Endocrine3- Thyroid Phys

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Endocrine3- Thyroid Phys
2015-11-17 20:11:16
vetmed endocrine3

vetmed endocrine3
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  1. Describe the blood supply to the thyroid gland.
    large blood supply, collateral branches, venous drainage is complex
  2. There are __________ nerve fibers to the thyroid gland.
  3. The fetal thyroid gland develops from the __________ and descends from the ___________, forming the ___________, which usually closes and atrophies before birth.
    endoderm; floor of the mouth; thyroglossal duct
  4. Sometimes, you can find active thyroid cells from the ___________ to the ___________, which has implications in thyroid cancer treatment.
    base of the tongue; base of the heart
  5. Where are receptors for thyroid hormones?
    nuclei in almost all body cells
  6. What is the general function of thyroid hormone?
    increase transcription of a large number of genes, affecting rate of growth, metabolism, etc.
  7. Biologic functions of thyroid hormone. (6)
    increase cellular metabolic activity (increase mitochondria, increase Na+K+ATPase pump, increase O2 consumption), increase calorigenesis (increase basal metabolic rate, heat production, and O2 consumption), increase neurotransmission, increase HR/BF, increase appetite/GI motility, stimulate carb and fat metabolism
  8. Affects of increased thyroid hormone in the nervous system. (2)
    increase neurotransmission, required for development
  9. Affects of increased thyroid hormone in the cardiovascular system. (3)
    increased HR, cardiac output, and blood flow
  10. Affects of increased thyroid hormone in the GI system. (3)
    increased appetite, food intake, and GI motility
  11. How does increased thyroid hormone affect the GI?
    diarrhea (increased motility)
  12. How does decreased thyroid hormone affect the GI?
    constipation (decreased motility)
  13. Affects of increased thyroid hormone on carbohydrate and fat metabolism. (6)
    increase rate of glycolysis, increased glucose uptake by cells, increased gluconeogenesis, increased insulin secretion/demand, increased lipid mobilization (decreased fat stores), decreased plasma cholesterol and TG
  14. Hyperthyroidism is associated with ________ BMR and _________ body weight in the face of ___________.
    increased; decreased; increased appetite
  15. Hypothyroidism is associated with ________ BMR and _________ body weight in the face of ___________.
    decreased; increased; decreased appetite
  16. Describe the structure of the thyroid gland.
    numerous follicles bordered by follicular cells, b/w follicles are parafollicular cells
  17. Follicles contain __________ that serves as storage of thyroid hormone precursors.
    colloid (thyroglobulin)
  18. The thyroid is the only endocrine organ that ______________.
    stores hormones extracellularly
  19. Function of follicular cells.
    synthesis and secretion of thyroid hormones
  20. Parafollicular cells are aka __________, and they are responsible for ____________.
    C cells; calcitonin production
  21. Iodide is taken up from the ________ and rapidly secreted from the _________; it is selectively removed from circulation by _________.
    diet; kidneys; thyroid follicular cells
  22. Selective removal of iodide from circulation by thyroid follicular cells by pumping it against its concentration gradient.
    iodide trapping
  23. The abbreviation for tyrosine is ______.
  24. __________ is synthesized in the ER and Golgi of the follicular cells and secreted into the follicle; it is the main component of __________.
    Thyroglobulin; colloid
  25. Describe thyroid hormone synthesis.
    • 1. iodide trapping- iodide removed from blood and secreted into follicle
    • 2. iodide oxidized to iodine by thyroperoxidase
    • 3. iodine organification- iodination of thyroglobulin by peroxidase
    • 4. coupling reaction- cleaves iodinated tyrosine for production of thyroid hormones
  26. The rate of iodide trapping is dependent on ________.
    TSH conc
  27. What is the significance of the multiple possible iodination sites of tyrosine?
    where it is iodinated dictates what hormone it will eventually be cleaved to, T4, T3, or inactive reverse T3
  28. Describe the coupling reaction that takes place during thyroid hormone synthesis.
    cleaves iodinated tyrosine and transfers the ring to a second iodinated tyrosine to produce thyroid hormone
  29. Follicular colloid is phagocytised and fuses with the lysosomes, where...
    T4, T3, and rT3 are formed by cleavage from thyroglobulin
  30. How do thyroid hormones get to the blood?
    diffuse out of the follicular cells
  31. Drugs to treat hyperthyroidism, methimazole in the US, affect the thyroid by...
    inhibiting thyroid peroxidase, and therefore, inhibit iodide organification (iodination of thyroglobulin)
  32. The body has a ________ store of thyroid hormones; therefore,...
    huge; release of thyroid hormones can occur for a few days after initiation of treatment of hyperthyroidism.
  33. What processes does TSH affect? (3)
    iodide uptake, hormone synthesis (increase TG and thyroperoxidase), hormone release
  34. Almost all T3 and T4 are _________.
    bound to plasma proteins
  35. Only __________ enters cells to produce a biologic effect in target tissues and a negative feedback on the pituitary and hypothalamus.
    unbound thyroid hormone
  36. T4 has ________ activity in the blood; it binds to __(2)__.
    modest; thyroxine binding globulin (TBG) and transthyretin (TTR)
  37. T3 has ________ activity in the blood; it binds to __(2)__.
    high; TBG and albumin
  38. rT3 has ________ activity in the blood; it binds to ______.
    no (inactive); nothing
  39. T4:fT4 molar ratio ~ ________, meaning...
    100:1; there is not a large fraction that is free to enter cells/ produce a biologic effect.
  40. T4 is converted to __(2)__ in ________ by __________.
    T3 or rT3; peripheral tissue; deiodination
  41. ____________ removes iodine from T4, converting it to _________.
    5'-deiodinase; more active T3
  42. Why thyroid hormone activates target cells at lower concentrations due to its higher potency?
  43. How are thyroid hormones degraded?
    T4 glucuronidated (limited in cats), T3 is sulfated [both in the liver]
  44. HPT axis regulation from hypothalamus. (2)
    TRH, somatostatin
  45. Somatostatin ________ TSH secretion.
  46. What regulatory effect does T3 have?
    down-regulates TRH and TSH release (negative feedback)
  47. HPT axis regulation from pituitary. (1)
  48. HPT axis regulation from the thyroid. (2)
    T3, T4
  49. Physiological regulation of the thyroid. (6- 3 increase, 3 decrease)
    • increases activity with: cold stress, estrogen, leptin
    • decreases in activity with: glucocorticoids, GH, ghrelin
  50. What is euthyroid sick syndrome?
    down-regulation of TSH by inflammatory cytokines and glucocorticoids--> animals sicks with inflammatory/chronic disease have suppressed HPT axis and low thyroid hormones, even though the thyroid (and entire HPT axis) is functioning normally
  51. Low plasma T4 can be caused by... (2)
    hypothyroidism, non-thyroidal illness causing euthyroid sick syndrome
  52. What is the gold standard for differentiating b/w hypothyroidism and euthyroid sick syndrome?
    TSH stimulation test; thyroid gland non-responsive to TSH, hypothyroidism (primary and secondary) is diagnosed
  53. Describe secondary hypothyroidism.
    abnormality in the pituitary, causing low TSH and low stimulation of the thyroid gland
  54. TSH is ________ in primary hypothyroidism; it is _________ in secondary hypothyroidism (this measurement is seldom useful in dogs).
    high; low
  55. High TSH is consistent with....
    lack of feedback from T3, T4, or both.
  56. Why are TSH measurements are frequently not reliable?
    TSH secretion is pulsatile
  57. Describe interpretation of T3 suppression test. [this test is seldom used]
    decrease in T4 in health, no change in T4 with hyperthyroidism
  58. What is the premise of the T3 suppression test?
    Exogenous T3 cannot be converted to T4
  59. Describe the premise of the TRH stimulation test.
    causes suppression of TSH secretion in primary hyperthyroidism
  60. How do you diagnose hyperthyroidism?
    measure T4
  61. The biologic activity of TRH and TSH can be estimated by...; in contrast, T3/T4...
    measuring their blood concs; have a nuclear receptor and the biologic activity can last hours/days after they are removed from circulation.