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Physiology Block 8 > Hypothalamic-pituitary-thyoid Axis > Flashcards

Hypothalamic-pituitary-thyoid Axis Flashcards

1
Q

What is the HPT axis concerned with

A

Production of thyroid hormone (T3 and T4)

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2
Q

In the HPT axis what does the hypothalamus release

A

TRH

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3
Q

In the HPT axis, what does the anterior pituitary gland release

A

TSH

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4
Q

In the HPT axis, what does the thyroid release

A

T4 and T3

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5
Q

Thyroxine

A

T4

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6
Q

What happens to T4 in target tissues

A

Converted to T3

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7
Q

Which is more potent, T3 or T4

A

T3

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8
Q

Which is easier to make in the thyroid, T3 or T4

A

T4

That’s why T4 gets released from thyroid and then it gets converted to T3

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9
Q

Feedback of the HPT axis

A

Both short and long loop

  • TSH inhibits TRH
  • T3-T4 inhibit TSH and TRH
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10
Q

What are the master metabolic hormones

A

T3/T4

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11
Q

What are the metabolic functions of T3 and T4

A
  • signals through steroid pathway (but its amine)
  • bone growth and maturation
  • CNS maturation
  • increases BMR and heat production
  • increases all body metabolism
  • increases cardiac output
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12
Q

A consequence of hypothyroidism would be

A

Cold intolerance

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13
Q

What does the thyroid contain

A

Follicular epithelial cells

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14
Q

What do the follicular epithelial cells of the thyroid do

A

Make thyroid hormone

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15
Q

Where does the basal membrane of the follicular epithelial cells of the thyroid face

A

Blood

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16
Q

Where do the apical membrane of the cells of the follicular epithelium of the thyroid face

A

Colloid

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17
Q

Mixture of forming thyroid hormone attached to thyroglobulin

A

Colloid

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18
Q

Protein that binds and helps form thyroid hormone

A

Thyroglobulin

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19
Q

What does thyroid hormone synthesis require

A

Dietary iodine

-iodized salt and other sources

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20
Q

How Is thyroid hormone synthesized

A

By stepwise addition of iodine to tyrosine

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21
Q

What thyroid hormones have biological functions

A

T3 and T4 only

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22
Q

How are thyroid hormones synthesized

A
  • iodide pumped into the cell from blood (forms iodine when entering the colloid)
  • tyrosine on thyroglobulin are iodinated (iodine is added to tyrosine, makes MIT and DIT)
  • iodinated thyroglobulins combine
  • thyroglobulin with T3, T4, DIT, and MIT stay in colloid
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23
Q

What are the combinations of iodinated thyroglobulins that make up thyroid hormones

A
  • two DITs make T4
  • one DIT and one MIT make T3
  • remainder of other MIT and DIT remained attached
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24
Q

What makes MIT and DIT

A

Tyrosine on thyroglobulins are iodinated

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25
Q

When does iodide turn to iodine?

A

When entering the colloid

26
Q

How are T3 and T4 secreted into circulation

A
  • upon stimulation thyroglobulin and the attached thyroid hormones is brought into the cell
  • T3 and T4 are removed from thyroglobulin and secreted into circulation
  • MIT and DIT on thyroglobulin are deiodinated and recycled to repeat the process
27
Q

What is the average intake of iodine a day

A

500ug, only need 150

28
Q

What happens to most of the iodine

A

Taken up by the thyroid to make T3 and T4

29
Q

Degradation of T3 and T4

A

Liver degrades T3 and T4 and releases iodine back into blood

30
Q

Where is most iodine secreted

A

By the kidney

31
Q

T3/4 in blood

A
  • not easily dissolved in blood
  • is carried bound to proteins
  • albumin and specific thyroxine binding proteins
32
Q

What kind of T3 and T4 can be sensed and regulated

A

Only free T3 and T4

33
Q

What kind of T3 and T4 can be degraded

A

Only free T3/T4

34
Q

Yo are researching a new drug and discover it rapidly increases albumins affinity for T3/T4. What affect would this have on TSH levels?

A

Increase

35
Q

Why kind of steroid is T3/T4

A

Amine, but acts like a steroid

36
Q

How are T3 and T4 carried

A

Binding proteins

37
Q

What do T3 and T4 activate

A

Intracellular receptors

38
Q

What does T3 and T4 do once it binds intracellular receptors

A

Directly initiates transcriptional changes

39
Q

What converts T4 to T3?

**

A

Iodinase in tissues

**

40
Q

Thyroid hormone action

A
  • primarily a metabolic regulator
  • causes bone formation and growth
  • allows for maturation of CNS
  • increases basal metabolic rate
  • increases metabolism
  • increases cardiac output
41
Q

Thyroid hormone and bone growth

A
  • causes bone formation and growth
  • required for growth to adult stature
  • act with growth hormones to regulate growth of bones
42
Q

Thyroid hormone and maturation of CNS

A
  • allows for maturation of CNS
  • required in the prenatal period for full CNS development
  • in adults altered T3/T4 levels cause central defects
43
Q

How does thyroid hormone increase BMR?

A
  • increase Na/K ATPase
  • increase O2 consumption (except brain, gonads, and spleen)
  • increase heat production
  • increase BMR
44
Q

Thyroid hormone and metabolism

A

Increases availability and uptake of glucose, proteins, and fats

45
Q

Thyroid hormone and cardiac output

A
  • higher O2 consumption requires more CO to meet demand

- also activates SNS

46
Q

Hyperthyroidism

A

Too much T3/T4

47
Q

What are some examples of disorders that cause hyperthyroidism

A
  • Graves’ disease
  • thyroid tumor
  • TSH/TRH secreting tumor
48
Q

What us Graves’ disease

A

Autoimmune disease where Ab activate the receptors on the thyroid causing it to release too much T3/T4

49
Q

What are some symptoms of hyperthyroidism

A
  • weight loss
  • heath intolerance
  • sweating
  • increased cardiac output
  • exopthalamos
  • goiter
50
Q

Hypothyroidism

A

Not enough T3/T4

51
Q

What are some disorders that can cause hypothyroidism

A
  • Hashimotos
  • iodine deficiency
  • congenital (cretinism)
52
Q

What is Hashimotos

A

Destruction of thyroid gland by immune system

53
Q

What are some symptoms of hypothyroidism?

A
  • weight gain
  • cold intolerance
  • low cardiac output
  • mental slowness
  • lack of energy
  • myxedema (swollen, puffy, skin)
54
Q

Treatment for hypothyroidism

A

Give them exogenous T3/T4 or iodine

55
Q

What is the treatment for hyperthyroidism

A

Remove the thyroid/ tumor

56
Q

Congenital hypothyroidism

A
  • cretinism
  • due to necessity of T3/T4 for normal development prenatal hypothyroidism causes both physical and metal developmental abnormalities
  • learning disabilities
  • growth is inhibited
57
Q

Enlarged thyroid gland

A

Goiter

58
Q

When can you get a goiter

A

Can occur in either hyper or hypothyroidism

59
Q

Why is the thyroid enlarged when you have a goiter

A

High TSH levels

60
Q

What kind of hypothyroidism would you not have a goiter

A

Hashimotos

61
Q

What kind of hypothyroidism could give you a goiter

A

Iodine deficiency

62
Q

What kind of hyperthyroidism could give you a goiter

A

Graves
Thyroid tumor
TSH/TRH secreting tumor

Physiology Block 8 (7 decks)

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