Thyroid Physiology Flashcards
what does the thyroid gland secrete?
thyroxine
tri-iodothyronine (T3)
calcitonin
what do the parathyroid glands secrete and what is this involved in?
parathyroid hormone
involved in calcium
what does thyroid hormone do?
allows you to convert what you eat into energy
at what vertebral level does the thyroid lie?
C5 - T1
2nd-4th tracheal rings
what nerve supplies the thyroid?
parasympathetic from vagus nerves
sympathetic from superior middle and inferior ganglia of the sympathetic trunk (with blood vessels)
what nerves can be damaged in thyroid surgery?
recurrent laryngeal nerves
- hoarse voice
what supports the thyroid?
strap muscles
ligaments
- berry ligament (posterior suspensory ligament) attaches the posteromedial aspect of the gland
is thyroid hormone stored?
yes
means if you give drug to stop producing it, its still present for 3 weeks
what are the components of thyroid tissue?
follicles - contain
- follicular cells
- colloid
- parafollicular C cells
what do parafollicular cells do?
secrete calcitonin
what is a colloid?
tyrosine containing thyroglobulin filled sphere enclosed by follicular cells
where is T3 and T4 secreted from?
colloids
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
what is the most abundant thyroid hormone secreted?
T4 (thyroxine) = 90%
which is the more potent thyroid hormone?
T3 (triiodothyrodine)
= major biologically active thyroid hormone
can T4 be converted to T3?
yes
by any cell by removing iodine via de-iodinase enzyme
usually in the liver and kidney
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
bound/unbound is the biologically active form of thyroid hormone?
unbound
which stimulates release of thyroid hormones?
TSH
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
what causes increased total T4?
pregnancy newborn oestrogen hepatitis/cirrhosis porphyria heroin
what causes decreased total T4?
androgens glucocorticoids, cushings acromegaly systemic illness chronic liver disease nephrotic syndrome phenytoin carbamzepine
what systems does thyroxine affect?
CNS URT gynaecology MSK GI cardio Derm
what mechanisms does thyroxine effect?
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
