Structure and Function of Thyroid Gland Flashcards Preview

Z OLD ESA 1- Metabolism > Structure and Function of Thyroid Gland > Flashcards

Flashcards in Structure and Function of Thyroid Gland Deck (118):
1

Where is the thyroid gland located?

In the neck, in front of the lower larynx and upper trachea

2

When is the thyroid gland visible and palpable?

When enlarged

3

What is the thyroid gland called when its enlarged?

A goitre

4

What lies in close proximity to the thyroid gland?

Two nerves

5

What nerves lie in close proximity to the thyroid gland?

The recurrent laryngeal and the external branch of the superior laryngeal

6

What is the problem with thyroid surgery?

During surgery, the close nerves are at risk, and must be avoided

7

What is the consequence of damage to the nerves close to the thyroid?

They supply the larynx, and are involved in speech

8

Describe the vascular supply to the thyroid

Very good, with three arteries supplying it and three veins draining it

9

What are the three arteries and veins supplying the thyroid gland called?

Superior, muddle and inferior thyroid arteries/veins

10

What shape is the thyroid gland?

Butterfly, with two lateral lobes joined by a central isthmus

11

How big is the thyroid gland?

Varies in size, but usually 2-3cm across, and weighs 15-20g

12

What are the the major cell types found in the thyroid gland?

- Follicular 
- Parafollicular (C-cells)

13

How are the follicular cells of the thyroid gland arranged?

In numerous functional units called follicles, separated by connective tissue

14

Where are the parafollicular cells found?

ln the connective tissue

15

What shape are follicles?

Spherical

16

Describe the structure of follicles

Lined with epithelial (follicular) cells, surrounding a central lumen

17

What do the follicle lumens contain?

Colloid

18

What is colloid rich in?

The protein thyroglobulin

19

What does the thyroid gland produce?

3 hormones

20

What hormones are produced in the follicular cells?

#NAME?

21

What hormone is produced in the parafollicular cells?

Calcitonin

22

What are T3 and T4?

Small molecules derived from the amino acid tyrosine

23

How are T3 and T4 derived from tyrosine?

With the addition of atoms of iodine

24

What is calcitonin?

A polypeptide hormone

25

What is calcitonin involved in?

Calcium metabolism

26

What are the basic steps of synthesis of T3 and T4 in the thyroid follicles?

- Transport of iodide into the epithelial cells against a concentration gradient 
- Synthesis of a tyrosine rich protein (thyroglobulin) in epithelial cells 
- Secretion (exocytosis) of thyroglobulin into the lumen of follicle 
- Oxidation of iodide to produce an iodinating species 
- Iodination of side chains of tyrosine residues in thyroglobulin to form MIT (mono-iodotyrosine) and DIT (di-iodotyrosine)
- Coupling of DIT with MIT or DIT to form T3 and T4 respectively within the thyroglobulin.

27

What ratios are T3 and T4 produced in?

~1:10

28

How are T3 and T4 stored?

Extracellularly in the lumen of the follicles as part of the thyroglobulin molecules

29

How much T3 and T4 is stored?

Normally considerable- enough so would last for several months at normal rates of secretion

30

How is T3 and T4 secreted?

Thyroglobulin is taken into the epithelial cells from the lumen of the follicles by the process of endocytosis. Here, proteolytic cleavage of the thyroglobulin occurs to release T3 and T4, and these diffuse from the epithelial cells into the circulation

31

What is the synthesis and secretion of T3 and T4 under the control of?

The hypothalamus and the anterior pituitary gland

32

What is the hypothalamic factor for T3 and T4 secretion control?

Thyrotropin-releasing hormone (TRH)

33

What kind of molecule is TRH?

Tri-peptide

34

Where is TRH released from?

Cells in the dorsomedial nucleus of the hypothalamus

35

What is TRH release under the influence of?

#NAME?

36

What do increased stress levels cause regarding TRH?

Increases release

37

What do increased temperatures cause regarding TRH?

Decreases releases

38

What does TRH do once released?

Travels in the hypothalamic/pituitary portal system to stimulate the secretion of Thyroid Stimulating Hormone (TSH)

39

Where is TSH secreted from?

The thyrotropes in the anterior pituitary

40

What does TSH do once secreted?

Travels in the blood to affect the follicular cells of the thyroid gland

41

What kind of molecule is TSH?

Glycoprotein hormone

42

What does TSH consist of?

Two covalently linked subunit s (α and ß subunit s)

43

In what manner is TSH released?

In low-amplitude pulses following a diurnal rhythm

44

When are higher levels of TSH released?

At night

45

What does TSH interact with?

Receptors on the surface of the follicle cells

46

What does TSH do when it has interacted with the receptors on the follicle cells?

Stimulates all aspects of the synthesis and secretion of T3 and T4

47

What effects does TSH have?

Trophic effects on the thyroid gland

48

What are the trophic effects of TSH on the thyroid gland?

- Increased vascularity 
- Increase in size 
- Number of follicle cells

49

How can the trophic effects of TSH have negative consequences?

May result in an enlarged thyroid (goitre) that may or may not be overactive

50

Are T3 and T4 hydrophilic or hydrophobic?

Hydrophobic

51

How are T3 and T4 transported in the blood?

Bound to the proteins thyronine binding globulin, pre-albumin and albumin

52

How much T3 and T4 is free in solution?

53

What is important about the free T3 and T4?

It is biologically active

54

How do T3 and T4 differ in affinity?

T3 has a slightly lower affinity for transport proteins than T4

55

What is the result of T3 having a lower affinity to the receptor than T4?

A greater percentage is free, and it’s half-life in the circulation is shorter

56

What increases the synthesis of thyronine binding globulin (TBG)?

Oestrogen during pregnancy

57

What is the result in the increased TBG during pregnancy?

Fall in the amount of T3 and T4 in the blood, as more in bound

58

What is the result in the fall of free T3 and T4 in pregnancy?

Removes the inhibitory feedback on the pituitary and hypothalamus, and so more TRH and TSH are produced, so thyroid gland secretes more T3 and T4

59

What is the result in the increased T3 and T4 synthesis in pregnancy?

Amount of free T3 and T4 returns to normal, but total amount in the blood increases

60

Where do T3 and T4 exert their influences?

They have general effects on the metabolic activity of most tissues, and more specific effects on particular tissues

61

How long does the response of target tissues to T3 and T4 generally take?

Generally occurs slowly, taking day’s/weeks to manifest themselves

62

What are the effects of T3 and T4?

#NAME?

63

Of what nature are the metabolic effects of T3 and T4?

Generally catabolic

64

What do the metabolic effects of T3 and T4 lead to?

- Increase in BMR
- Heat production 
- Increased oxygen consumption

65

What is T3 and T4 important for?

Normal growth and development

66

What are their effects on physical growth related to?

In part, their metabolic effects on tissues

67

What specific effects do T3 and T4 have that affects growth?

- Affect bone mineralisation 
- Increase synthesis of heart muscle protein

68

What is particularly sensitive to T3 and T4?

The nervous system

69

When is the nervous system particularly sensitive to T3 and T4?

During development

70

Why is the nervous system especially sensitive to T3 and T4 during development?

Because T3 and T4 are required for the development of the cellular processes of nerve cells, hyperplasia of cortical neurones and myelination of nerve fibres

71

What happens in the absence of thyroid hormones from birth to puberty?

The child remains mentally retarded (cretinism)

72

What happens if a thyroid hormone deficiency is not corrected within a few weeks of birth?

Irreversible damage occurs

73

What happens to newborns soon after birth?

They have their thyroid function assessed

74

What is lack of thyroid hormones characterised by in adults?

- Poor concentration
- Poor memory 
- Lack of initiative

75

What is true of some of the actions of T3 and T4?

They are indirect

76

What are the indirect actions of T3 and T4 related to?

Important interactions with other hormones and neurotransmitters

77

What effects do T3 and T4 have on hormones and neurotransmitters?

They stimulate them

78

Where do T3 and T4 stimulate hormones and neurotransmitters?

In a variety of tissues, e.g. heart muscle, GI tract

79

What may the stimulation of hormones and neurotransmitters produce?

Increased responsiveness of these tissues to regulatory factors

80

What does T3 and T4 stimulation lead to in the heart muscle?

Tachycardia

81

What does T3 and T4 stimulation lead to in the GI tract?

Increased motility

82

What role does T3 and T4 have in the actions of hormones such as FSH and LH?

Permissive

83

What happens in the absence of thyroid hormones due to their permissive roles?

Ovulation fails to occur

84

How do T3 and T4 exert their actions?

The cross the plasma membrane of target cells, and interact with specific high affinity receptors

85

Where are the T3 and T4 receptors found?

In the nucleus and possibly the mitochondria

86

What are the receptors made up of?

Protein

87

How does receptor affinity for T3 differ from that of T4?

The affinity for T3 is 10-fold that of T4

88

What do the receptors have?

A number of domains

89

What happens when T3 binds to the hormone-binding domain of it’s receptor?

It is thought it produces a conformational change in the receptor that unmasks the DNA binding domina

90

What happens on interaction of the hormone-receptor complex with DNA?

It increases the rate of transcription of specific genes, that are then translated into proteins

91

What DNA does the T3 hormone-receptor complex interact with?

Nuclear or mitochondrial

92

What does the increased rate of protein synthesis due to the action of T3 stimulate?

Oxidative energy metabolism in the target cells

93

What is the purpose of T3 stimulation of oxidative energy metabolism in the cells?

Provides the extra energy required for protein synthesis

94

What does increased protein synthesis by T3 produce?

Increased amounts of specific functional proteins

95

What does increased amounts of functional proteins result in?

- Increased cell activity 
- Increased demand for energy

96

How can T4 be converted to T3 in tissues?

By removal of the 5’-iodine

97

Where is T4 to T3 an important mechanism?

In regulating the amount of active hormone in cells

98

Why is T4 to T3 conversion an important regulatory mechanism?

T3 has 10 times the activity of T4

99

What does the removal of the 3’-iodide produce?

Inactive reverse T3 (rT3)

100

What can rT2 do?

Bind to thyroid hormone receptors without stimulating them, but blocks the effect of T3

101

What is hyperthyroidism?

When there is too much physiological active thyroid hormones

102

What is hypothyroidism?

When there is too little physiologically active thyroid hormone

103

What is the most common form of hypothyroidism?

Hashimotos’ disease

104

How much of the population does Hasimotos’ disease affect?

1%, mostly women

105

What is Hashimotos’ disease?

An autoimmune disease, that results either in the destruction of thyroid follicles, or production of an antibody that blocks the TSH receptor on follicle cells, preventing them from responding to TSH

106

How are patients with Hashimotos’ disease generally treated?

With oral thyroxine

107

What does the dosage of thyroxine in treatment of Hashimotos disease depend on?

Patient’s signs and symptoms, and TSH levels

108

What is the most common form of hyperthyroidism?

Graves disease

109

How many people does Graves disease affect?

~1% of population, mostly women

110

What is Grave’s disease?

An autoimmune disease in which antibodies (thyroid stimulating immunoglobulin (TSI)) are produced that stimulate the TSH receptors on follicle cells, resulting in increased production and release of T3 and T4

111

Why do TSH levels fall in Grave’s disease?

Due to the negative feedback exerted by T3 and T4 on the anterior pituitary gland

112

Does the fall in TSH affect thyroid hormone secretion?

No

113

Why does the fall in TSH secretion not affect thyroid hormone secretion?

The stimulus for thyroid hormone release is TSI, rather than TSH

114

How are patients with Graves disease treated?

With carbimazole

115

What is carbimazole?

A drug that inhibits the incorporation of iodine into thyroglobulin

116

How does carbimazole work?

It inhibits the enzyme thyroid peroxidase, and therefore prevents coupling and iodination of tyrosine residues on thyroglobulin

117

What are the signs and symptoms of hypothyroidism in adults?

- Cold intolerance and reduced BMR
- Weight gain tiredness and lethargy 
- Bradycardia 
- Neuromuscular system signs
- Weakness 
- Muscle cramps 
- Cerebellar ataxia 
- Dry and flaky skin 
- Alopecia
- Deep, husky voice

118

What are the signs and symptoms of hyperthyroidism?

- Heat intolerance, increased oxygen consumption and increased BMR
- Weight loss 
- Physical and mental hyperactivity 
- Tachycardia 
- Intestinal hyper-mobility 
- Skeletal and cardiac myopathy giving rise to tiredness, weakness and breathlessness
- Osteoporosis due to increased bone turnover and preferential resorption