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Year 2- Endocrine System > The Thyroid Gland > Flashcards

The Thyroid Gland Flashcards

1
Q

What s the shape of the thyroid gland?

A

Butterfly

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2
Q

Where does the thyroid gland lie?

A

Across the trachea at the base of the larynx.

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3
Q

What is the function of the thyroid gland?

A

Synthesises the thyroid hormones

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4
Q

What are the two physiologically active forms of the thyroid hormone?

A

T3 (triiodothyronine)
T4 (thyroxine)

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5
Q

The thyroid gland is mostly composed of two types of cell. Name them.

A

C cells
Follicular cells

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6
Q

What do C cells secrete?

A

Calcitonin

->calcitonin is a calcium regulating hormone

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7
Q

What is the function of the follicular cells?

A

Support thyroid hormone synthesis and surround hollow follicles

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8
Q

Describe the structure of thyroid follicles.

A

Spherical structures with walls made of follicular cells. The centre is filled with colloid

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9
Q

What is colloid?

A

Sticky glycoprotein matrix.

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10
Q

How many months worth of TH do the thyroid follicles contain?

A

2-3 months worth

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11
Q

Where in the thyroid follicles would you find the precursor form of TH?

A

Within the colloid

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12
Q

What do follicular cells manufacture?

A

The enzymes which make TH
Thyroglobulin- large protein rich in tyrosine residues.

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13
Q

Where do the enzymes and thyroglobulin created in the follicular cells get secreted into?

A

The colloid

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14
Q

What do the follicular cells concentrate?

A

Iodide

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15
Q

What does the concentrated iodide from the plasma combine with in the colloid to make the TH?

A

Tyrosine residues

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16
Q

How does the body get tyrosine and iodide?

A

Through the diet

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17
Q

How does iodide enter the follicular cells from the pladms?

A

Via a sodium/iodide transporter

->the coupling of sodium enables the follicular cells to take up the iodide against a concentration gradient

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18
Q

Now that iodide is in the follicular cell, how does it get into the colloid?

A

Via pendrin transporter

Basically iodide goes from plasma to follicular cells via Na/I transporter and then into the colloid via pendrin transporter

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19
Q

Enzymes which are exocytosed into the colloid alongside the thyroglobulin carry out what process?

A

Oxidation of iodide into iodine

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20
Q

After the iodide has been oxidised into iodine, what happens to the iodine?

A

Iodine is added to the thryoglobulin

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21
Q

What is it called if one iodine gets added onto tyrosine?

A

MIT (monoiodotyrosine)

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22
Q

If a second iodine is added onto tyrosine, what is this called?

A

DIT (diiodotyrosine)

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23
Q

MIT and DIT undergo conjugation where what is formed?

A

T3 aka triiodothyronine

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24
Q

If two DIT conjugate together, what is formed?

A

T4 aka tetraiodithyronine

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25
Q

In T3 aka triiodothyronine, how many:
a. tyrosines
b. iodines

A

a. 2 tyrosine
b. 3 iodine

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26
Q

In T4 aka tetraiodothyronine, how many:
a. tyrosines
b. iodines

A

a. 2 tyrosine
b. 4 iodine

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27
Q

Which hormones are the active form of thyroid hormone?

A

T3 and T4

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28
Q

RECAP- which type of hormone are thyroid hormones?

A

Amine hormones, however, they behave like steroid hormones

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29
Q

Why can we not store TH in their active forms of T3 or T4?

A

They are lipophilic and would cross the cell membrane and get into the blood.

30
Q

TSH= thyroid stimulating hormone.
What happens in response to TSH which releases TH?

A

Portions of the colloid are taken back into the follicular cell by endocytosis.
Within the cells, they are packaged into vesicles containing proteolytic enzymes.
These enzymes cut the thyroglobulin to release TH

31
Q

Are T3 and T4 lipid or water soluble?

A

Lipid soluble

32
Q

As T3 and T4 are lipid soluble, they pass across the membrane and into the plasma. What happens to T3 and T4 when they are in the plasma?

A

They bind to plasma proteins, mainly thyroxine-binding globulin

33
Q

Does thyroxine-binding globulin have a higher affinity for T3 or T4?

A

T4

->hint in the name, THYROXINE-binding globulin. Therefore there is more T4 circulating in the plasma than T3

34
Q

How much of the TH in our plasma is physiologically active?

A

0.2%

-> the rest is bound to plasma proteins. However, only tiny concentrations of hormones are required by the body hence why this 0.2% is enough

35
Q

Which has a longer half life, T3 or T4?

A

T4

->this is because of the greater affinity of the thyroxine-binding globulin for T4. Therefore, it releases it slowly meaning it has a longer half life

36
Q

TH circulating in the plasma feeds back via negative feedback loops to switch off the release. Explain the pathway.

A

The circulating TH feeds back and antagonises the release of TSH from the anterior pituitary AND antagonises the release of TRH from the hypothalamus.

37
Q

How much more T4 is protein bound than T3?

A

50x more

38
Q

Which type of TH binds to 90% of the TH receptors?

A

T3

->there is a higher affinity for T3 by the TH receptor than T4

39
Q

So therefore, which TH is most physiologically active?

A

T3

40
Q

How does T4 become deiodinated to form T3?

A

By deiodinase enzymes

41
Q

T4 can be deiodinated in two places, where?

A

Half is deiodinated in the plasma, half in the target cells

42
Q

What are those with hypothyroid supplemented with?

A

T4 as T3 has too short a half life.
However, T3 levels remain normal as the deiodinase enzymes convert T4 into T3

43
Q

List some of the main stimulating factors which release TRH from the hypothalamus.

A

Cold, exercise and pregnancy

44
Q

Glucocorticoids and somatostatin are both inhibitory when it comes to the release of TH.
How is somatostatin inhibitory?

A

Inhibits TSH

45
Q

Glucocorticoids and somatostatin are both inhibitory when it comes to the release of TH.
How are glucocorticoids inhibitory?

A

Inhibit TSH and the conversion of T4 into T3

46
Q

How do TH alter protein synthesis?

A

They bind to receptors in the target cells and change transcription which alters protein synthesis

47
Q

TH raise metabolic rate. What happens as a result?

A

Promotes thermogenesis (increase in temperature of the body)

48
Q

How does TH promote thermogenesis?

A

By promoting futile cycles of simultaneous catabolism and anabolism, which produces heat energy

49
Q

Hyperthyroidism?

A

Over activity of the thyroid gland

50
Q

What are the main two causes of hyperthyroidism?

A

Graves Diseases- common
Thyroid adenoma- less common

51
Q

What happens in Graves disease?

A

Body produces antibodies which mimic the action of TSH so there is continual activation of the thyroid gland

52
Q

How does the thyroid gland change in those with graves disease?

A

Increases in size by 2-3x due to hyperplasia

53
Q

How can a thyroid adenoma cause hyperthyroidism?

A

The tumour secretes hormones

54
Q

What are the symptoms of hyperthyroidism?

A

Weight loss/heat intolerance
Muscle weakness
Hyperexcitable reflexes and psychological disturbances
Increased HR, cardiac failure

55
Q

Why is there weight loss/heat intolerance in those with hyperthyroidism?

A

Increased metabolic rate and heat production

56
Q

Why is there muscle weakness in those with hyperthyroidism?

A

Increased protein catabolism

57
Q

Why is there hyperexcitable reflexes and psychological disturbances in those with hyperthyroidism?

A

Altered functioning of the NS

58
Q

Why is there increased HR and potentially cardiac failure in those with hyperthyroidism?

A

Elevated cardiovascular function as TH is permissible to epinephrine on the beta receptors

59
Q

What happens in hypothyroidism?

A

Thyroid gland is underperforming

60
Q

List some of the causes of hypothyroidism.

A

Hashimoto’s disease
Deficiency in dietary iodine

61
Q

What happens in Hashimoto’s disease?

A

Autoimmune attack of the thyroid gland preventing the release of thyroxine and triiodothyonine

62
Q

List some food sources of iodine :)))

A

Milk
Fish
Seafood
Seaweed

63
Q

What are some of the symptoms of hypothyroidism?

A

Weight gain, cold intolerance
Brittle nails, thin skin
Slower reflexes, fatigue
Reduced CVS function- slow HR and weak pulse

64
Q

What causes weight gain and cold intolerance in those with hypothyroidism?

A

Decreased metabolic rate and heat production

65
Q

What causes the brittle nails and thin skin in those with hypothyroidism?

A

Disrupted protein synthesis

66
Q

What causes the slow reflexes and fatigue in those with hypothyroidism?

A

Altered functioning of NS

67
Q

Goitre?

A

Significant enlargement of the thyroid gland

68
Q

What can cause goitre formation relating to hypothyroidism?

A

Increased trophic action of TSH on thyroid follicular cells

69
Q

What can cause goitre formation relating to hyperthyroidism?

A

Graves disease- autoimmune

70
Q

If someone is deficient in iodine, what can they not produced?

A

T3/4

71
Q

If there’s not enough T3/, which hormone is released from the hypothalamus?

A

TRH

->this can cause the thyroid gland to enlarge

72
Q
A

Year 2- Endocrine System (27 decks)

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