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MEH > 7. Thyroid > Flashcards

7. Thyroid Flashcards

1
Q

The parafollicular cells within the thyroid gland secrete which substance?

A

Calcitonin

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2
Q

What is thyroxine?

A

tetraiodothyronine

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3
Q

The release of thyroid hormone from the thyroid gland is triggered by which hormone?

A

TSH

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4
Q

Where is thyroid stimulating hormone released from?

A

Anterior pituitary gland

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5
Q

What type of receptor does thyroid hormone bind to?

A

Nuclear receptor - bound to bind DNA causing transcriptional repression. Hormone binding causes conformational changet to function as a transcriptional activator rather than a repressor.

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6
Q

Which form of thyroid hormone has the longest half-life?

A

T4

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7
Q

Which protein serves to transport thyroid hormone (T3 & T4) in plasma?

A

Thyroxin-binding globulin

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8
Q

Outline the structure of the thyroid gland.

A

2 lobes joined by an isthmus

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9
Q

Where is the thyroid gland located?

A

Anterior to trachea, Inferior to thyroid cartilage, superior to sternal notch.

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10
Q

When does thyroid development take place?

A

At 4-7 weeks

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11
Q

Briefly explain the development of the thyroid gland.

A
  1. Epithelial proliferation at the base of tongue, floor of pharynx.
  2. Descends down thyroglossal duct, attached to tongue.
  3. Thyroglossal duct generates
  4. Thyroid gland continues descending for 2 weeks to it’s final position.
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12
Q

Outline the histological structure of the thyroid gland.

A

Arranged in follicles, lined with follicular cells and filled with extracellular colloid.
Parafollicular cells between follicles.

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13
Q

What is the contents of colloid?

A

Thyroglobulin store

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14
Q

What do parafollicular/C-cells secrete?

A

Calcitonin

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15
Q

What cells in the parathyroid gland secrete PTH?

A

Chief cells/principal cells

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16
Q

By what transporter does iodine enter the thyroid follicle cells?

A

Sodium/iodide symporter

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17
Q

Once in the follicular cell, what happens to iodide?

A

Oxidised to iodine by thyroid peroxidase

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18
Q

What 3 reactions does the membrane-bound thyroid peroxidase catalyse?

A
  1. Oxidation of iodide
  2. Iodination of tyrosine residues on thryogloblin
  3. Coupling of iodinated residues to form bound T3 and T4
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19
Q

How are thyroid hormones excreted from the colloid when needed?

A

Pinocytosis into the follicular cell, fusion with lysosome which degrades the thyroglobin protein. T3 and T4 released into plasma.

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20
Q

What happens to uncoupled T3 and T4?

A

Recycled back to iodide by enzyme deiodinase

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21
Q

What cofactor is required for oxidation of iodide?

A

H202

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22
Q

90% of thyroid hormone secreted is ____.

A

T4

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23
Q

The most potent thyroid hormone is ____.

A

T3

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24
Q

Where is most of T4 converted to T3 in circulation?

A

Liver and kidneys

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25
Q

What is the structure of TSH?

A

Glycoprotein
2 subunits - alpha shared with LH and FSH
beta subunit provides biological activity.

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26
Q

What receptor does TSH bind to?

A

GPCR - alpha q and alpha s

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27
Q

What does TSH binding stimulate?

A 
iodide uptake
iodideoxidation
thyroglobulin iodination
colloid pinocytosis
proteolysis of thyroglobulin
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28
Q

What are the 3 general actions of thyroid hormone?

A
  1. Increase BMR and heat production
  2. Stimulation of metabolic pathways
  3. Sympathomimetic
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29
Q

How does thyroid hormone increase BMR?

A

Increase size and number of mitochondria

Synthesis of enzymes in respiratory chain

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30
Q

Which metabolic pathways does thyroid hormone stimulate?

A
  1. Lipolysis and beta-oxidation of fatty acids

2. insulin-dependent entry of glucose into cells via GLUT4, glycogenolysis, gluconeogenesis.

31
Q

How does thyroid hormone have sympathomimetic effects?

A

Increases cell response to catecholamines by increasing receptor number on target cells.

32
Q

How does thyroid hormone affect the CVS?

A 
Increase CO (HR and contractility)
Peripheral vasodilation to carry heat to body surface
33
Q

How does thyroid hormone affect the nervous system?

A

Myelination of nerves and development of neurones. Essential for both development and adult function.

34
Q

Which genes are activated by thyroid hormone?

A

PEPCK
Ca2+ ATPase
Na+/K+ATPase
cytochrome oxidase (ETC)

35
Q

What is a goitre?

A

Enlargement of thyroid gland - can be present in both hypo and hyper-thyroidism.

36
Q

What might cause hypothyroidism?

A
  • Iodine deficiency
  • TSH or TRH deficiency
  • Autoimmune destruction
  • Congenital
  • Anti-Thyroid drugs
  • Radioactive iodine
37
Q

What symptoms are associated with hypothyroidism?

A 
Obesity
Intolerance to cold 
Bradycardia 
Menorrhagia 
Pallor 
Dry skin
Loss of lateral third of eyebrow
38
Q

What condition is caused by hypothyroidism in children?

A

Cretinism

39
Q

What condition is caused by hypothyroidism in adults?

A

Myxedema

40
Q

What autoimmune condition causes hypothyroidism?

A

Hashimoto’s disease

41
Q

What antibodies are produces in hashimoto’s?

A

Against thyroglobulin and thyroid peroxidase

42
Q

What is the treatment for hashimoto’s disease?

A

Oral T4 (longer half life)

43
Q

What symptoms and signs are associated with myxoedema?

A 
Thick puffy skin - around eyes
Intolerance to cold 
Mental deterioration
Slow speech
Muscle weakness
44
Q

What is the effect of cretinism in children?

A 
Dwarfed stature
Mental deficiency
Poor bone development
Muscle weakness 
Slow pulse
45
Q

What can cause hyperthyroidism?

A 
Autoimmune - Graves' disease
Toxic multinodular goitre
Toxic adenoma 
Excessive T4 therapy
Thyroid carcinoma
Ectopic thyroid tissue
46
Q

What ar the symptoms associated with hyperthyroidism?

A 
Weight loss
Intolerance to heat
Irritability 
Fatigue
Sweating
Tremor
Anxiety
May have - goitre, bulging eyes, bounding pulse
47
Q

What autoantibodies are produced in Graves’ disease?

A

Thyroid stimulating immunoglobulin (TSI) which binds to TSH receptor and constantly stimulates thyroid hormone secretion.

48
Q

What would you expect plasma T3, T4 and TSH levels to be in hyperthyroidism?

A

Elevated T3 and T4

Very low TSH

49
Q

What would you expect plasma T3, T4 and TSH levels to be in hypothyroidism?

A

TSH elevated

Low T3 and T4

50
Q

Which two signs would be indicative of Graves’ disease over other types of hyperthyroidism?

A

Exopthalmos - eye bulging
Pre-tibial myxoedema
Related to auto-immunity rather than thyroid levels.

51
Q

What isotope is used in thyroid scintigraphy?

A

Technetium-99m

52
Q

What anti-thyroid drug can be used in hyperthyroidism and Graves’ disease?

A

Carbimazole

53
Q

How does carbimazole work?

A

Pro-drug, converted to methimazole which inhibits thyroid peroxidase from iodinating and coupling tyrosine on thyroglobulin.

54
Q

Why does carbimazole have a delayed affect (2-4 weeks)?

A

There is a considerable store of thyroglobulin which takes 2-4 weeks to diminish.

55
Q

What happens to the thyroid upon swallowing, what fascia explains this?

A

moves upwards upon swallowing as the pre-trachial fascia attaches the thyroid gland to the trachea and larynx.

56
Q

What is lingual thyroid?

A

Developmental problem leaving thyroid gland tissue on the tongue.

57
Q

What is a thyroglossal cyst, where is it located?

A

Remnants of the thyroglossal duct epithelium.

Always in the mid-line

58
Q

When does a thyroglossal cyst move upwards?

A

Upon tongue protrusion

59
Q

Why is the normal range for TSH so wide?

A

Hypothalamus is constantly responding to environmental changes so levels change accordingly.

60
Q

When are physiological goitres most likely to occur?

A

Menarche
Pregnancy
Menopause

61
Q

What is the commonest cause of a goitre globally?

A

Iodine deficiency

62
Q

What is the commonest cause of a goitre in the UK?

A

Multinodular goitre

63
Q

Does a multi-nodular goitre affect thyroid function?

A

No, only if it becomes toxic after many years and the patient may develop hyperthyroidism.

64
Q

Which geographical areas are at a higher risk of goitre?

A

Mountainous areas - greater iodine deficiency

65
Q

Why does iodine deficiency lead to goitre formation?

A

Decreased thyroxine levels lead to increased TSH secretion leading to stimulation and enlargement of the thyroid gland.

66
Q

Why is iodine deficiency a particular concern during pregnancy?

A

If foetus is iodine deficient, can lead to cretinism.

67
Q

What is a retrosternal multi nodular goitre and it’s main complication?

A

Multinodular goitre can enlrge inferiorly behind the sternum.
Can cause tracheal compression.

68
Q

What proportion of plasma thyroid hormone is “free” in plasma?

A

1% - this is the only biological active hormone

69
Q

What is subclinical hyperthyroidism characterised by?

A

Low or undetectable concentration of TSH with free T3 and T4 within laboratory reference ranges.

70
Q

Which condition always results in goitre formation?

A

Dietary iodine deficiency

71
Q

Which anti-arrythmic drug is commonly associated with side effects that disrupt thyroid function?

A

Amiodarone - similar structure to thyroxine so can disrupt thyroid function - both hypo and hyper reported.

72
Q

Why does the thyroid gland move up on swallowing?

A

It is invested by the pre-tracheal fascia.

73
Q

How can feeling a patients hands help to differentiate between possible hyperthyroidism and anxiety?

A

Warm and sweaty - hyperthyroid

Cold and clammy - anxiety

MEH (19 decks)

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