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Systems: Endocrine AB > Thyroid Gland > Flashcards

Thyroid Gland Flashcards

1
Q

What is the thyroid gland?

A
  • One of the larger endocrine glands that is shaped like a butterfly and 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

Synthesises the Thyroid Hormones of which there are two physiologically active forms:

  • 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

Thyroid follicles are 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
  • Support thyroid hormone synthesis
  • Surround hollow follicles
  • 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

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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

Follicular cells manufacture the 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 packaged into vesicles and exported 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). The 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

What inhibits transport of iodide into the thyroid gland?

A

Iodide transport into thyroid gland is inhibited by thiocyanates, compounds formed from detoxification of cyanide. Common origin is cigarette smoke

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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

In response to TSH, 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

-They 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
  • Most TH circulates in the form of protein bound T4 ~100nmoles/l, while T3 is only ~2.3nmoles/l (note: free TH is in picomolar range (1000x smaller)).
  • 50x more total T4 in plasma than T3
32
Q

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

A

The TH receptor has a much higher affinity for T3 than T4 making T3 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?

A

TSH

37
Q

What do THs trigger when the 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
  • Anabolic

- 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 bind 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 may be 2-3x normal size due to hyperplasia.
  • Hyperactivity of cells also apparent.
44
Q

What are the symptoms of hyperthyroidism?

A
  • Weight loss/ heat intolerance
  • Muscle weakness/ weight loss
  • Hyperexcitable reflexes and psychological disturbances
  • 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?

A

50mg/year

53
Q

What may idiopathic hypothyroidism be linked to?

A

Thyroiditis

54
Q

What are the symptoms of hypothyroidism?

A
  • Weight gain/cold intolerance
  • Brittle nails/thin skin
  • Slow speech/reflexes, fatigue
  • 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) or over-activity as a result of autoimmune disease (Graves Disease)

Systems: Endocrine AB (25 decks)

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