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D&E Physiology > The Thyroid Gland > Flashcards

The Thyroid Gland Flashcards

1
Q

What shape is the thyroid gland?

A

Butterfly shape

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2
Q

What are the thyroid hormones?

A

T3 - triiodothyronine

T4 - Thyroxine

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3
Q

What type of cells are contained in the thyroid gland?

A

C cells

Follicular cells

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4
Q

What are C-cells?

A

Clear cells - secrete calcitonin

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5
Q

What are follicular cells?

A

Support thyroid hormone synthesis and surround follicles

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6
Q

What are follicles?

A

Spherical structures whose walls are made of follicular cells
Centres filled with colloid

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7
Q

What is colloid?

A

Fluid in the centre of a thyroid follicle

Sticky glycoprotein matrix

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8
Q

How much TH is contained within a follicle?

A

2-3mths worth

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9
Q

What is the function of follicular cells?

A

TH production enzymes
Thyroglobulin production
Concentrate iodide (I-)

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10
Q

What is thyroglobulin?

A

A large protein rich in tyrosine residues

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11
Q

How do follicular enzymes and thyroglobulin enter the colloid?

A

Vesicles

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12
Q

How do we get iodide and tyrosine?

A

Diet

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13
Q

How does Iodide enter the cells?

A

Blood

Na+/I- transporter (symporter)

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14
Q

What is the benefit of coupling I- to Na+?

A

Allows iodide to be taken up against a concentration gradient

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15
Q

How is iodide transported into colloid?

A

Pendrin transporter

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16
Q

How is iodide added to tyrosine?

A

One iodine + tyrosine:
MIT (monoiodotyrosine)
Add a second iodine:
DIT (diiodotyrosine)

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17
Q

What is the fate of MIT and DIT?

A

MIT + DIT = T3

DIT + DIT = T4

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18
Q

How is thyroid hormone synthesis catalysed?

A

Thyroid peroxidase on the apical membrane of follicular cells

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19
Q

TSH stimulates what?

A

Portions of colloid to be taken up into the follicular cells by endocytosis

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20
Q

What happens to colloid taken back up into the follicular cells?

A

Form vesicles with proteolytic enzymes to cut up thyroglobulin into thyroid hormones

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21
Q

How is colloid cut into thyroid hormones?

A

Packaged into vesicles in the follicular cells with enzymes

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22
Q

How do T3/4 pass into the blood?

A

Passive - lipid soluble

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23
Q

How do T3/T4 travel in the blood?

A

Bound to thyroxine-binding globulin

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24
Q

What additional proteins are involved in TH movement?

A

Transporter proteins

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25
Q

What aspect of TH control is stimulated by TSH?

A

Exocytosis of iodinated/conjugated thyroglobulin

Release of TH into the plasma

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26
Q

Where does conjugation of thyroglobulin take place?

A

Colloid

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27
Q

TH stimulates follicular cells to do what?

A

Endocytose colloidal thyroglobulin

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28
Q

What is the fate of thyroid hormones in the absence of TSH?

A

Thyroid hormones are stored in the colloid in the absence of TSH

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29
Q

What portion of plasma T3/T4 is bound?

A

99.8%

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30
Q

Thyroxine binding globulin has a particularly high binding affinity for what? What does this mean?

A

T4 - it is released more slowly, has a longer half life

31
Q

What is the half life of thyroid hormone?

A

T4 - 6 days

T3 - 1 day

32
Q

Which fraction of the hormone inhibits TSH and TRH?

A

FREE/unbound TH

33
Q

Most TH circulates in which form?

A

Protein bound T4 - 100nmoles/L

T3 - 2.3nmoles/L

34
Q

What is the ratio of total (free and bound) TH?

A

50x more T4 than T3

35
Q

The majority of TH binding to TH receptors inside cells is what?

A

T3 (90%)

36
Q

Which TH is most physiologically active?

A

T3 - 3-5x higher than T4

37
Q

What is the fate of T4?

A

Used by cells

Deiodinated to T3 by deiodinase enzymes

38
Q

What is the role of deiodinase enzymes?

A

Convert T4 to T3

39
Q

Where does Deiodination take place?

A

50% in plasma

50% in cells

40
Q

What environments stimulate TRH release

A

PREGNANCY
Cold
Exercise

41
Q

Where is TRH released from?

A

The hypothalamus

42
Q

Which hormones are inhibitory to TH?

A

Somatostatin

Glucocorticoids

43
Q

Thyroid hormones cause an increased excretion of what?

A

Glucocorticoids

44
Q

How do somatostatins effect TH?

A

Inhibit TSH

45
Q

How do glucocorticoids effect TH?

A

Inhibit TSH and conversion of T4 to T3

46
Q

What is the function of thyroid hormone?

A 
Increased metabolic rate
Promotion of thermogenesis
Increase hepatic gluconeogenesis
Net Increase proteolysis and lipolysis
Growth 
Brain development in utero
47
Q

What are futile cycles?

A

Counteracting cycles of hormones simultaneously causing catabolism and anabolism

48
Q

What is the effect of TH increasing hepatic gluconeogenesis?

A

No effect on plasma [Glucose] so long as pancreas is supplying adequate insulin

49
Q

What is the effect of TH on proteins?

A

Simultaneous protein synthesis and proteolysis but NET EFFECT IS PROTEOLYSIS

50
Q

What is cretinism?

A

Congenital hypothyroidism caused by maternal iodine deficiency

51
Q

Why is TH essential for growth?

A

TH stimulates GH receptor expression

52
Q

What are the causes of hyperthyroidism?

A

Graves’ disease

Thyroid adenoma

53
Q

What is a thyroid adenoma?

A

A hormone secreting thyroid tumour

54
Q

What is Graves’ disease?

A

Antibodies produced that mimic TSH and cause the thyroid to be constantly on
Plasma[TSH] is very low
Often goitre
Cell hyperactivity

55
Q

What are the symptoms of hyperthyroidism?

A 
Weight loss/heat intolerance
Muscle weakness
Excitable reflexes
Psychological disturbances 
Increased HR
Cardiac failure
56
Q

What is the cause of weight loss in hyperthyroidism?

A

Increased protein catabolism with increased metabolic rate

57
Q

What is the cause of hyper-excitable reflexes in hyperthyroidism?

A

Altered nervous system function

58
Q

What is the cause of increased CO/HR in hyperthyroidism?

A

TH is permissive to epinephrine (beta receptors) - increased cardiovascular function

59
Q

What are the causes of hypothyroidism?

A

IDIOPATHIC
Hashimoto’s disease
Iodine deficiency

60
Q

Idiopathic hypothyroidism may be linked to what?

A

Thyroiditis

61
Q

What are the dietary sources of iodine?

A

Milk
Seafood
Seaweed

62
Q

What is Hashimoto’s disease?

A

Autoimmune attack on thyroid

63
Q

What are the symptoms of hypothyroidism?

A 
Weight gain/cold intolerance
Brittle nails/hair/skin
Slow speech/reflexes
Fatigue
Slow HR, weak pulse
64
Q

How does hypothyroidism cause weight gain/cold intolerance?

A

Decreased metabolic rate and heat production

65
Q

How does hypothyroidism cause brittle nails/thin skin?

A

Disrupted protein synthesis

66
Q

How does hypothyroidism cause slow speech/reflexes, fatigue?

A

Altered nervous system function

67
Q

How does hypothyroidism cause slow HR/Weak pulse?

A

Reduced cardiovascular function

68
Q

What is a goitre?

A

Hypertrophy of the thyroid gland

69
Q

What causes a goitre?

A

Hyper/hypothyroidism

70
Q

How does hypothyroidism cause a goitre?

A

Increased trophic action of TSH on the thyroid follicular cells

71
Q

How does hyperthyroidism cause a goitre?

A

Overactivity due to autoimmune disease (Graves’ disease)

72
Q

Low levels of TH stimulates what?

A

Activity in the anterior pituitary

73
Q

What is a primary disorder of TH release?

A

Diseases of the thyroid directly

D&E Physiology (10 decks)

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