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Physiological concepts > Wk11 - thyroid > Flashcards

Wk11 - thyroid Flashcards

1
Q

Describe the position and basic anatomy of the thyroid gland.

A

The thyroid gland is found inferior to the larynx, with it’s left and right lobes on either side of the trachea. The two lobes are connected by the isthmus.

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2
Q

The thyroid gland receives (poor/ rich) bloody supply.

A

Rich.

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3
Q

t/f: the thyoid gland doesn’t possess the ability to store hormones.

A

FALSE - the thyroid gland possesses the ability to store hormones and release them over a long duration of time.

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4
Q

Describe the composition of the thyroid gland on a cellular level, including molecule(s) of particular relevance.

A

The tissue of the thyroid gland is organised into follicles, with a dense capillary network surrounding each follicle. Each follicle contains a colloid molecule thyroglobulin, which contains the amino acid tyrosine.

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5
Q

State the basic steps involved with the formation of thyroxine (thyroid hormone). (5)

A
  1. Iodide ions absorbed from GIT, transported to thyroid gland via blood.
  2. Follicular cells concentrate iodide ions through use of a Na-Iodide symporter.
  3. Pendrin (iodide transporter) pushes iodide into the colloid.
  4. At the apical surface of each cell, iodide ions are activated by thyroid perioxidase (enzyme).
  5. This reaction either attaches one (MIT) or two (DIT) iodide ions to the tyrosine molecule of the thyroglobulin.
  6. Coupling takes place (2DIT = T4, DIT+MIT=T3), thyroxine molecules REMAIN INCORPORATED into the thyroglobulin.
  7. Each thyroglobulin ends up containing 4-8 molecules of T3/ T4.
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6
Q

Explain what is meant by the following term(s):

Colloid.

A

A homogenous mixture of 2 or more substances, however the substance(s) are not chemically mixed (still exist as individual particles).
In our case, the colloid of the thyroid follicle is a glycoprotein.

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7
Q

Expand the following acronym:

MIT.

A

Monoiodotyrosine.

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8
Q

Expand the following acronym:

DIT.

A

Di-iodotyrosine.

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9
Q

What is the major influencing factor of thyroxine secretion?

A

TSH release, from the anterior pituitary.

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10
Q

List the MECHANISMS which TSH stimulates.

A
  1. Iodide transport into follicular cells.
  2. Thyroglobulin and thyroid peroxidase production.
  3. Release of thyroid hormones.
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11
Q

t/f: in the absence of TSH thyroid molecules become inactive.

A

True.

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12
Q

Briefly explain the steps involved with the stimulation of thyroid follicles to release thyroxine.
(note: this is NOT PRODUCTION of thyroxine, this is the basic steps involved with stimulus and release)

A
  1. Follicular cells remove thyroglobulin from the follicles via endocytotic process.
  2. Lysosomal enzymes process TG and AA, and cause thyroid hormones to enter the cytoplasm.
  3. Released T3/ T4 diffuse out of the cell, and enter the circulation.
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13
Q

State the composition of thyroxine hormone secretion (what T3%, T4%).

A

TH is usually 90% T4, 10% T3.

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14
Q

What percentage of the following molecules become bound to Thyroid binding Globulins (TBG’s):

a. ) T3.
b. ) T4.

A

a. ) T3 - 70% bound to TBG.

b. ) T4 - 75% bound to TBG.

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15
Q

75% of T4, and 70% of T3 is found bound to TBG within circulation. How does the remainder travel?

A

The remainder is found:

  1. Bound to thyroid-binding pre-albumin (TBPA).
  2. Bound to plasma albumin.
  3. Found unbound (“free”, 0.3% T3 and 0.03% T4).
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16
Q

What portion of thyroxine is considered to be the ‘reserve supply’?

A 
The TH (thyroid hormone) which is found bound within circulation is considered the 'reserve supply'; as the primary supply is that which is stored within the thyroid gland. 
(remember: the thyroid gland has a high capacity to store hormones)
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17
Q

Which of the two, T3 or T4, is found at a higher %[] UNBOUND within the blood?

A

T3, found at 0.3% [] (whereas T4 is found at 0.03%).

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18
Q

t/f: thyroid hormones affect almost every cell in the body.

A

True.

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19
Q

Once inside a cell, where may thyroid hormones bind? State the functional consequence of this. (3)

A
  1. Receptors in the cytoplasm - thyroxine storage.
  2. Receptors in the nucleus - activating certain genes involved with metabolic enzyme synthesis, and therefor stimulating metabolic pathways for energy usage.
  3. Receptors in the mitochondria - increasing rates of mitochondrial ATP production.
    (note: the combined effect of (2) and (3) is the increase in rate of oxidative phosphorylation)
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20
Q

Discuss the role of TH during foetal/ neonatal growth periods.

A

TH plays important role in CNS development.

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21
Q

What occurs should a prenatal/ postnatal individual be deficient in TH?

A

Mental retardation and growth impairments occur.

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22
Q

t/f: TH is an important regulator for growth of the CNS, but not the rest of the body.

A

FALSE - thyroid hormones are important regulators of growth of the entire body.
(note: this is partly because TH stimulate GH production!)

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23
Q

TH is important in regulating the growth of the entire body. Describe three (3) ways in which this is true.

A
  1. Stimulates GH production (ant. pituitary).
  2. Stimulates protein (structural or enzyme) production in sk muscle, heart, liver.
  3. Promotes calcification and closure of cartilage growth plates of bones.
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24
Q

Discuss the thermogenic action of TH release, and it’s consequences for the body.

A

Thyroid hormones essentially increase the basal rate for oxidative phosphorylation, and therefor, energy production. Consequently there is increased heat production and oxygen consumption.

Simply put, thyroid hormones increase the metabolic rate of cells.

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25
Q

T3 in particular is known to stimulate the synthesis of what? (2)

A
  1. Cytochromes - such as cytochrome oxidase.
  2. Na-K ATPase.
    (note: not in all cells, certain cells only)
26
Q

Which of the two, T3 or T4, is the hormone which is primarily responsible for the effects of TH’s?

A

T3.
(note: counter-intuitive as 90% of TH released is T4, however this is because ~90% of T4 released is then converted into T3)

27
Q

What’re the 2 sources of T3, in peripheral tissues?

A
  1. Converted T4 (90%) - enzymes in the liver, kidneys and other tissues convert T4 to T3.
  2. T3 released by thyroid gland (10%).
28
Q

What hormone(s) does the thyroid gland release?

A
  1. Thyroxine (T3 or T4).

2. Calcitonin.

29
Q

The thyroid gland contains follicular cells which secrete thyroid hormone, and a second group of endocrine cells which secrete calcitonin.

a. ) What is the name of these cells?
b. ) How do these cells compare in size to follicular cells?

A

a. ) C cells (‘C’ for clear), AKA parafollicular cells.

b. ) C cells are far larger.

30
Q

State the stimulus for release, and effect of calcitonin.

A

Calcitonin lowers [Ca+2], it’s stimulus for release is high [Ca+2] being detected by C cells directly.

31
Q

During what states or stages of life is calcitonin most relevant? (3)

A
  1. Childhood - for bone modelling and growth.
  2. Pregnancy - reducing bone mass loss.
  3. Starvation - reducing bone mass loss.
32
Q

Describe the basic structure and position of the parathyroid glands.

A

The parathyroid glands are 4 small, round glands found on the posterior surface of the thyroid gland (2 on each lobe).

33
Q

State the 2 types of cells of the parathyroid gland, including their secretion and relative abundance.

A
  1. Chief (principal) cells - produce PTH, more numerous.

2. Oxyphil cells - produce the ‘reserve capacity’ of PTH, less numerous.

34
Q

State the function(s) of PTH.

A
  1. Increases [Ca+2] - via increasing osteoclast activity in bone resorption.
  2. Ca+2, Mg+2 and PO4(3-) absorption - from GIT into blood, ONLY IN PRESENCE OF VITAMIN D.
35
Q

Describe, in terms of %’s, where the body’s calcium is stored.

A

99% - crystalline form.

  1. 9% - intracellular.
  2. 1% - ECF.
36
Q

How does the 0.1% of calcium which is found in the ECF (plasma) travel?

A

50% - freely diffuseable (“free”, unbound).

50% - bound to protein or complexed with PO4(3-).

37
Q

Briefly discuss the vital roles which calcium plays in the body. (7)

A
  1. Neuromuscular excitability - decreased [Ca+2] leads to over excitability of nerves/ muscles.
  2. Excitation-contraction - for cardiac and SM, changes in [Ca+2] effects contractility.
  3. Stimulus-secretion coupling - Ca+2 entry triggers exocytosis.
  4. Maintenance of tight junctions - Ca+2 forms the intercellular cement between cells.
  5. Blood clotting - Ca+2 acts as cofactor.
  6. Ca+2 acts as a secondary messenger.
  7. Ca+2 is involved in cell motility and cilia action.
38
Q

How is it that changes in [Ca+2] can affect membrane permeability? Discuss the relevance of this in a clinical setting.

A

Decreased [Ca+2] increases membrane permeability to Na+. This means low [Ca+2] can induce muscle spasms and affect respiration.

High [Ca+2] can cause cardiac arrythmias.

39
Q

The regulation of calcium metabolism depends upon the hormonal control of calcium exchanges between the ECF and 3 compartments.
State these 3 compartments.

A

ECF with…

  1. Bone.
  2. Gut (SI).
  3. Kidneys.
40
Q

The control of calcium metabolism can be considered in 2 main ways; state these.

A
  1. Regulation of Ca+2 homeostasis (minute to minute).

2. Regulation of Ca+2 balance (maintaining constant overall amount).

41
Q

Regulation of calcium homeostasis occurs primarily between which compartments, and on what sort of a time-scale?

A

Homeostatic regulation for Ca+2 occurs on a minute to minute basis, primarily through adjustments between ECF and bone. Adjustments of Ca+2 levels between ECF and urinary output (thus kidneys) also helps to maintain Ca+2 homeostasis, although to a lesser extent.

42
Q

Regulation of overall calcium balance occurs (rapidly/ slowly), and involves adjustments between which compartments? State the hormones relevant to this exchange.

A

Regulation of overall Ca+2 balance occurs slowly, in order to maintain an overall constant amount of Ca+2 in the body. These exchanges occur primarily between the ECF and GIT (small intestine), and involve hormonal control from PTH and vitamin D.

43
Q

Regulation of calcium balance (long term) occurs primarily through intestinal Ca+2 absorption. How does PTH influence this?

A

PTH is used to absorb Ca+2 from the gut, however requires presence of vitamin D to do so.

44
Q

Explain what is meant by the following term(s):
Hypercalcaemia.
Which hormone is most important should an individual experience this condition?

A

Elevated Ca+2 levels; calcitonin is very important to bring levels back within tolerance limits.

45
Q

Bone remodelling involves 2 control loops; state them.

A
  1. Negative feedback loops (hormonal) - due to PTH and calcitonin.
  2. Mechanical and gravitational forces acting on the skeleton.
46
Q

Which hormone is released when blood [Ca+2] is decreased, and what is the effect of this on bone cell(s)?

A

Decreased [Ca+2] stimulates PTH release, which increases osteoclast activity causing more Ca+2 to be released from bone. Thus, restoring [Ca+2] levels.

47
Q

Which hormone is released when blood [Ca+2] is increased, and what is the effect of this on bone cell(s)?

A

Increased [Ca+2] stimulates calcitonin release, which inhibits osteoclast activity and therefor limits bone resorption. Thus, helping lower Ca+2 levels back to homeostatic levels.

48
Q

State the target tissue(s) of PTH.

A

Bone, kidneys, intestines.

49
Q

t/f: PTH mobilises some Ca+2 stores to give increased [Ca+2] within the ECF.

A

True.

50
Q

State the 2 main actions of PTH, which are used to achieve it’s overall goal of increasing [Ca+2].

A
  1. Induce fast Ca+2 efflux - from small pool of Ca+2 in “bone fluid”.
  2. Stimulates bone dissolution - promoting osteoclast activity.
51
Q

PTH promotes bone dissolution, in order to increase [Ca+2]. What is a potential problem associated with this?

A

Increased blood [PO4(3-)] levels, as calcium is stored in the bone combined with PO4(3-) as hydroxyapetite crystals.

52
Q

What is the name of the membrane separating the bone-fluid compartment from the osteocyte?

A

Osteocytic-osteoblastic bone membrane.

53
Q

Discuss the effect of PTH on the kidneys.

A

PTH increases Ca+2 retention from the kidneys, by increasing the renal threshold for Ca+2. Inadvertently, this will increase the amount of PO4(3-) which is excreted, as it means less PO4(3-) can be reabsorbed.

54
Q

Discuss the effect of PTH on the Intestines.

A

PTH indirectly increases Ca+2 and PO4(3-) absorption. Vitamin D directly increases absorption of Ca+2 and PO4(3-).

55
Q

Calcitonin has effects can be considered as short and long term; elaborate on these.

A

Short term - induces decreased Ca+2 efflux across osteocytic-osteoblastic membrane.

Long term - induce decreased bone resorption (via osteoclast inhibition) and promoting Ca+2 excretion at the kidneys.

56
Q

Which of the two, PTH or calcitonin, plays the more dominant role in Ca+2 homeostasis?

A

PTH.

57
Q

Vitamin D is AKA what?

A

Cholecalciferol.

58
Q

t/f: Vitamin D contributes to both calcium homeostasis and balance.

A

FALSE - vitamin D only influences calcium balance, as it’s actions are too slow to influence calcium homeostasis.

59
Q

What can occur to the body in the absence of vitamin D?

A

In the absence of vitamin D, PTH will maintain Ca+2 levels. However this comes at the expense of bones; which due to de-mineralisation become soft and deformed.
This leads to ricketts (children), or osteomalacia (adults).

60
Q

Describe the steps involved with PTH, vitamin D and calcium metabolism.

A
  1. Vitamin D (cholecalciferol) can come from dietary sources, or due to the conversion of 7-dehydrocholesterol when it is exposed to UV light.
  2. In the liver, a hydroxyl group is added onto the 25th carbon forming 25-hydroxycholecalciferol.
    NOW THE EFFECT DEPENDS ON WHETHER PTH IS PRESENT OR NOT…
    3.a.) PTH is present - substance is converted into 1, 25-dihydroxycholecalciferol. This is a very active form of vitamin D. Thus, PTH enhances the effect of vitamin D.
    3.b.) PTH absent - in the kidneys the substance is converted into 24, 25-dihydroxycalciferol. This is a form of vitamin D with very little effect. This pathway may occur due to high calcium levels, so the body wants the effect of vitamin D to be nullified.

(hint: remember you get Vitamin D from the sun or vitamin D tablets!)

Physiological concepts (12 decks)

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