into follicular cells, and digestive enzymes cleave T3 and T4 from the TGB molecule.
    • T3 and T4 are lipid soluble, so they can diffuse through the plasma membrane and bind to the carrier protein thryroxin-binding globulin (TBG) in the blood.
  6. How is thyroid hormone produced / secreted?
    • Low T3 and T4 levels or a low metabolic rate stimulates the hypothalamus to secrete thyrotropin-releasing hormone (TRH),
    • which results in the anterior pituitary producing thyroid-stimulating hormone (TSH).
    • TSH binds to TSH receptors in the follicular cells and activates a number of processes required to synthesize T3 N T
  7. What are the required steps for T3 and T4 synthesis?
    • Iodide trapping
    • synthesis of thyroglobulin (TGB)
    • oxidation of iodide
    • Iodination of tyrosine
    • Coupling of T1 and T2 to make T3 and T4
    • Pinocytosis and digestion of colloid
    • Secretion of thyroid hormones
    • Transport of T3 and T4 in the blood
  8. Required steps for T3 and T4 synthesis:
    1. Iodide trapping
    Iodine circulates in the blood as iodide (I-).  Iodide is actively transported into the follicular cells. Because of the process, the thyroid gland contains most of the iodide in the body.
  9. Required steps for T3 and T4 synthesis:
    2. Synthesis of thyroglobulin (TGB)
    TGB is a glycoprotein produced by follicular cells.  It contains large numbers of the amino acid tyrosine.  Tyrosine is the site on the TGB molecule that will bind with iodine.
  10. Required steps for T3 and T4 synthesis:
    3.  oxidation of iodide
    Before iodide can bind to tyrosine, it must be oxidized and combine with another iodide to form an iodine molecule (I2).
  11. Required steps for T3 and T4 synthesis:
    4.  Iodination of tyrosine
    The side chain of tyrosine may pick up one (T1) or two (T2) iodine molecules.
  12. Required steps for Tand T4 synthesis:
    5.  Coupling of T1 and T2 to make T3 and T4
    As pm pf tje ;ast ste[s. two tyrosine molecules are joined to form either T3 (T1+T2) or T4 (T2+T2).
  13. Required steps for T3 and T4 synthesis:
    6.  Pinocygtosis and digestion of colloid
    Once synthesized, the iodine-containing TGB reenters the follicular cells and digestive enzymes break down the molecule, releasing the formed T3 and T4.
  14. Required steps for T3 and T4 synthesis:
    7.  Secretion of thyroid hormones
    T3 and T4 are lipid soluble, so they freely pass the cell membrane into the interstitial fluid and into he blood.
  15. Required steps for T3 and T4 synthesis:
    8.  Transport of T3 and T4 into the blood
    Once in the blood stream, 99% of the secreted hormone binds to transport proteins, mainly thyrosine-binding globulin (TBG), not to be confused with TGB.
  16. True or False
    The thyroid gland is the only gland to store a large supply of its products.

    approximately 100 days worth.
  17. Thyroid sequence
    • Thyroid hormones
    •   -Regulate oxygen use
    •   -Increase the basal metabolic rate (BMR)
    •   -increase protein synthesis
    •   -Increase carbohydrate and fatty acid catabolism
    •   -Increase reactivity of the nervous system by increasing sensitivity to ephinephrine and norephinephrine.
    •   -Control tissue growth and development along with hGH.
  18. What are the Thyroid hormone actions?
    • Increases the basal metabolic rate
    • Stimulates the synthesis of additional sodium-potassium (Na+/K+) pumps
    • Increases protein synthesis
    • Increases fatty acid and glucose catabolism
    • Decrease blood cholesterol
    • Increases the effects of epinephrine and norepinephrine
    • Accelerates body growth, especially during fetal life and adolescence.
  19. Thyroid hormone actions:
    Increases the basal metabolic rate
    • The basal metabolic rate is the rate fo oxygen consumption while awake, at rest, and fasting.  
    • When the need for ATP increases, the use fo all nutrients increases.
  20. Thyroid hormone actions:
    Stimulates the synthesis of additional sodium -potassium (Na+/K+) pumps
    • this major action has a cascade of effects.
    • With the increase in Na+/K+ pumps, the demand for ATP is greater
    • As ATP is produced, calories are used, and more heat is produced (exothermic). 
    • This is how thyroid hormones help a person regulate their normal body temperature.
  21. Thyroid hormone actions:
    Increase protein sysnthesis
    Encourages growth
  22. Thyroid hormone actions:
    Increases fatty acid and glucose catabolism
    Fatty acids and glucose are used to synthesize ATP
  23. Thyroid hormone actions:
    Decreases blood cholesterol
    Reduces blood cholesterol by increasing cholesterol excretion
  24. Thyroid hormone actions:
    Increases the effects of epinepherine and norepinepherine.
    Enhances the sympathetic nervous response (heart rate, force of heart contraction, and blood pressure)
  25. Thyroid hormone actions:
    Accelerates body growth, especially during fetal life and adolescence
    Work synergistically with human growth hormone and insulin to develop the skeletal and nervous systems
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Biomed mod.14 obj.17-18
2013-02-14 21:39:46
Biomed mod 14 obj 17 18

Biomed mod.14 obj.17-18
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