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endocrine system 1 > Thyroid Gland > Flashcards

Thyroid Gland Flashcards

1
Q

What is the thyroid gland?

A
  • one of the larger endocrine glands
  • shaped like a butterfly
  • lies across the trachea at the base of larynx
  • it isn’t visible or palpable in health
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2
Q

How much does the thyroid gland weigh?

A

15-20g

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3
Q

What does the thyroid gland synthesise?

A

thyroid hormones

  • T3 (triiodothyronine)
  • T4 (thyroxine).
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4
Q

What are the 2 cell types that the thyroid gland contains?

A
  • C (clear) cells

- Follicular cells

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5
Q

What are thyroid follicles?

A

spherical structures whose walls are made of follicular cells

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6
Q

What do C cells secrete?

A

Calcitonin (Ca regulating hormone)

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7
Q

What do follicular cells do?

A
  • manufacture enzymes that make thyroid hormones as well as thyroglobulin
  • actively concentrate iodide from the plasma and transport it into the colloid where it combines with the tyrosine residues to form the thyroid hormones
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8
Q

What is colloid?

A

Sticky glycoprotein matrix at centre of follicle

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9
Q

What do thyroid follicles contain?

A

2-3 months supply of TH

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10
Q

What do follicular cells manufacture?

A

enzymes that make thyroid hormones as well as thyroglobulin (a large protein rich in tyrosine residues)

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11
Q

What are packaged into the colloid?

A

The enzymes and thyroglobulin are secreted from the follicular cells into the colloid.

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12
Q

What are tyrosine and iodine derived from?

A

Diet

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13
Q

What catalyses the reaction of tyrosine to T4 and T3?

A

Thyroid peroxidase

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14
Q

Where is thyroid peroxidase located?

A

On the apical membrane of the follicular cells

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15
Q

How does iodide enter the follicular cell?

A
  • Iodide enters the follicular cells from the plasma
  • via a Na+/I- transporter (symport)
  • coupling to Na+ enables the follicular cells to take up iodide against a concentration gradient.
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16
Q

How is iodide transported into the colloid?

A

Via the pendrin transporter

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17
Q

.

A

.

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18
Q

What does addition of iodine to tyrosine result in?

A

MIT (monoiodotyrosine)

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19
Q

What doe a second iodine to MIT result in?

A

DIT (diiodotyrosine)

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20
Q

MIT+ DIT=

A

T3 (trriodotyronine)

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21
Q

DIT + DIT =

A

T4 (tetraiodothyronine)

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22
Q

How does colloid respond to TSH?

A
  • portions of the colloid are taken back up into the follicular cell by endocytosis
  • within the cells they form vesicles which contain proteolytic enzymes that cut the thyroglobulin to release thyroid hormones
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23
Q

How do T3 and T4 exit the cell?

A

Both T3 and T4 are lipid soluble and so pass across the follicular cell membrane into the plasma

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24
Q

How are T3 and T4 circulated in the blood?

A

bind to plasma proteins, mainly thyroxine binding globulin (TBG)

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25
Q

What is movement of TH from colloid to plasma under the influence of?

A

TSH released from the pituitary

26
Q

What does TSH stimulate?

A

Stimulates the follicular cells to endocytose colloidal thyroglobulin

27
Q

What happens to thyroid hormones when follicular cells are not stimulated?

A

They are stored in the colloid

28
Q

Why does T4 have a longer half life than T3?

A

Thyroxine Binding Globulin (TBG) has particularly high affinity for T4 releasing it only slowly into the plasma. This accounts for the longer half life of T4:

  • T4 ~ 6 days
  • T3 ~ 1 day
29
Q

What percentage of T3 and T4 circulate in the plasma bound to plasma protein?

A

More than 99.8%

30
Q

What form of hormone exerts an inhibitor effect on TSH and TRH

A

Free hormone

31
Q

In what form does most TH circulate?

A

-50x more total T4 in plasma than T3

protein bound
T4: ~100nmoles/l
T3: ~2.3nmoles/l

(note: free TH is in picomolar range (1000x smaller)).

32
Q

Why is 90% of TH binding to TH receptors inside cells T3?

A

TH receptor has a much higher affinity for T3 than T4

T3 is 3-5 times more physiologically active than T4

33
Q

How does T4 become T3?

A

T4 is deiodinated to T3 by deiodinase enzymes

34
Q

Where is T4 converted to T3?

A

Around half the T4 is deiodinated in plasma, the remaining fraction being deiodinated inside target cells

35
Q

What do glucocorticoids inhibit?

A

TSH and conversion of T4 to T3

36
Q

What does somatostatin inhibit?

37
Q

What do THs trigger when they bind to nuclear receptors in the target cells?

A

They change transcription and translation to alter protein synthesis.

38
Q

What are the 6 functions of thyroid hormone?

A
  • Raises metabolic rate and promotes thermogenesis
  • Increase hepatic gluconeogenesis
  • Net increase in proteolysis
  • Net increase in lipolysis
  • Critical for growth (lack of TH results in retarded growth)
  • Required for foetal brain development (deficiency= congenital hypothyroidism)
39
Q

What can cause congenital hypothyroidism?

A

Dietary iodine deficiency in the mother

40
Q

Why is TH critical for growth?

A

stimulates GH receptor expression

41
Q

Give 2 causes of hyperthyroidism.

A
  • Grave’s disease

- Thyroid adenoma

42
Q

What is a thyroid adenoma?

A

Hormone secreting thyroid hormone

43
Q

What happens in Grave’s disease?

A
  • antibodies produced that mimic TSH and continually activate the thyroid gland
  • increased release of TH switches off TSH release from anterior pituitary so [TSH]plasma very low
  • thyroid gland hyperplasia. hyperactivity of cells also apparent.
44
Q

What are the symptoms of hyperthyroidism?

A

increased metabolic rate and heat production
-> weight loss/ heat intolerance

increased protein catabolism
-> muscle weakness/ weight loss

altered nervous system functions
-> hyperexcitable reflexes and psychological disturbances

elevated cardiovascular function
-> increased HR/contractile force, high output, cardiac failure

45
Q

Why do you get weight loss/ heat intolerance in hyperthyroidism?

A

Increased metabolic rate and heat production

46
Q

Why do you get muscle weakness/ weight loss in hyperthyroidism?

A

Increased protein catabolism

47
Q

Why do you get Hyperexcitable reflexes and psychological disturbances in hyperthyroidism?

A

Altered nervous system function

48
Q

Why do you get increased HR/contractile force, high output and cardiac failure in hyperthyroidism?

A

Elevated cardiovascular function. TH is permissive to epinephrine, B receptors

49
Q

Give 3 causes of hypothyroidism.

A
  • hashimoto’s disease
  • deficiency in dietary iodine
  • idiopathic
50
Q

What is Hashimoto’s disease?

A

Autoimmune attack of thyroid gland

51
Q

What are sources of iodine?

A
  • milk
  • fish
  • seafood
  • seaweed
52
Q

How much iodine do we need a year?

53
Q

What may idiopathic hypothyroidism be linked to?

A

Thyroiditis

54
Q

What are the symptoms of hypothyroidism?

A

decreased metabolic rate and heat production
-> weight gain/cold intolerance

disrupted protein synthesis
-> brittle nails/thin skin

altered nervous system function
-> slow speech/reflexes, fatigue

reduced cardiovascular function
-> slow heart rate/weaker pulse

55
Q

Why do you get weight gain/cold intolerance in hypothyroidism?

A

Decreased metabolic rate and heat production

56
Q

Why do you get brittle nails/thin skin in hypothyroidism?

A

Disrupted protein synthesis

57
Q

Why do you get slow speech/reflexes and fatigue in hypothyroidism?

A

Altered nervous system function

58
Q

Why do you get slow heart rate/weaker pulse in hypothyroidism?

A

Reduced cardiovascular function

59
Q

Goitre

A

Significant enlargement of the thyroid gland (hypertrophy)

60
Q

What is goitre associated with?

A

Thyroid pathologies (hypo and hyperthyroidism)

61
Q

What may cause goitre formation?

A
  • increased trophic action of TSH on thyroid follicular cells (hypothyroidism)
  • over-activity as a result of autoimmune disease (Graves Disease)

endocrine system 1 (25 decks)

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