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GI Block > Endo: Regul of T3/4 Synthesis > Flashcards

Endo: Regul of T3/4 Synthesis Flashcards

1
Q

Describe the structure of thyroid hormone

A

T3/4 is composed of two tyrosine residues iodinated in 3 or 4 places

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2
Q

What kind of receptor is used by T3/T4?

A

The receptor for T3/4 exists bound to DNA (intracellular receptor!)

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3
Q

What are two major sources of iodine?

A

Water

Salt with added Iodide

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4
Q

How does the Iodine get from blood into thyroid colloid? (3 steps)

A
  1. Na/I symporter on basolateral side of follicular cell pumps Iodide into cell
  2. Na then pumped back out by Na/K ATPase
  3. On luminal side, Thyroperoxidase converts I- into I2 and pumps it into follicular lumen
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5
Q

Where is Thyroglobulin made?

Where does it go next?

A

Thyroglobulin is synthesized in follicular cell

TG is then pumped into the follicular lumen (colloid)

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6
Q

Describe the overall process of Thyroid hormone synthesis (4)

A
  1. Iodine is taken up by follicular cell and pumped across into follicular lumen (colloid)
  2. Thyroglobulin is made by follicular cell and pumped into lumen
  3. In lumen, TPO adds Iodine to Thyroglobulin
  4. Iodinated Thyroglobulin gets endocytosed by follicular cell and converted to T3/4
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7
Q

Name the three reactions catalyzed by Thyroid Peroxidase

A

Thyroid Peroxidase…

  • Oxidizes I- into I2
  • Adds Iodide to Tyr residues of thyroglobulin
  • Connects Tyrosine residues (couples MIT and DIT)
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8
Q

What happens after TPO adds Iodine to thyroglobulin in the lumen?

A
  1. Thyroglobulin-Iodide gets endocytosed by follicular cell
  2. Thyroglobulin-Iodide gets degraded by proteases
  3. This releases T3 and T4
  4. T3, T4 released into blood
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9
Q

How is thyroid hormone transported in the blood?

A

T3/4 exists in blood bound to TBG

*But remember, only free hormone exerts effect

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10
Q

Which is functional: T3 or T4?

A

T3 is functional

(T4 is like a pro-hormone, and gets converted to T3 within cells)

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11
Q

Name the functions of thyroid hormone (6)

A
  1. Brain maturation
  2. Bone growth (permissive effect on GH)
  3. Beta adrenergic stimulation
  4. Basal metabolic rate increases
  5. Blood sugar increases (gluconeogenesis, glycogenolysis)
  6. Break down of fats (lipolysis)
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12
Q

What is cretinism?

Name 5 symptoms

A
  • Cretinism = severe growth and mental retardation due to low thyroid hormone (congenital hypothyroidism)
  • Signs: pot-belly, puffy-faced, protruding umbilicus, protruding tongue, poor brain development
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13
Q

Describe the hormone axis for thyroid, including negative feedback

A
  1. Hypothalamus releases TRH
  2. TRH stimulates ant pituitary to release TSH
  3. TSH stimulates thyroid to release T3/4

*T3 provides negative feedback on hypothalamus and pituitary

*Somatostatin also inhibits TSH release

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14
Q

How does TSH cause T3/4 release? (3)

A

TSH

  • Stimulates growth of thyroid gland and follicular cells
  • Stimulates protein synthesis (thyroid peroxidase and thyroglobulin)
  • Stimulates iodide uptake
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15
Q

What are the TSH and T3/4 levels in

  • Endemic hypothyroidism
  • Hypopituitarism
  • Hashimoto’s thyroiditis
A
  • Endemic hypothyroidism: High TSH, low T3/4
    • Insufficient Iodine in water
  • Hypopituitarism: Low TSH, low T3/4
  • Hashimoto’s thyroiditis: High TSH, low T3/4
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16
Q

What are the TSH, T3/4 levels in

  • Thyroid tumor
  • Pituitary tumor
  • Grave’s disease
A
  • Thyroid tumor: low TSH, high T3
  • Pituitary tumor: high TSH and high T3
  • Grave’s disease: low TSH, high T3
    • Autoimmune stimulation of TSH receptors on thyroid
17
Q

What enzyme converts T4 into T3 in peripheral tissues?

Name 3 things that inhibit this enzyme

A
  • 5’ deiodinase converts T4 into T3
    • T3 is active form
  • 5’ deiodinase is inhibited by glucocorticoids, beta blockers, and PTU
18
Q

What is the Wolff-Chaikoff effect?

A

Excess iodine temporarily inhibits thyroid peroxidase, decreasing T3/4 production

19
Q

What is one cause of increased thyroid binding globulin?

One cause of decreased TBG?

A
  • Increased TBG with oral contraceptives
  • Decreased TBG with hepatic failure

GI Block (77 decks)

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