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ZA - Endocrine System > The Thyroid Gland > Flashcards

The Thyroid Gland Flashcards

1
Q

IS the thyroid gland visible or palpable in health?

A

No

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2
Q

What does the thyroid gland make?

A

Thyroid hormones - T3 and T4

T3 = Trii-odo-thyronine
T4 = Thyroxine
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3
Q

What 2 cell types are in the thyroid gland?

A

C/Clear cells

Follicular cells

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4
Q

What do C cells do?

A

Secrete calcitonin

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5
Q

What do follicular cells do?

A

Support thyroid hormone synthesis by making the enzymes that make thyroid hormones

Also makes thyroglobulin - a large protein rich in tyrosine residues

Also actively concentrate’s iodide from plasma and transport it to the colloid where it combines with tyrosine to make thyroid hormones

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6
Q

How much supply of TH does the thyroid follicles hold?

A

2-3 months

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7
Q

Where are the follicular cells?

A

Surrounding the hollow follicles

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8
Q

What is the colloid?

A

The hollow space in the centre of sphere

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9
Q

What happens to the enzymes and thyroglobulin made by follicular cells?

A

They are packaged into vesicles are exported from follicular cells into the colloid

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10
Q

Where is tyrosine and iodide derived from?

A

The diet

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11
Q

How is T4 - Thyroxine made?

A

2 tyrosines and 4 Iodides

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12
Q

How is T3 - triiodothyronine made?

A

2 tyrosines and 3 Iodides

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13
Q

What catalyses the thyroid hormone synthesis reaction and where is it found?

A

Thyroid peroxidase located on the apical membrane of the follicular cells

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14
Q

How does iodide enter the follicular cells?

A

Via Na+/I- transported (symport)

The coupling to Na+ enables the follicular cells to take up iodide against a concentration gradient

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15
Q

How is iodide transported into the colloid?

A

Pendrin transporter

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16
Q

What inhibits iodide transport into thyroid gland?

A

Thiocyanates

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17
Q

What are Thiocyanates? Where do they come from?

A

Compounds formed from detoxification of cyanide

Common origin is cigarette smoke

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18
Q

What catalyses the addition of iodide to tyrosine residues on the thyroglobulin molecule?

A

Thyroid peroxidase

and Thyroglobulin itself

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19
Q

How exactly is T3 - triiodothyronine made?

A

Iodine is added to tyrosine to make monoiodotyrosine and again to make Diiodotyrosine

DIT and MIT are reacted together t make T3

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20
Q

How exactly is T4 - Thyroxine made?

A

DIT and DIT reacted together

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21
Q

What happens in the thyroid gland in response to TSH?

A

Thyroglobulin + TH complex is endocytosed into the follicular cells

Once in the follicular cells they form vesicles containing proteolytic enzymes that cut the thyroglobulin to release the thyroid hormones

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22
Q

How do T3 and T4 pass throught the cellular membrane?

A

They are both hydrophobic/lipophilic so pass through the membrane into the plasma where they bind to plasma proteins

23
Q

What plasma protein does thyroid hormones mainly bind to?

A

Thyroxine-binding globulin

24
Q

Does T3 or T4 circulate in the plasma?

A

Both do

25
Q

What is movement of TH from colloid to plasma influenced by?

A

TSH

26
Q

Where is TSH released from?

A

Pituitary gland

27
Q

Why does T4 have a longer half life than T3?

A

Thyroxine binding globulin (TBG) has a higher affinity for T4 than T3 releasing it slowly into the plasma

28
Q

Can bound hormone exert inhibitory effects on TSH and TRH so stop TH release?

A

No - only free hormone can

29
Q

Out of T3 and T4, which one is more numerous and by how much?

A

T4 - ~100 nmoles/L
T3 - ~2.3 nmoles/L

These values are totals and not free hormones

30
Q

What binds more to receptors inside cells, T3 or T4? What does this mean about each hormones physiological activity?

A

T3 does 90% of the binding as receptors have a much higher affinity for it -

This means even though T4 is more numerous, T3 is more physiologically active

31
Q

How is T4 converted to T3?

A

Deiodinase enzymes deiodinate T4 - 50% in the plasma and the rest inside cells

32
Q

What does deiodinase activity depend on?

A

How in demand T3 is to tissues

33
Q

Explain the pathway of TH release.

A

Stimulus such as cold, pregnancy or exercise causes hypothalamus to release TRH onto AP gland which releases TSH onto thyroid which stimulates release of TH

34
Q

What hormones inhibits TH release?

A

Glucocorticoids and somatostatin

35
Q

How does Glucocorticoid inhibit TH?

A

Inhibits TSH and conversion of T4 to T3

36
Q

How does somatostatin inhibit TH?

A

Inhibits TSH

37
Q

Thyroid hormone functions?

A

Change transcription and translation to alter protein synthesis

Increases metabolic rate and promotes thermogenesis

Increases hepatic gluconeogenesis

Net increase in proteolysis and lipolysis

Critical for growth and foetal brain development

38
Q

How does TH influence growth?

A

Has anabolic effects

Has permissive effects on GH

39
Q

TH/Iodine deficiency in a pregnant mother causes?

A

Congenital hypothyroidism

40
Q

What the causes for hyperthyroidism?

A

Grave’s disease

Thyroid Adenoma

41
Q

Is graves common?

A

Yes

42
Q

Pathophysiology behind Graves?`

A

Antibodies produced that bind and 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

43
Q

Is thyroid adenoma common?

A

No - its rare

44
Q

What is a thyroid adenoma?

A

A hormone secreting thyroid tumour

45
Q

Hyperthyroidism symptoms and why these occur?

A

Increased metabolic rate and heat production = weight loss and heat intolerance

Increased protein catabolism = muscle weakness/weight loss

Altered nervous system function = hyperexcitable reflexes and psychological disturbances

Elevated CV function - TH is permissive to adrenaline B receptors = Increased HR and contractile force, high output and cardiac failure

46
Q

Causes of hypothyroidism?

A

Hashimoto’s - autoimmune
Deficiency in dietary iodine
Idiopathic

47
Q

How much iodine is needed per year?

A

Only 50mg

48
Q

Hypothyroidism symptoms and causes of symptoms?

A

Weight gain and cold intolerance = decreased metabolic rate and head production

Brittle nails/think skin = disrupted protein sysnthesis

Slow speech/reflexes and fatigue = alterned NS function

Slow HR and weak pulse = impaired CV function

49
Q

What is goitre?

A

Enlargement of thyroid gland

50
Q

What can cause goite?

A

Hypo and hyper-thyroidism as well as other thyroid pathologies

51
Q

What causes goitre?

A

Increased trophic action of TSH on thyroid follicular cells (hypothyroidism)

Overactivity as a result of an autoimmune disease (Grave’s disease/hyperthyroidism)

52
Q

What is a 1y disorder?

A

Dirrect effect on endocrine gland making the end hormone

53
Q

What is a 2y disorder

A

Pituitary issue

54
Q

What is a 3y disorder?

A

Hypothalamic issue

ZA - Endocrine System (21 decks)

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