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Endocrinology > Thyroid gland > Flashcards

Thyroid gland Flashcards

1
Q

What is the structure of the thyroid gland?

A

Two lobes, right and left

Joined together in the middle by the isthmus

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2
Q

What is the pyramidal lobe?

A

Extends upwards from the middle of the thyroid

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3
Q

How many people have the pyramidal lobe?

A

Only a few

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4
Q

What does the thyroid gland lie in front of?

A

The larynx and the trachea

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5
Q

What does the thyroid gland lie below?

A

Thyroid and cricoid and cartilage

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6
Q

What does the thyroid gland lie above?

A

Suprasternal notch

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7
Q

What does the isthmus lie in front of?

A

2nd - 3rd rings of cartilage of trachea

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8
Q

What is the pretracheal fascia?

A

Fascia that joins the thyroid gland

to the trachea and larynx

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9
Q

Why does the thyroid gland moves upward on swallowing?

A

Because the larynx moves up when swallowing
pulls on pretracheal fascia
pulls up thyroid gland

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10
Q

How many parathyroid glands are there?

A

Four

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11
Q

Where are the parathyroid glands located?

A

Behind the thyroid gland

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12
Q

What are thyroid follicles made up of?

A

Follicular cells
arranged in spheres

Extracellular colloid within the spheres

Parafollicular cells located between follicular cells

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13
Q

What do follicular cells do?

A

Produce thyroglobulin, thyroid hormones

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14
Q

What does the colloid store?

A

Thyroglobulin

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15
Q

How do the follicular cells transport thyroglobulin into the colloid?

A

Exocytosis on their apical domain

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16
Q

What do parafollicular cells produce?

A

Calcitonin

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17
Q

What does calcitonin have a role in?

A

Calcium homeostasis

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18
Q

What is the histology of the parathyroid gland?

A

Principle/chief cells

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19
Q

What do principle/chief cells do?

A

Produce parathyroid hormone

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20
Q

What happens to iodine in the GI tract?

A

Reduced to iodide I-

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21
Q

What happens to iodide in the GI tract?

A

Taken up by epithelial cells in the small intestine

Absorbed into blood

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22
Q

Where is iodine found in the body?

A

Mostly in the thyroid gland

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23
Q

Which molecules of the body contain iodine?

A

monoiodotyrosine, MIT
diiodotyrosine, DIT
triiodothyronine, T3
thyroxine, T4

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24
Q

What is MIT made up of?

A

Tyrosine

One iodine attached to its aromatic ring

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25
Q

What is DIT made up of?

A

Tyrosine

Two iodines attached to its aromatic ring

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26
Q

What is T3 made up of?

A

MIT and DIT joined together

Two tyrosines, three iodines

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27
Q

What is T4 made up of?

A

Two DIT molecules joined together

Two tyrosines, four iodines

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28
Q

How is iodide taken up by follicular cells?

A

By the sodium iodide trap

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29
Q

Where is the sodium iodide trap located on follicular cells?

A

Basolateral domain

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30
Q

What happens to iodide in the follicular cells? What type of reaction is this?

A

I- —–> I2

oxidation

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31
Q

What enzyme catalyses the reaction
I- —–> I
in the follicular cells?

A

Thyroid peroxidase

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32
Q

What additional molecule is required to be present for the oxidation of iodine?

A

Hydrogen peroxide H2O2

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33
Q

What happens to iodine in the follicular cells?

A

It diffuses into the colloid

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34
Q

What happens to iodine and thyroglobulin in the colloid?

A

Thyroglobulin tyrosine residues are iodinated

forming MIT and DIT on thyroglobulin

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35
Q

What enzyme catalyses the iodination of thyroglobulin in colloid?

A

Thyroid peroxidase

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36
Q

Thyroid peroxidase catalyses oxidation, iodination and….

A

Coupling

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37
Q

What happens in the coupling reaction?

A

MIT and DIT are joined together
to form T3 and T4
on thyroglobulin

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38
Q

How do follicular cells take up thyroglobulin from the colloid?

A

Pinocytosis in the apical domain

39
Q

What is the pinocytosed thyroglobulin contained in inside the follicular cell?

A

Vesicle

40
Q

What happens to the vesicle containing thyroglobulin?

A

Lysosome fuses with it

41
Q

What happens to the thyroglobulin inside the phagolysosome?

A

It is digested by proteases

releasing T3 and T4

42
Q

What happens to the isolated T3 and T4 in the follicular cells?

A

Diffuse across the basolateral domain of the follicular cells
enter the blood

43
Q

Which is secreted into the blood more - T3 or T4?

A

T4

44
Q

Why do T3 and T4 not dissolve in the blood?

A

Are hydrophobic

Insoluble in water

45
Q

How are T3 and T4 carried in the blood?

A

By binding to thyroxine-binding globulin!

46
Q

Which is more biologically active - T3 or T4?

A

T3

47
Q

Where is T4 converted into T3?

A

Liver

Kidneys

48
Q

Where are thyroid hormone receptors located?

A

Inside the cell

Inside the nucleus - are nuclear receptors

49
Q

How does thyroid hormone enter cells?

A

Through thyroid hormone transporters

50
Q

How does thyroid hormone enter the nucleus?

A

Through the nuclear pores

51
Q

Where is the thyroid hormone receptor inside the nucleus?

A

Is pre-bound to specific DNA sequences

52
Q

What are the specific DNA sequences that the thyroid hormone receptor is pre-bound to? What are they called?

A

Promoter region of thyroid regulated gene

Thyroid hormone response elements

53
Q

How does the thyroid hormone receptor affect the thyroid regulated gene?

A

It represses transcription of the gene

54
Q

When thyroid hormone binds its receptor, what happens to the receptor?

A

Undergoes conformational change

Becomes transcriptional activator

55
Q

How does the changed thyroid hormone receptor affect the thyroid regulated gene?

A

Relieves its repression

Results in the gene being transcribed, translated, protein produced

56
Q

What are some examples of proteins produced by thyroid hormone activated genes?

A

Ca2+ ATPase
Na+ K+ ATPase
Cytochrome oxidase
PEPCK

57
Q

How do thyroid hormones affect cells?

A

Affect cell differentiation
cell development
both linked to metabolism

58
Q

What are the general effects of thyroid hormone?

A

Increase in BMR, heat production
Stimulation of metabolic pathways
Sympathomimetic effects

59
Q

How does thyroid hormone increase BMR, heat production?

A

Increasing number AND size of mitochondria

Stimulating synthesis of enzymes in electron transport chain

60
Q

Which tissues do not undergo and increase in BMR, heat production by thyroid hormone?

A

Brain
Spleen
Testes - for sperm production

61
Q

What general type of metabolic pathway do thyroid hormones tend to stimulate?

A

Catabolic

62
Q

How does thyroid hormone affect carbohydrate metabolism?

A

Increase in GLUT4 transporters, insulin dependent entry of glucose

Gluconeogenesis

Glycogenolysis

63
Q

How does thyroid hormone affect lipid metabolism?

A

Stimulates lipolysis

B-oxidation of fatty acids

64
Q

How do thyroid hormones have sympathomimetic effects?

A

Increase catecholamine receptors on cells

larger response to catecholamines

65
Q

Which tissues are specifically affected by thyroid hormone?

A

CVS

Nervous sustem

66
Q

How is the CVS affected by thyroid hormones?

A

Increased response to catecholamines gives…

-increased heart rate
-increased force of contraction
=increased cardiac output

-peripheral vasodilation
to carry extra heat to body surface

67
Q

How is the nervous system affected by thyroid hormones?

A

Required for development and function of neruones

e.g. their myelination

68
Q

How is the release of thyroid hormone regulated?

A

HPT axis

69
Q

Outline the HPT axis

A

Hypothalamus releases….

Thyrotropin releasing hormone (TRH) …. which stimulates….

Anterior pituitary …which releases….

Thyroid stimulating hormone …which stimulates…

Thyroid gland …which releases…

T3 and T4 …which inhibit the anterior pituitary, inhibit the hypothalamus - long loop

TSH inhibits the hypothalamus via the short loop

70
Q

What type of hormone is TSH?

A

Glycoprotein hormone

71
Q

What is the structure of TSH?

A

Two subunits
a and B
joined by covalent bonds

72
Q

What other hormones is the a subunit present in?

A

FSH

LH

73
Q

What does the B subunit confer?

A

Specifity

74
Q

What type of receptor is the TSH receptor?

A

GPCR

75
Q

What G proteins is the TSH GPCR coupled to? What are the final outcomes of these G proteins?

A

Gas - gives PKA

Gaq - gives PKC, Ca2+ increase

76
Q

What is the outcome of TSH binding to its receptor?

A

Stimulate thyroid hormone synthesis and release

77
Q

How exactly does TSH binding to its receptor stimulate thyroid hormone synthesis and release?

A 
Iodide uptake
Iodide oxidation to iodine
Thyroglobulin synthesis
Thyroglobulin iodination
Thyroglobulin pinocytosis
Thyroglobulin proteolysis
78
Q

Why might free T3 and T4 levels show variation that’s not due to disease?

A

Factors to do with method
Changes in albumin levels - also carries thyroid hormones in blood, but to lesser extent
Dilution effects

79
Q

Why might free TSH levels show variation that’s not due to disease?

A

Variation depending on time of day
Pulsatile secretion
Medications

80
Q

What is the first endocrine gland to develop in the embryo?

A

Thyroid gland

81
Q

When does the thyroid gland begin to develop?

A

3-4 weeks

82
Q

How does the thyroid gland first appear?

A

Epithelial proliferation

83
Q

Where does the thyroid gland first appear?

A

Floor of pharynx

Base of tongue

84
Q

What direction does the developing thyroid gland move in?

A

Descends downwards

85
Q

What does the developing thyroid gland descend as?

A

Thyroid diverticulum

86
Q

What does the descending thyroid gland move through?

A

The thyroglossal duct

87
Q

How does the developing thyroid gland move in relation to the hyoid bone?

A

In front of the hyoid bone

Then behind the hyoid bone

88
Q

What happens to the thyroglossal duct?

A

It degenerates

89
Q

What is lingual thyroid?

A

Remnant of thyroid tissue on tongue

90
Q

What is a thyroglossal duct cyst?

A

Remnant of thyroglossal duct

Shows up as swelling in anterior neck

91
Q

How does a thyroglossal duct cyst move when the tongue us protruded? Why?

A

Moves inwards and upwards

because is connected to tongue

92
Q

How many cells in the body does thyroid hormone affect?

A

All of them!

93
Q

Where is a thyroglossal duct cyst located?

A

Near hyoid bone

Always in the midline

Endocrinology (14 decks)

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