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Endocrinology > Thyroid Physiology > Flashcards

Thyroid Physiology Flashcards

1
Q

what does the thryoid gland secrete

A

thyroxine -T4
tri-iodothyroxine -T3
calcitonin

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2
Q

what do the parathryoid glands secrete

A

parathyroid hormone

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3
Q

what is the isthmus of the thyroid

A

connects the two lobes across the trachea

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4
Q

where is the thyroid gland

A

between the 5th cervical and 1st thoracic vertebrae / 2-4th tracheal rings

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5
Q

how big and heavy is the normal thyroid gland

A

12-15mm

25-30 grams

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6
Q

is there always an isthmus

A

no- can just be 2 separate lobes

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7
Q

what can cause the thyroid to change in size without being pathological

A

increases in size during pregnancy/ menstruation

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8
Q

what innervates the thyroid gland

A

autonomic nerve supply- parasympathetic from vagus nerves

sympathetic fibres of superior, middle and inferior ganglia of the sympathic trunk

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9
Q

what is the blood supply to the thyroid (+ veins)

A

superior and inferior thyroid arteries (branch of external carotid) +/- thyroidea ima

3 pairs of veins- superior/ middle thyroid vein (goes into internal jugular)
inferior thyroid vein (goes into brachiocephalic veins)

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10
Q

what supports the thryoid gland

A

ligaments and strap muscles

posteromedial aspect of the gland is attached by the posterior suspensory ligament (berry ligament)

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11
Q

what is the relevance of the anatomy of the thyroid gland for any surgical proceudres

A

Isthmus encountered during routine tracheostomy (needs retracted)
Recurrent Laryngeal Nerve/ vascularity - surgery

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12
Q

what happens if the recurrent laryngeal nerve is bruised

A

causes temporary hoareness of voice

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13
Q

what are the componnents of the thryoid gland

A

follicle, follicular cells, colloid, parafollicular cells

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14
Q

what is a colloid thryoglobulin

A

tyrosine containing thyroglobulin filled spheres enclosed by follicular cells

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15
Q

what do parafollicular cells do

A

secrete calcitonin

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16
Q

what is the function of thyroid follicles

A

store the thyroid hormones produced by the follicular cells

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17
Q

why does it take 2-3 weeks to notice that a hormone has stopped being produced

A

as hormones stored in follicles and slowly released into blood stream

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18
Q

what mineral is need to make thyroid hormones

A

iodine

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19
Q

how is iodine taken into follicular cells

A

from blood stream via iodine transporters

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20
Q

what does iodine get trapped in to make thryoid hormones

A

pinocytes

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21
Q

what does iodine form when in follicular cells

A

attaches to tyrosine residues on thyroglobulin to form MIT (monoiodotyrosine unit) and DIT (di-iodotyrosine unit)

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22
Q

how do MIT and DIT form thyroid hormones

A

MIT and DIT couple to make triiodothyronine= T£

DIT and DIT couple to make thryoxine= T4

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23
Q

what drugs can stop the formation of thyroid hormones from iodine and how

what are they used to treat

A

carbimazole and propylthioracil

stops iodine binding to tyrosine residues on thryoglobulin

used to treat hyperthyroidism

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24
Q

what makes up the majority of thyroid hormones secreted

A 
T4 thyroxine (90%)
(T3 10%)
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25
Q

which thyroid hormone is more potent

A

T3 is 4x more potent than T4

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26
Q

what makes T4 more potent

A

is converted to T3 by any cell (take off the iodine)

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27
Q

what is the major biologically active thyroid hormone

A

T3- triiodothyronine

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28
Q

what causes the release of thyroid hormones

A

thyroid stimulating hormone (from pituitary gland)

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29
Q

how do T3 and T4 travel in the blood

A

are hyrdophobic/ lipophobic so bind to plasma proteins

30
Q

what plasma proteins do T3 and T4 bind to

A

70% thyroxine binding globulin (TBG)

20% thryoxine binding prealbumin (TBPA)

5% albumin

31
Q

what receptors do T3 and T4 work on

A

nuclear receptors

32
Q

wha is the biologically active form of thyroid of thryoid hormone- bound or unbound to plasma proteins

why

A

unbound

as only the free hormone is available to the tissues

33
Q

why is T3 the more active thyroid hormone

A

as is bound 10-20 times less by TBG and not significantly by the others so has a more rapid onset and offset of actions

metabolic state correlates more closely with the free than the total concentration in the plasma

34
Q

what is the role of the bound plasma

A

acts a buffer to maintain free hormone concentration

35
Q

what can increase total T4 but not free T4 by increasing TBG

A

pregnancy, newborn state, contraceptive pill/ other sources of oestrogen

hepatitis A and chronic active hepatitis, billiary cirrhosis

36
Q

what can decrease total T4 but not free T4 by decreasing TBG

A

androgens
larges doses of glucocortcoids, cushings syndrome, active acromegaly

severe systemic illness, chronic liver disease, nephrotic syndrome

37
Q

what are the effects of thyroxine

A 
affects every tissue
moist, sweaty skin 
increased HR, palpitations 
anxiety 
loose stools 
lessens/ loss of periods 
loss of muscle mass as you burn up stores
38
Q

what do thryoid hormones do to the basal metabolic rate?

how?

A

increase it

increase:

  • number and size of mitochondria
  • oxygen use an rate of ATP hydrolysis
  • synthesis of respiratory chain enzymes
39
Q

how do thyroid hormones affect thermogenesis

A

increase it

40
Q

how do thyroid hormones affect carbohydrate metabolism

A

increase blood glucose- stimulate glyconeogenesis and gluconeogenesis

increase insulin dependent glucose uptake to cells

41
Q

how do thyroid hormones affect lipid metabolism

A

mobilise fats from adipose tissue

increase fatty acid oxidation in tissues

42
Q

how do thyroid hormones affect protein metabolism

A

protein synthesis

43
Q

how do thyroid hormones affect growth

A

growth hormone releasing hormone (GHRH) production and secretion requires thyroid hormones

glucocorticoid-insulin GHRH release also dependent on thyroid hormones

GH/ somatomedins require presence of thyroid hormone for activity

44
Q

how do thyroid hormones influence development of foetal and neonatal brain

A

myelinogenesis and axonal growth require thyroid hormones

45
Q

what are the effects of hypothyroidism on the CNS activity

A

slow intellectual functions

46
Q

what are the effects of hyperthryoidism on the CNS activity

A

nervousness, hyperkinesis and emotional lability (quick changes in mood)

47
Q

what is the permissive sympathetic action of thyroid hormones

A

thyroid hormones increase responsiveness to adrenaline and sympathetic NS neurotransmitter, noradrenaline- increases number of receptors

cardiovascular responsiveness also increased due to this effect - increased force of rate of contraction of the heart

48
Q

what drug is used to treat symptoms of the permissive sympathomimetic action of thyroid hormones in hyperthyriodism

A

beta blockers (propanalol)

49
Q

what stimulates the release of TSH

A

thyrotrophin releasing hormone from the hypothalamus

50
Q

what releases TSH

A

anterior pituitary

51
Q

what is the negative feedback loop of the thyroid hormone regulation

A

T3 and T4 exert negative feedback control of release of TRH and TSH

52
Q

what effect does temperature have on thyroid hormone regulation

A

in babies and young children cold temperatures stimulate TRH release (stimulates TSH release- T3 and T4 release from thyroid)

53
Q

what effect does stress have on thyroid hormone regulation

A

inhibits TRH and TSH release

54
Q

what is the ‘rhythm’ of thyroid hormone release

A

circadian rhythm- highest late at night, lowest in the am

55
Q

what are de-iodinase enzymes

A

3 enzymes (type 1,2,3), important in the activation (t4 - t3) and deactivation (t3- t4, t3- t2, t4-rt3) of thyroid hormone

add or remove iodine atom in the outer ring

56
Q

where is de-iodinase type 1 found

A

in the liver and kidney (acts a reserve enzyme)

57
Q

where is de-iodinase type2 found

A

heart, skeletal muscle, CNS, fat, thyroid and pituitary

58
Q

where is de-iodinase type 3 found

A

fetal tissue and placenta and brain (except pituitary)

59
Q

what is the most active de-iodinase ezyme

A

type 2

60
Q

what can hypothyroidism be associated with

A

primary gland failure can be associated with enlarged thyroid gland (goitre)
secondary to TRH or TSH (no goitre)
lack of iodine in diet (goitre)

61
Q

what are the symptoms of hypothyroidism

A 
reduced BMR
slow pulse rate 
fatigue 
lethargy 
slow response times 
mental sluggishness 
cold intolerance 
put on weight easily
62
Q

what is myxoedema

A

sign of hypothyroidism in adults - puffy face, hands and feet

63
Q

what is cretinism

A

sign of hypothyroidism in babies- dwarfism and limited mental function due to deficiency of thyroid hormones present at birth

64
Q

what is the most common cause of hypothyroidism

A

autoimmune disease- destroys cells of the thyroid

65
Q

what is graves disease

A

hyperthyroidism

66
Q

what is hyperthyroidism associated with

A

autoimmune disease- thyroid stimulating immunoglobulin (acts like TSH but unchecked T3 and T4)

exophthalmos- bulging eyes due to water retaining carbohydrate build up behind eyes

goitres

67
Q

what are the symptoms of hyperthyroidism

A 
increases BMR 
very fast pulse rate 
increased nervousness and excessively emotional 
insomnia 
sweating and heat intolerance 
lose weight easily
68
Q

does thyroxine affect affect cardiac muscle

A

yes- can affect every tissue in the body

69
Q

can thyroxine cause infertility

A

yes can cause loss of periods

70
Q

heat intolerance is in hyper/hypo thyroidism

A

hyperthyroidism

71
Q

cold intolerance is in hyper/hypo thyroidism

A

hypothyroidism

Endocrinology (46 decks)

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