Thyroid Physiology Flashcards by Joanne McEwan

what does the thyroid gland secrete?

thyroxine
tri-iodothyronine (T3)
calcitonin

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

parathyroid hormone

involved in calcium

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what does thyroid hormone do?

allows you to convert what you eat into energy

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at what vertebral level does the thyroid lie?

C5 - T1

2nd-4th tracheal rings

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what nerve supplies the thyroid?

parasympathetic from vagus nerves

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

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what nerves can be damaged in thyroid surgery?

recurrent laryngeal nerves

- hoarse voice

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what supports the thyroid?

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

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is thyroid hormone stored?

yes

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

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what are the components of thyroid tissue?

follicles - contain

follicular cells
colloid
parafollicular C cells

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what do parafollicular cells do?

secrete calcitonin

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what is a colloid?

tyrosine containing thyroglobulin filled sphere enclosed by follicular cells

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where is T3 and T4 secreted from?

colloids

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

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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what is the most abundant thyroid hormone secreted?

T4 (thyroxine) = 90%

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which is the more potent thyroid hormone?

T3 (triiodothyrodine)

= major biologically active thyroid hormone

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can T4 be converted to T3?

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

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how are most thyroid hormones found in the blood?

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

unbound

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which stimulates release of thyroid hormones?

TSH

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which hormone level is important, free or bound?

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

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what causes increased total T4?

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pregnancy 
newborn
oestrogen
hepatitis/cirrhosis
porphyria
heroin

what causes decreased total T4?

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

what systems does thyroxine affect?

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CNS
URT
gynaecology
MSK
GI
cardio
Derm

what mechanisms does thyroxine effect?

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metabolism
growth
development
reproduction
behaviour

what does thyroid hormone increase?

``` basal metabolic rate thermogenesis carbohydrate metabolism lipid metabolism protein metabolism ```

how does thyroid hormone increase basal metabolic rate

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

how does thyroid hormone affect growth?

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

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

myelinogenesis and axonal growth require thyroid hormone

how does TH affect normal CNS activity?

``` hypothyroidism = slow intellectual functions hyperthyroidism = nervousness, hyperkinesis and emotional lability ```

what is permissive sympathomimetic action?

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

how does temperature stimulate TH release?

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

how does stress effect TH regulation?

stress inhibits TRH and TSH release

how does circadium rhythm affect TH regulation?

TH levels highest late at night and lowest in the morning

what are deiodinase enzymes and what do they do?

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

examples of deiodinase functions?

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)

where is type 1 deiodinase (D1) found?

liver and kidney

where is type 2 deiodinase (D2) found?

``` heart skeletal muscle CNS fat thyroid pituitary ```

where is type 3 deiodinase (D3) found?

fetal tissue placenta brain (except pituitary)

what can cause a deficiency of thyroid hormones?

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)

what are the symptoms of hypothyroidism?

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

what is hypothyroidism called in adults and babies?

``` adults = myxoedema (puffy face, hands and feet) babies = cretinism ```

what are the features of cretinism?

dwarfism | limited mental functioning due to deficiency of thyroid hormones at birth

what is the most common cause of hyperthyroidism?

graves disease

what causes graves and what are 2 common features?

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

what are the symptoms of hyperthyroidism?

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

does thyroxine affect cardiac muscle?

yes | can affect any tissue in the body

how can thyroxine affect reproductive tissue?

change in periods

how does thyroid change in hypothyroidism?

initially large | will then shrink

how does Thyroid hormone affect carbohydrate metabolism?

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

how does thyroid hormone affect lipid metabolism?

mobilises fats from adipose tissue | increases fatty acid oxidation in tissues

how does thyroid hormone affect protein metabolism?

increases protein synthesis

why is T3 more biologically active than T4?

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

Thyroid Physiology Flashcards

(52 cards)