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Physiology > Thyroid Hormones > Flashcards

Thyroid Hormones Flashcards

1
Q

What hormones does the thyroid produce?

A
  • thyroxine (T4)
  • triodothyronine (T3)
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2
Q

What are the overall functions of T3 and T4?

A
  • increase rate of basal oxygen use, basal metabolism, and rate of heat production
  • modulate delivery of substrates and oxygen by cardiovascular and respiratory systems to sustain metabolic rate
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3
Q

Thyroid hormone levels change based on what?

A
  • energy need
  • calorie supply
  • environmental temperature
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4
Q

What do you need for thyroid hormone synthesis?

A
  • 2 tyrosines
  • iodine
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5
Q

What is the major thyroid hormone produced?

A

thyroxine (T4)

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6
Q

What is the biologically active thyroid hormone secreted in lesser quantities?

A

triiodothyonine (T3)

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7
Q

What enzyme converts T4 to T3?

A

5’ deiodinase

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8
Q

What is the alternative product of T4 (inactive form of T3) that is produced when less thyroid hormone action is needed?

A

Reverse T3

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9
Q

The parafollicular cells (C cells) of the thyroid gland secrete what?

A

calcitonin

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10
Q

Under stimulation, colloidal thyroid hormone is absorbed where?

A

into follicular cells

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11
Q

How is iodine supplied for thyroid hormone synthesis?

A

supplied by diet via iodide

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12
Q

Thyriod hormones are stored where until stimulated to secrete?

A

extracellularly in follicular lumen

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13
Q

What is the ratio of T4:T3 within the thyroid gland?

A

10:1

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14
Q

What is the Wolff-Chaikoff effect?

A

transient reduction in thyroid hormone levels cause by ingestion of a large amount of iodine and lasts about 10 days

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15
Q

Excessively high levels of dietary iodide (like in the Wolff-Chaikoff effect) will suppress the activity of what?

A

2 Na/1 I pump

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16
Q

Thyroglobulin is stored in follicles as what until stimulation for release of thyroid hormones?

A

colloid

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17
Q

When stimulus of thyroid gland is received (thyroid stimulating hormone), what happens?

A
  • retrieval of thyroglobulin from follicle lumen into endocrine cell via endocytosis
  • lysoomes fuse with colloid droplet and release T3, T4, free MIT, and free DIT from thyroglobulin
  • T3 and T4 enter blood
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18
Q

How are thyroglobulins retrieved from follicle lumen and into endocrine cell via endocytosis?

A

cell membrane forms pseudopods which engulf a pocket of colloid

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19
Q

What happens to MIT and DIT when thyroid hormones are secreted into circulation?

A

rapidly deiodinated by deiodinase to conserve iodide for recylcing

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20
Q

What are the effects of hepatic failure on circulating thyroid hormones?

A
  • decreased liver protein synthesis
  • decreased thyroxine binding globulin
  • increase in circulating thyroid hormone
  • decrease in hormone synthesis
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21
Q

What proteins does circulating T3 and T4 bind to?

A
  • thyroxine binding globulin
  • transthyretin
  • albumin
22
Q

What conditions cause sick euthyroid syndrome?

A
  • fasting/starvation
  • stress (cortisol)
  • catabolic diseases - cancer
  • hepatic diseases
  • renal diseases
23
Q

What enzyme is downregulated in sick euthyroid syndrome?

A

5’ deiodinase

24
Q

Sick euthyroid syndrome is due to dysregulation of what?

A

thyrotropic feedback control

25
Q

Describe the regulation of the thyroid gland.

A
  • thyrotropin-releasing hormone is released from hypothalamus
  • causes release of thyroid stimulating hormone from anterior pituitary
  • causes release of T4 and T3 from thyriod gland
  • negative feedback of T4 and T3 to anterior pituitary and hypothalamus
26
Q

Since T4 cannot cause negative feedback itself, what must occur?

A

it has to be converted to T3 via 5’ deiodinase since T3 is the active form

27
Q

Thyroid releasing hormone is a _____ hormone.

A

tripeptide

28
Q

Thyroid stimulating hormone is a ____ hormone with ____ subunits.

A

glycoprotein, 2

29
Q

What are the biologically active sites of TSH?

A

beta subunit

30
Q

Describe the steps in the synthesis of thyroid hormones.

A
  • need to synthesize protein molecule called thyroglobulin using tyrosine
  • iodide transported from blood into follicular epithelial cells via 2 Na+ / 1 symporter
  • iodide transported across apoical membrane into colloid of follicle via pendrin and oxidized to iodine via thyroid peroxidase
  • iodine incorporated into tyrosines at specific sites in thyroglobulin via thyroid peroxidase
  • coupling reaction – results in MIT and DIT whcih remain attached to thyroglobulin until stimulus for secretion
31
Q

Negative feedback exerted predinantly at the ______ to decrease TSH.

A

anterior pituitary

32
Q

What enzyme converts T4 to T3?

A

5’ deiodinase

33
Q

What enzyme converts T4 to rT3?

A

5 deiodinase

34
Q

What are the major sites of degradation of thyroid hormones?

A
  • liver
  • kidneys
  • skeletal muscle
35
Q

What hormones provide tonic inhibition for TSH release?

A
  • dopamine
  • somatostatin
36
Q

How does T4 enter cells?

A

facilitated diffusion

37
Q

T4 is deiodinated to T3 where?

A

inside cells

38
Q

T3 binds the receptors within what of the cell?

A

nucleus

39
Q

Why does T3 bind to the receptor within the nucleus of the cell?

A
  • T3 receptor complex binds to thyroid regulatory element on DNA to stimulate transcription of genes
40
Q

What are the effects of thyroid hormones on basal metabolic rate and metabolism?

A
  • increased oxygen consumption and increased Na/K ATPase
  • increased cardiac output and pulse pressure
  • increased thermogenesis
  • increase supply of substrates for energy production
  • increase appetite, gut absorption, motility
  • increase mobilization of carbs and fat
41
Q

Protein synthesis vs degradation is dependent on thyroid hormone levels. Explain.

A
  • if thyroid hormone levels are normal then protein synthesis predominates
  • if thyroid hormone levels are high then protein degradation predominates to use amino acids for energy
42
Q

Throid hormones act with what to promote growth of bone, cartilage, teeth, epidermis, and hair?

A
  • growth hormone
  • IGFs
43
Q

What effects do thyroid hormones have on the central nervous system?

A
  • increases blood flow and glucose metabolism in the brain
  • enhances peripheral nerve reflexes
44
Q

What effects do thyroid hormones have on the autonomic nervous system?

A

interact with sympathetic nervous system to have similar effects on increasing cardiac output, heat production, etc

45
Q

What is hypoerthyroidism?

A

overactive thyroid causing excess thyroxine

46
Q

If hyperthyroidism is in thyroid gland, TSH is high or low?

A

low

47
Q

If hyperthyroidism is in hypothalamus or pituitary, TSH is high or low?

A

high

48
Q

What is hypothyroidism?

A

underactive thyroid causing low thyroid hormones

49
Q

If hypothyroidism is in thyroid gland, TSH is high or low?

A

high

50
Q

If hypothyroidism is in hypothalamus or pituitary gland, TSH is high or low?

A

low

Physiology (18 decks)

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