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Endocrine > Thyroid Physiology > Flashcards

Thyroid Physiology Flashcards

1
Q

what does the thyroid gland secrete?

A

thyroxine
tri-iodothyronine (T3)
calcitonin

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2
Q

what do the parathyroid glands secrete and what is this involved in?

A

parathyroid hormone

involved in calcium

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3
Q

what does thyroid hormone do?

A

allows you to convert what you eat into energy

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4
Q

at what vertebral level does the thyroid lie?

A

C5 - T1

2nd-4th tracheal rings

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5
Q

what nerve supplies the thyroid?

A

parasympathetic from vagus nerves

sympathetic from superior middle and inferior ganglia of the sympathetic trunk (with blood vessels)

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6
Q

what nerves can be damaged in thyroid surgery?

A

recurrent laryngeal nerves

- hoarse voice

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7
Q

what supports the thyroid?

A

strap muscles
ligaments
- berry ligament (posterior suspensory ligament) attaches the posteromedial aspect of the gland

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8
Q

is thyroid hormone stored?

A

yes

means if you give drug to stop producing it, its still present for 3 weeks

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9
Q

what are the components of thyroid tissue?

A

follicles - contain

  • follicular cells
  • colloid
  • parafollicular C cells
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10
Q

what do parafollicular cells do?

A

secrete calcitonin

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11
Q

what is a colloid?

A

tyrosine containing thyroglobulin filled sphere enclosed by follicular cells

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12
Q

where is T3 and T4 secreted from?

A

colloids

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13
Q

what are the steps in the synthesis and storage of T3 and T4?

A

TRH released from hypothalamus
TRH stimulates release of TSH from anterior pituitary
TSH stimulates uptake of iodine by follicle cells
iodine attached to tyrosine residues via de-iodinase enzyme
- tyrosine + 1 iodine = MIT
- tyrosine + 2 iodine = DIT
coupling of MIT/DIT
- MIT + DIT = T3
- DIT + DIT = T4
T3 and T4 stored in colloid thyroglobulin until required
T3 and T4 exert negative feedback control on hypothalamus and anterior pituitary

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14
Q

what is the most abundant thyroid hormone secreted?

A

T4 (thyroxine) = 90%

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15
Q

which is the more potent thyroid hormone?

A

T3 (triiodothyrodine)

= major biologically active thyroid hormone

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16
Q

can T4 be converted to T3?

A

yes
by any cell by removing iodine via de-iodinase enzyme
usually in the liver and kidney

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17
Q

how are most thyroid hormones found in the blood?

A 
bound to plasma proteins as hydrophobic/lipophillic
- thyroxine binding globulin (70%)
- thyroxine binding prealbumin (20%)
- albumin (5%)
small amount is unbound
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18
Q

bound/unbound is the biologically active form of thyroid hormone?

A

unbound

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19
Q

which stimulates release of thyroid hormones?

A

TSH

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20
Q

which hormone level is important, free or bound?

A

free
only free correlates to metabolic state and is available to tissues
only free shown on blood test

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21
Q

what causes increased total T4?

A 
pregnancy 
newborn
oestrogen
hepatitis/cirrhosis
porphyria
heroin
22
Q

what causes decreased total T4?

A 
androgens
glucocorticoids, cushings
acromegaly
systemic illness
chronic liver disease
nephrotic syndrome
phenytoin
carbamzepine
23
Q

what systems does thyroxine affect?

A 
CNS
URT
gynaecology
MSK
GI
cardio
Derm
24
Q

what mechanisms does thyroxine effect?

A 
metabolism
growth
development
reproduction
behaviour
25
Q

what does thyroid hormone increase?

A 
basal metabolic rate
thermogenesis
carbohydrate metabolism
lipid metabolism
protein metabolism
26
Q

how does thyroid hormone increase basal metabolic rate

A

increase in size and number of mitcohcondria
increase oxygen use and rate of ATP synthesis
increase synthesis of respiratory chain enzymes

27
Q

how does thyroid hormone affect growth?

A

growth hormone releasing hormone production requires thyroid hormones
growth hormone/somatomedins require presence of thyroid hormone for activity

28
Q

how does thyroid hormone affect development of foetal and neonatal brain?

A

myelinogenesis and axonal growth require thyroid hormone

29
Q

how does TH affect normal CNS activity?

A 
hypothyroidism = slow intellectual functions
hyperthyroidism = nervousness, hyperkinesis and emotional lability
30
Q

what is permissive sympathomimetic action?

A

TH increase responsiveness to adrenaline and sympathetic NS neurotransmitter, noradrenaline by increasing numbers of receptors
CV responsiveness also increased due to this effect - increased force and rate on contraction

31
Q

how does temperature stimulate TH release?

A

low temp stimulates TRH release which stimulates TSH release which increases T3 and T4 release

32
Q

how does stress effect TH regulation?

A

stress inhibits TRH and TSH release

33
Q

how does circadium rhythm affect TH regulation?

A

TH levels highest late at night and lowest in the morning

34
Q

what are deiodinase enzymes and what do they do?

A

subfamily of 3 enzymes (type 1, 2, and 3) which are involved in the activation and deactivation of thyroid hormone by adding or removing an iodine atom in the outer ring of TH

35
Q

examples of deiodinase functions?

A

remove iodine from T3 > T2 making it inactive
adding iodine to T3 > T4 (still active)
adding iodine to T4 > rT3 (inactive)
removal of iodine from T4 > T3 (still active)

36
Q

where is type 1 deiodinase (D1) found?

A

liver and kidney

37
Q

where is type 2 deiodinase (D2) found?

A 
heart
skeletal muscle
CNS
fat
thyroid
pituitary
38
Q

where is type 3 deiodinase (D3) found?

A

fetal tissue
placenta
brain (except pituitary)

39
Q

what can cause a deficiency of thyroid hormones?

A

primary gland failure (can be due to goitre)
secondary to TRH or TSH (not goitre associated)
lack of iodine in diet (can be associated with goitre)

40
Q

what are the symptoms of hypothyroidism?

A 
reduced BMR
slow pulse
fatigue, lethargy, slow response time and mental sluggishness
cold intolerance
tendency to put weight on easily
41
Q

what is hypothyroidism called in adults and babies?

A 
adults = myxoedema (puffy face, hands and feet)
babies = cretinism
42
Q

what are the features of cretinism?

A

dwarfism

limited mental functioning due to deficiency of thyroid hormones at birth

43
Q

what is the most common cause of hyperthyroidism?

A

graves disease

44
Q

what causes graves and what are 2 common features?

A

autoimmune (thyroid stimulating immunoglobulin acts like TSH but unchecked by T3 and T4)
features
- exophthalmos (bulging of eyes due to water retaining carbs build up behind eyes)
- goitre

45
Q

what are the symptoms of hyperthyroidism?

A 
increased BMR
very fast pulse
increased nervousness and excessively emotional
insomnia
sweating and heat intolerance
tendency to lose weight easily
46
Q

does thyroxine affect cardiac muscle?

A

yes

can affect any tissue in the body

47
Q

how can thyroxine affect reproductive tissue?

A

change in periods

48
Q

how does thyroid change in hypothyroidism?

A

initially large

will then shrink

49
Q

how does Thyroid hormone affect carbohydrate metabolism?

A

increases blood glucose due to stimulation of glycogenolysis and gluconeogenesis
increases insulin dependant glucose uptake into cells

50
Q

how does thyroid hormone affect lipid metabolism?

A

mobilises fats from adipose tissue

increases fatty acid oxidation in tissues

51
Q

how does thyroid hormone affect protein metabolism?

A

increases protein synthesis

52
Q

why is T3 more biologically active than T4?

A

because it is bound 20X less avidly by TBG and not significantly bound by TTR

Endocrine (45 decks)

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