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Endocrine Physiology > The Thyroid Gland > Flashcards

The Thyroid Gland Flashcards

1
Q

Which two hormones does the thyroid gland synthesise?

A

T3 - triiodothyronine

T4 - Thyroxine

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2
Q

What are the two cell types in the thyroid gland?

A

C (clear) cells

Follicular cells

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3
Q

What do C (clear) cells secrete?

A

Calcitonin

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4
Q

What do follicular cells do?

A

Support thyroid hormone synthesis
Surround hollow follicles
Actively concentrate iodide from the plasma and transport it into the colloid

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5
Q

What are the walls of thyroid follicles made from?

A

Follicular cells

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6
Q

What is the centre of the thyroid follicle filled with?

A

Colloid

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7
Q

What is colloid?

A

A sticky glycoprotein matrix

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8
Q

How much supply of thyroid hormone (TH) is in a thyroid follicle?

A

2 - 3 months

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9
Q

What do follicular enzymes make?

A

Enzymes that make thyroid hormone

Thyroglobulin

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10
Q

What is thyroglobulin rich in?

A

Tyrosine residues

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11
Q

Enzymes and thyroglobulin are packaged into vesicles and exported from the follicular cells into where?

A

The colloid

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12
Q

What happens to iodide once it is transported into the colloid?

A

Combines with tyrosine resides to form thyroid hormones

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13
Q

Where are both tyrosine and iodide derived from?

A

The diet

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14
Q

What is thyroid peroxidase?

A

Enzyme present on the colloidal side of the cells

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15
Q

Function of thyroid peroxidase

A

Catalyses the addition of iodide to tyrosine residues is thyroglobulin
In this process iodide loses an electron to become iodine

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16
Q

Addition of one iodine to tyrosine leads to….

A

MIT (monoiodotyrosine)

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17
Q

Addition of a second iodine to MIT leads to……

A

DIT (diiodotyrosine)

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18
Q

MIT + DIT = ?

A

Triiodothyronine or T3

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19
Q

DIT + DIT = ?

A

Thyroxine or T4

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20
Q

How are thyroid hormones released in response to TSH?

A

Portions of the colloid are taken back up into the follicular cell by endocytosis. Within the cells they form vesicles which contain proteolytic enzymes that cut the thyroglobulin to release the thyroid hormones

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21
Q

Solubility of both T3 and T4

A

Lipid soluble

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22
Q

Binding of both T3 and T4

A

They are lipid soluble and so pass across the follicular cell membrane into the plasma, where they bind to plasma proteins, mainly thyroxine-binding globulin

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23
Q

Which of T3 and T4 circulate in the plasma?

A

Both

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24
Q

What molecules is thyroxine made up of?

A

2 tyrosine

4 iodine

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25
Q

What molecules is triiodothyronine made up of?

A

2 tyrosine

3 iodine

26
Q

Where is thyroxine peroxidase located?

A

On the apical membrane of the follicular cells

27
Q

How does iodide enter the follicular cells from the plasma?

A

Via a Na/I- transporter (symport).

The coupling to Na+ enables the follicular cells to take up iodide against a concentration gradient

28
Q

How is iodide transported into the colloid?

A

Via the pendrin transporter

29
Q

What is iodide transport into the thyroid gland inhibited by?

A

Thiocyanates

30
Q

Common origin of thiocyanates

A

Cigarette smoke

31
Q

How does TH release into the plasma?

A

Under influence of TSH from pituitary

TSH stimulates the follicular cells to endocytose colloidal thyroglobulin

32
Q

How does T3 and T4 circulate in the plasma?

A

99.8% bound to plasma protein

33
Q

What does thyroxine binding globulin (TBG) have a particularly high affinity for? What does this result in?

A

T4
Releasing it slowly into the plasma
This accounts for the longer half life

34
Q

What is the half life of T4?

A

Approx. 6 days

35
Q

What is the half life of T3?

A

Approx. 1 day

36
Q

Which of T3 and T4 is there more of in the plasma?

A

50x more total (free + bound) T4 in plasma than T3

37
Q

Which of T3 and T4 is more physiologically active?

A

T3 3-5x more active than T4

38
Q

90% of TH binding to TH receptors inside cell is either T3 or T4?

A

T3

39
Q

Which of T3 or T4 does the TH receptor have a much higher affinity for?

A

T3

40
Q

How is T4 turned into T3?

A

Deiodinated to T3 by deiodinase enzymes

41
Q

Where is T4 deiodinated into T3?

A

Half in plasma

Remaining being inside target cells

42
Q

Effects on TRH from hypothalamus

A

Cold
Exercise
Pregnancy

43
Q

What hormones have an inhibitory effect on TRH?

A

Glucocorticoids

Somatostatin

44
Q

What do glucocorticoids inhibit?

A

TSH

Conversion of T4 to T3

45
Q

What does somatostatin inhibit?

A

TSH

46
Q

Effects of thyroid hormones

A 
Raises metabolic rate 
Promotes thermogenesis
Net increase in proteolysis
Increase in hepatic gluconeogenesis
Net increase in lipolysis
Critical for growth 
- anabolic
- stimulates GH receptor expression
47
Q

Causes of hyperthyroidism

A

Graves disease

Thyroid adenoma

48
Q

Pathology of graves disease

A

Antibodies produced that bind mimic TSH and continually activate the thyroid gland
Increase release of TH switches off TSH release from anterior pituitary so [TSH] in the plasma is very low.

49
Q

How common is graves disease?

A

Common

50
Q

How common is thyroid adenoma?

A

Rare

51
Q

Symptoms of hyperthyroidism

A 
Weight loss
Heat intolerance
Muscle weakness
Hyperexcitable reflexes 
Psychological disturbances
Increased HR/Contractable force 
Cardiac failure
52
Q

Causes of hypothyroidism

A

Hashimotos disease
Deficiency in dietary iodine
Idiopathic

53
Q

What is hashimotos disease?

A

Autoimmune attack of thyroid gland

54
Q

How much dietary iodine do we need a year?

A

50mg/year

55
Q

Sources of iodine

A

Mild
Fish
Seafood
Seaweed

56
Q

Symptoms of hypothyroidism

A 
Weight gain 
Cold intolerance
Brittle nails
Thin skin 
Slow speech/reflexes
Fatigue 
Slow HR
Weak pulse
57
Q

What are thyroid pathologies often accompanied by?

A

Significant enlargement of thyroid gland = goitre

58
Q

What is goitre?

A

Significant enlargement of thyroid gland

59
Q

Pathology of goitre in hypothyroidism

A

Increased trophic action of TSH on thyroid follicular cells

60
Q

Pathology of goitre in graves disease

A

Over activity as a result of autoimmune disease

Endocrine Physiology (8 decks)

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