thyroid gland Flashcards
how many lobes is the thyroid gland made up of
2
what is the middle part of the thyroid gland called
the isthmus
what are the cells of the thyroid gland called
follicular cells
have colloid in the middle
and parafollicular
what is embedded in the thyroid gland
parathyroid glands (2 on each lobe - superior and inferior parathyroid glands)
what runs close to the thyroid gland
left recurrent laryngeal nerve (supplies vocal chords)
what does the parathyroid gland do
calcium metabolism
hypoparathyroidism
what is the embryology of the thyroid gland
midline outpouching from the floor of pharynx (originates from base of tongue) development of thyroglossal duct divides into 2 lobes duct disappears leaving foramen caecum final position by week 7 thyroid gland then develops
what is thyroid agenesis
no thyroid gland
STEPS for production of T3 and T4
1) TSH binds to TSH-R and sets off a series of events
- activates Na-I transporter allowing I to enter cell
- activates TG (thyroglobulin) production
- activation of TPO enzyme
2) TG (a prohormone) enters the colloid
3) TPO (enzyme) enters colloid and catalyses
- iodination with hydrogen peroxide
- coupling reaction
4) TG converted to MIT and DIT (monoiodotyrosine and diiodotyrosine)
5) forms T3 and T4
what does an MIT and DIT make
T3
what do 2 DITs make
T4 (thyroxine)
what is the main product of the thyroid gland
thyroxine T4
what happens to T4
it is deiodinated to T3 (triiodothyronine) its bioactive form in target tissues
it can also be deiodinated in a different position to produce reverse T3 (inactive)
what does a healthy adult thyroid gland secrete
both T3 and T4
what is T4
the bubble tea shop duh
jk
it is a prohormone converted to T3 by deiodinase enzyme into T3
what are the percentages for how T3 is made
80% by deiodination of T4
and 20% from direct thyoridal secretion
T3 provides almost all the thyroid hormone activity in target cells
how is T3 transported in the blood
mostly bound to plasma proteins when entering circulation
- thyroid binding globulin TBG (70-80%)
- albumin (10-15%)
- prealbumin (aka. transthyretin)
only 0.05% T4 and 0.5% T3 is unbound (bioactive components)
what does T3 do inside target cells
enters the nucleus and directly alters gene expression
what is thyroid hormone essential for
foetal growth and development esp the CNS
what can untreated congenital hypothyroidism be called
cretinism (unable to make own thyroid hormone)
what is measured in newborn infants during the heel prick test on day 5 and what can it be a sign of
TSH
can be a sign of congenital hypothyroidism
what does thyroid hormone do
increases basal metabolic rate (amount of energy each cell requires to function)
protein, carbohydrate and fat metabolism
potentiate actions of catecholamines (eg tachycardia and lipolysis)
effects on GI, CNS and reproductive system (fertility)
half life of T4 and T3
T4 ~ 7-9 days
T3 ~ 2 days
summary of thyroid hormone actions
thyroid hormone binds to nuclear receptor > synthesis of new proteins > leads to
growth - bone maturation
CNS - maturation of CNS
BMS - ^ Na+/K+ ATPase, ^O2 consumption, ^heat production, ^BMR
metabolism - ^glucose absorption, ^glycogenolysis, gluconeogenesis, ^lipolysis, ^protein degradation and synthesis
cardiovascular - ^cardiac output
control of thyroid hormone production (what inhibits anterior pituitary production of TSH)
somatostatin
there is also negative feedback to switch off TSH production when there is too much T3 and T4 produced
what is primary hypothyroidism
autoimmune damage to the thyroid gland
thyroxine/T4 level decline
TSH levels climb
what are the commonest forms of autoimmune thyroid disease
Hashimotos thyroiditis
Grave’s disease (tho normally associated with hyper)
the presence of 1 autoimmune disease increases the risk of others
presence of autoantibodies damaging thyroid
symptoms and signs of hypothyroidism
deepening voice depression and tiredness cold intolerance weight gain with reduced appetite constipation bradycardia eventual myxoedema coma
pharmacology and therapeutics : levothyroxine
indications of hypo and hyperthyroidism
hypo - inability to make thyroid hormone
hyper - blocking and replacement regimen
dosing for levothyroxine is adjusted to?
according to TSH (aim to get in normal range) and low T4 and high TSH if hypothyroidism due to negative feedback
most common dose of levothyroxine
100 micrograms
most common way of administering levothyroxine
orally
when do you need to give levothyroxine via IVF
myxoedema coma
what are some potential complications with levothyroxine use
weight loss
headaches
heart attack
rapid HR
what are some symptoms of toxicity associated with combined thyroid hormone replacement
palpitations, tremor, anxiety
often combination treatment suppresses TSH
what is hyperthyroidism
thyroid makes too much thyroxine
T4 levels rise
TSH levels drop
causes of hyperthyroidism
Graves disease - whole gland is smoothly enlarged and whole gland is overactive
toxic multinodular goitre
solitary toxic nodule
which both overproduce T4
what is Graves disease
an autoimmune disease
antibodies bind to and stimulate TSH receptor in the thyroid gland
smooth goite - no nodules
no negative feedback = continual stimulation
other antibodies bind to muscles behind the eye causing bulging - exopthalamos
other antibodies stimulate growth of soft tissue of shins - pretibial myxoedema - thickening of skin on shin
signs and symptoms of hyperthyroidism
heat intolerance weight loss with increased appetite myopathy mood swings diarrhoea tremor of hands palpitations sore eyes can occur through surgeries
