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ENDOCRINE > 6. Thyroid > Flashcards

6. Thyroid Flashcards

1
Q

where is the thyroid gland located

A

lies across the trachea at the base of the larynx

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2
Q

is the thyroid palpable in health

A

no - one of the larger endocrine glands (15-20g) but should NOT be palpable in health

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3
Q

what does the thyroid gland do

A

synthesises thyroid hormones

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4
Q

how many physiologiclly active forms of thyroid hormone are there

A

2:
T3 - triiodothyronine
T4 - thyroxine

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5
Q

what are the 2 cell types found in the thyroid

A
  1. C (clear) cells

2. Follicular cells

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6
Q

what do C cells do

A

secrete calcitonin (a Ca2+ regulating hormone)

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7
Q

what do follicular cells do

A
  • support thyroid hormone synthesis
  • manufacture enzymes to make TH
  • produce THYROGLOBULIN (large protein rich in tyrosine residues)
  • actively concentrate iodide from the plasma
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8
Q

what are thyroid follicles

A

spherical structures whose walls are made of follicular cells - centre of the follicle is filled with colloid (where TH stored)

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9
Q

what is colloid

A

the sticky glycoprotein matrix in the centre of thyroid follicles

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10
Q

how much TH does the thyroid contain in the colloid

A

2-3 months supply

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11
Q

how does iodide enter into follicular cells

A

enters from the plasma vi Na+/I- transporter = symport (i.e. Na and I travelling in the same direction)

coupling to Na allows I to move against the concentration gradient into the cell

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12
Q

once in follicular cells, what happens to iodide

A

transported into the colloid via the pendrin transporter

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13
Q

what inhibits transport of iodide into the thyroid gland

A

thiocyanates - compounds formed from detoxification of cyanide

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14
Q

what is thyroid peroxidase (aka thyroperoxidase)

A

enzyme present on the colloidal side of the cells - catalyses addition of iodide to tyrosine resides in thyroglobulin

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15
Q

what does the addition of one iodide to tyrosine form

A

MIT - monoiodotyrosine

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16
Q

what does the addition of two iodides to tyrosine form

A

DIT - diiodotyrosine

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17
Q

when iodide is added to tyrosine it looses an electron to form

A

iodine

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18
Q

how is T3 formed

A

when MIT and DIT join

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19
Q

how is T4 formed

A

when DIT and DIT join

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20
Q

what does TSH stimulate

A

portions of colloid to be taken back up into follicular cells by endocytosis

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21
Q

what happens to the colloid taken back into the follicles by endocytosis

A

form vesicles which contain proteolytic enzymes

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22
Q

what do the proteolytic enzymes in the vesicles do

A

cut through thyroglobulin to release thyroid hormones

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23
Q

what allows T3 and T4 to pass from the follicular cell into the plasma

A

the fact they are both lipid soluble means they can easily pass through cell membranes

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24
Q

what do T3 and T4 do once in the plasma

A

bind to plasma proteins- mainly thyroxine-binding globulin

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25
Q

what TH circulate in the plasma

A

BOTH T3 and T4

26
Q

what percent of T3 and T4 circulate bound to plasma protein

A

99.8%

27
Q

why does T4 have a longer half life than T3 - what are theses half lifes

A

thyroxine binding globulin (TBG) has a very high affinity for T4 and so releases it very slowly into the plasma

T4 half life - around 6 days
T3 half life - around 1 day

28
Q

what is the only hormone form that exerts an inhibitor effect on TSH and TRH

A

free hormone

29
Q

what is the most common form of TH in the plasma

A

protein bound T4

50x more total (free+bound) T4 in plasma than T3

30
Q

what TH binds the most to TH receptors inside cell

A

T3 - makes up 90% of TH that binds to TH receptors

31
Q

why does more T3 than T4 bind to TH receptors

A

TH receptors have a much higher affinity for T3 than T4 - means T3 3-5 times more PHYSIOLOGICALLY ACTIVE

32
Q

why is there more T3 to bind to TH receptors

A

T4 is deiodinated to T3 by deiodinase enzymes - half in the plasma, half inside the target cells

33
Q

can the level of deiodinase of T4 activity be altered

A

yes - can be altered at different times in different tissues to suit demand

34
Q

what drives continuous secretion and stable [plasma] of TH

A

thyrothrophic releasing hormone (TRH) released from the hypothalamus

35
Q

what factors increase TH secretion

A

cold, exercise, pregnancy

36
Q

what inhibit TH secretion and how

A
  1. glucocorticoids - inhibit TSH and conversion of T4 to T3

3. somatostatins -inhibit TSH

37
Q

How do TH work

A

to change transcription and translation to alter protein synthesis in target cells - do this by binding to nuclear receptors

38
Q

what are the 6 functions of TH

A
  1. raise metabolic rate/promote thermogenesis
  2. increase hepatic gluconeogenesis
  3. net increase in proteolysis
  4. net increase in lipolysis
  5. critical for growth (lack of TH = retarded growth)
  6. required for foetal brain development
39
Q

what is hyperthyroidism

A

thyroid over activated

40
Q

what are two causes of hyperthyroidism

A
  1. graves disease (common)

2. thyroid adenoma (rare)

41
Q

what is the mechanism behind hyperthyroidism in graves disease

A

antibodies are produced that mimic TSH so continually activate thyroid - increased release of TH switches off TSH release from ant. pit. - [TSH] plasma very low - thyroid may be 2-3x bigger due to hyperplasia (GOITRE) - hyperactivity in cells

42
Q

what is the mechanism behind hyperthyroidism in thyroid adenoma

A

rare thyroid tumour is hormone-secreting - leads to increased hormone levels

43
Q

what are the four effects of hyperthyroidism

A
  1. increased metabolic rate and heat production
  2. increased protein catabolism
  3. altered nervous system fucntion
  4. elevated cardiovascular function - TH is permissive to epinephrine, Beta receptors
44
Q

what symptoms do increased metabolic rate and heat production cause

A

weight loss and heat intolerance

45
Q

what symptoms does increased protein catabolism cause

A

weight loss and muscle weakness

46
Q

what symptoms does altered nervous system function cause

A

hyper-excitable reflexes and physiological disturbances

47
Q

what symptoms does elevated CV function cause

A

increased heart rate/contractile force, high output and cardiac failure

48
Q

what is hypothyroidism

A

under active thyroid

49
Q

what are three causes of hypothyroidism

A
  1. hashimoto’s disease
  2. deficiency in dietary iodine
  3. idiopathic
50
Q

what is the mechanism behind hypothyroidism in hashimoto’s disease

A

autoimmune attack of the thyroid gland

51
Q

what are good dietary sources of iodine

A

milk, fish, seafood, seaweed

52
Q

what are 4 effects of hypothyroidism

A
  1. decreased metabolic rate and heat production
  2. disrupted protein synthesis
  3. altered nervous system function
  4. reduced CV function
53
Q

what symptoms arise from decreased metabolic rate and heat production

A

weight gain and cold intolerance

54
Q

what symptoms arise from disrupted protein synthesis

A

brittle nails and thins skin

55
Q

what symptoms arise from altered nervous system function

A

slow speech/reflexes and fatigue

56
Q

what symptoms arise form reduced CV function

A

slow heart rate and weaker pulse

57
Q

what is a goitre

A

thyroid pathology (hyper/hypo) leading to a significant enlargement of the thyroid gland

58
Q

what is the possible cause of goitre

A

increased trophic action of TSH on thyroid follicular cells (Hypo)
OR
over activity as a result of autoimmune disease (graves - Hyper)

59
Q

what is a primary thyroid disease

A

disease resulting from gland not producing end hormone - could be from iodine deficiency e.g. hashimoto’s

60
Q

what is a secondary thyroid disease

A

disease resulting from issue at anterior pituitary e.g. defects

61
Q

what is a tertiary thyroid disease

A

disease resulting from issue in the hypothalamus e.g. defects

ENDOCRINE (13 decks)

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