The Thyroid Gland Flashcards
what vertebral level is the thyroid gland located at
between C5 and T1
what is the function of the thyroid gland
synthesises the thyroid hormones
what are the two thyroid hormones that are physiologically active
T3 (triiodothyronine)
T4 (thyroxine)
what are the two cell types that make up the thyroid gland
C (clear) cells
Follicular cells
what is the function of C cells
secrete calcitonin (Ca2+ regulating hormone)
what is the function of follicular cells
they manufacture the enzymes that make thyroid hormones and surround hollow follicles
thyroid follicles are special structures what are their walls made out of
follicular cells
what composes the centre of thyroid follicles
colloid (a sticky glycoprotein matrix) which contains 2-3 months supply of TH
once follicular cells have made enzymes and thyroglobulin where are these products stored
The enzymes and thyroglobulin are packaged into vesicles and exported from the follicular cells into the colloid
follicular cells also actively concentrate iodide from the plasma, where do they transport this to
into the colloid
when iodide is transported into the colloid what does it combine with
tyrosine residues to form thyroid hormones
where are both tyrosine and iodide derived from
the diet
how is iodide transported from the plasma into follicular cells
via Na+/I- transporter
why does iodide get coupled to Na+ to enter the follicular cells
as the coupling to Na+ enables the follicular cells to take up iodide against a concentration gradient
what is the role of the pendrin transporter
transports iodide into the colloid
what inhibits iodide transport into the thyroid gland
thiocyanates
what are thiocyanates
compounds formed from detoxification of cyanide (common origin is cigarette smoke)
what is the function of thyroid peroxidase
catalyses the addition of iodide to tyrosine residues in thyroglobulin
what is the product of the addition of one iodine to tyrosine
MIT (monoiodotyrosine)
what is the product of the addition of two iodine to tyrosine
DIT (diiodotyrosine)
what is the product of MIT + MIT
triiodothyronine or T3
what is the product of DIT + DIT
tetraiodothyronine or thyroxine T4
what happens to portions of the colloid in response to TSH
portions of the colloid are taken back up into the follicular cell by endocytosis
within the cells they form vesicles which contain proteolytic enzymes that cut the thyroglobulin to release thyroid hormones
can T3 ad T4 pass across the follicular cell membrane
yes as they are lipid soluble
T3 and T4 can pass across the follicular cell membrane into the plasma where they bind to plasma proteins, what protein do thy mainly bind to
thyroxine binding globulin
both T3 and T4 circulate in the plasma true or false
true
what percentage of T3 and T4 circulating in plasma is bound to plasma protein
99.8%
thyroxine binding globulin has a particularly high affinity for what TH
T4
why has T4 got a higher half life than T3
as TBG has a high affinity for T4 and only releases it slowly into the plasma this gives it a longer half life than T3
what TH is present in higher levels in the plasma T4 or T3
50x more total (free+bound) T4 in plasma than T3
90% of TH binding to TH receptors inside cells is T3, why is this
The TH receptor has a much higher affinity for T3 than T4 making T3 3-5 times more physiologically active than T4
what is TRH
thyroid releasing hormone
what is somatostatin also known as
growth hormone inhibiting hormone
what effects do glucocorticoids have on TSH and T4/T3
GLucocorticoids inhibits TSH and conversoion of T4 to T3
what is TSH
thyroid stimulating hormone
what effects does somatostatin have on TSH
Somatostatin inhibits Thyroid stimulating hormone
thyroid hormone is required for foetal brain development what can deficiency result in
congenital hypothyroidism
where do we get iodine from
from the diet
what is hyperthyroidism
is an overactive thyroid, where the thyroid gland produces too much thyroid hormones
what is Grave’s disease
Grave’s disease is a cause of hyperthyroidism, it is an autoimmune disease where the body produces antibodies that mimic TSH and continually activate the thyroid gland
why are [TSH]plasma levels low in hyperthyroidism
because there is increased release of thyroid hormone which switches off TSH release from the anterior pituitary
what is a thyroid adenoma
hormone secreting thyroid tumour (can cause hyperthyroidism)
what are the symptoms of hyperthyroidism
increased metabolic rate and protein catabolism:weight loss
increased metabolic rate causes heat production:heat intolerance
increased protein catabolism:muscle weakness
altered nervous system function:hyperexcitable reflexes and psychological disturbances
elevated cardiovascular function, TH is permissive to epinephrine and B receptors:
what is Hashimoto’s disease and what does it cause
it is an autoimmune attack of the thyroid gland and can cause hypothyroidism
how much iodine do we have to get from our diet per year in order not to become deficient
50mg
what can a deficiency in dietary iodine result in
hypothyroidism
what are the symptoms of hypothyroidism
decreased metabolic rate and heat production: weight gain/cold intolerance
disrupted protein synthesis: brittle nails/thin skin
altered nervous system function: slow speech/reflexes.fatigue
reduced cardiovascular function: slow heart rate/weaker pulse
what is goitre
significant enlargement of the thyroid gland
list 5 functions of thyroid hormone
- raises metabolic rate and promotes thermogenesis
- increases hepatic gluconeogenesis
- net increases in proteolysis
- net increase in lipolysis
- critical for growth
