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Semester 2-MEH > Session7-Thyroid Gland > Flashcards

Session7-Thyroid Gland Flashcards

1
Q

Describe the location of the thyroid gland

A
  • lies against and around front larynx and trachea
  • below thyroid cartilage
  • isthmus extends from 2nd to 3rd rings of trachea
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2
Q

What joins the two lobes of the thyroid gland?

A

Isthmus

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3
Q

True or false: thyroid gland is the first endocrine gland to develop

A

TRUE

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4
Q

How does the thyroid gland appear at 3-4 weeks of gestation?

A

As an epithelial proliferation in the floor of the pharynx at the base of the tongue

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5
Q

Complete the sentences:

The thyroid gland first descends as a ________________ through the ____________ duct and migrates downwards, passing in front of the hyoid bone. During migration, the gland remains connected to the tongue by the _____________ duct which subsequently ________________.

A

Diverticulum
Thyroglossal
Thyroglossal
Degenerates

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6
Q

What are thyroid follicles?

A

Follicular cells arranged in spheres

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7
Q

What are thyroid follicles filled with?

A

Colloid

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8
Q

What is colloid?

A

Deposit of thyroglobulin and is extracellular even though it is inside the follicle

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9
Q

What do thyroid follicular cells produce?

A

Thyroid hormone

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10
Q

What do thyroid parafollicular cells produce?

A

Calcitonin

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11
Q

What do parathyroid cells/chief cells produce?

A

Parathyroid hormone

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12
Q

What are the two thyroid hormones?

A 
T3 = triiodothyronine 
T4 = tetraiodothyronine (thyroxine)
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13
Q

What forms thyroid hormone T3?

A

Monoiodotyrosine (MIT) + diiodotyrosine (DIT)

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14
Q

What forms thyroid hormone T4?

A

Diiodotyrosine (DIT) + diiodotyrosine (DIT)

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15
Q

How does thyroglobulin play a role in thyroid hormone formation?

A

Acts as a scaffold on which hormones are formed

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16
Q

Which three reactions does thyroid peroxidase regulate?

A

1) oxidation of iodide to iodine
2) iodination of tyrosine acceptor residues on thyroglobulin
3) coupling of MIT or DIT to generate thyroid hormones within thyroglobulin protein

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17
Q

What does the oxidation of iodide to iodine require?

A

Hydrogen peroxide

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18
Q

How is iodide taken up from the blood?

A

By thyroid epithelial cells which have a sodium-iodide symporter or “iodine trap”

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19
Q

What are some common sources of iodine?

A
  • dairy products
  • grains
  • meat
  • vegetables
  • eggs
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20
Q

True or false: most T3 is converted to T4 outside the thyroid

A

FALSE - T4 converted to T3

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21
Q

Where is most T4 converted to T3?

A

In liver and kidneys

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22
Q

How are T3 and T4 transported in the blood?

A

Bound to protein thyroxine-binding globulin

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23
Q

Why do T3 and T4 need a protein in order to be transported in the blood?

A

They are lipid soluble

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24
Q

Complete the flow chart:

Hypothalamus -> ___________ _____________ hormone -> anterior pituitary -> ___________ ________________ hormone -> thyroid gland -> ____________ hormone -> target tissues

A 
Thyrotropin releasing (TRH)
Thyroid stimulating (TSH)
Thyroid
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25
Q

What are the two interconnected responses of thyroid hormones in the body?

A
  • effects on cellular differentiation and development

- effects on metabolic pathways

26
Q

What type of hormone is thyroid stimulating hormone (TSH) and what is it composed of?

A

Glycoprotein hormone

Composed of 2 non-covalently bound subunits (alpha and beta)

27
Q

Which subunit in TSH provides unique biological activity?

A

Beta

28
Q

True or false: TSH is the trigger for thyroid hormone release

A

TRUE

29
Q

What does TSH stimulate?

A
  • iodide uptake
  • iodide oxidation
  • thyroglobulin synthesis
  • thyroglobulin iodination
  • colloid pinocytosis into cell
  • proteolysis of thyroglobulin
  • cell metabolism and growth
30
Q

What type of receptor is TSH receptor?

A

GPCR

31
Q

What are the general actions of thyroid hormone?

A
  • increase in basal metabolic rate and heat production
  • stimulation of metabolic pathways
  • sympathomimetic effects
32
Q

How do thyroid hormones stimulate metabolic rate?

A
  • by increasing number and size of mitochondria

- by stimulating the synthesis of enzymes in the respiratory chain

33
Q

Which metabolic pathway is stimulated more by thyroid hormone?

A

Catabolic

34
Q

Which metabolic pathways do thyroid hormones stimulate?

A
  • lipid metabolism: stimulates lipolysis and beta oxidation of fatty acids
  • carbohydrate metabolism: stimulate insulin-dependent entry of glucose into cells and increase gluconeogenesis and glycogenolysis
35
Q

How do thyroid hormones increase target cell response to catecholamines?

A

By increasing receptor number on target cells

36
Q

What effect do thyroid hormones have on the cardiovascular system?

A

Increases heart’s responsiveness to catecholamines:

  • increases cardiac output (increase chronotropy and inotropy)
  • increased peripheral vasodilation to carry extra heat to body surface
37
Q

What effect do thyroid hormones have on the nervous system?

A

Increased myelination of nerves and development of neurones

38
Q

How do thyroid hormone receptors act as hormone-activated transcription factors?

A

By modulating gene expression - bind DNA in absence of hormone, leading to transcriptional repression

39
Q

Where is the thyroid hormone receptor pre-bound to specific DNA sequences?

A

On DNA called hormone response elements (HRE) in the promoter region of thyroid hormone regulated genes

40
Q

What is the normal level of plasma T4?

A

10-25 pM

41
Q

What is the normal level of plasma T3?

A

3-8 pM

42
Q

What is goitre?

A

Enlargement of thyroid gland

43
Q

True or false: goitre accompanies hyperthyroidism only

A

FALSE - hyper or hypothyroidism

44
Q

What are the causes of hypothyroidism?

A

1) failure of thyroid gland
2) TSH or TRH deficiency
3) inadequate iodine in diet
4) radioactive iodine
5) autoimmunity
6) post surgery
7) congenital
8) anti-thyroid drugs

45
Q

What are the symptoms of hypothyroidism?

A
  • obesity
  • lethargy
  • intolerance to cold
  • bradycardia
  • dry skin
  • alopecia
  • hoarse voice
  • constipation
  • slow reflexes
46
Q 
Describe the levels of:
1) T3
2) T4
3) TSH 
In hypothyroidism
A

1) low
2) low
3) elevated

47
Q

What can hypothyroidism cause in infants?

A

Cretinism - dwarfed stature, mental deficiency, poor bone development, slow pulse, muscle weakness, GI disturbances

48
Q

What can hypothyroidism cause in adults?

A

Myxedema - thick puffy skin, muscle weakness, slow speech, mental deterioration, intolerance to cold

49
Q

What is Hashimoto’s disease?

A

Autoimmune disease resulting in destruction of thyroid follicles -> hypothyroidism

50
Q

How can Hashimoto’s disease be treated?

A

-oral thyroid hormone: T4 usually used since longer half-life

51
Q

What are the causes of hyperthyroidism?

A
  • autoimmune Graves’ disease
  • toxic multinodular goitre
  • excessive T4 or T3 therapy
  • drugs
  • carcinoma
52
Q

What are some symptoms of hyperthyroidism?

A
  • weight loss
  • irritability
  • heat intolerance, sweaty hands
  • tachycardia
  • fatigue
  • increased bowel movements
  • possible tremor
  • hyper-reflexive
  • breathless
  • loss of libido
53
Q

What is Graves’ disease?

A

Autoimmune disease resulting in hyperthyroidism, caused by production of thyroid stimulating immunoglobulin (TSI)

54
Q

What does thyroid stimulating immunoglobulin (TSI) do?

A

Continuously stimulates thyroid hormone secretion outside normal negative feedback control

55
Q 
Describe the levels of:
1) T3
2) T4
3) TSH
In Graves' disease
A

1) elevated
2) elevated
3) low

56
Q

Which radioisotope is used for isotope scanning of the thyroid with a gamma camera?

A

Technetium-99m

57
Q

What are antithyroid drugs used to treat?

A

Overactive thyroid eg Graves’ disease

58
Q

How do antithyroid drugs work?

A

Block formation of thyroid hormone

59
Q

What is the most commonly used antithyroid drug in the UK?

A

Carbimazole

60
Q

What is carbimazole and how does it work?

A

Pro-drug and is converted to methimazole in the body

Prevents thyroid peroxidase from coupling and iodination get tyrosines on thyroglobulin

61
Q

What is a pro-drug?

A

Drugs converted to something else in the body which has activity in the body

62
Q

Why does carbimazole have a delayed effect?

A

Because there is a store of thyroglobulin which takes 2-4 weeks to diminish

Semester 2-MEH (21 decks)

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