thyroid axis + physiology Flashcards
where is TRH released from
hypothalamus
what does TRH do, what does it stand for
thyrotropin releasing hormone
stimulates the anterior pituitary to produce TSH
where is TSH released from
anterior pituitary
what does TSH stand for, what does it do
thyroid stimulating hormone
stimulates the thyroid gland to produce T4 and T3
what does TRH act on
anterior pituitary
what does TSH act on
thyroid
what does high TSH suggest
hypothyroidism
what does low TSH suggest
hyperthyroidism
when are TSH levels unreliable
during acute illness or if there is pituitary disease present
where is T4 and T3 produced
colloid thyroglobulins which are located within the follicular cells of the thyroid
what is T4
thyroxine
what is T3
triiodothyronine
what percentage of all thyroid hormones does T4 and T3 make up
T3 = 10% T4 = 90%
which is the more active thyroid hormone, how much more active? why?
T3
x4 more active than T4 as it remains mostly free within the blood
what happens to T4 in body cells
it is converted to T3
what is T4 made up of
DIT + DIT
what is T3 made up of
DIT + MIT
how does low temperature affect thyroid hormones, in who does this happen
low temp = high TRH
in children and babies
how does stress affect thyroid hormones
stress = decreased TSH & TRH
how does the circadian rhythm affect thyroid hormones
night = high levels of hormone morning = low levels of hormone
what does deiodinase enzymes do
adds or removes an iodine atom from the thyroid hormones
how many types of deiodinase enzymes are there
3
where is type 1 deiodinase enzyme found
liver & kidney
where is type 2 deiodinase enzymes found
everywhere! - heart, CNS, fat, brain pituitary
what does type 2 deiodinase enzyme do
turns T4 to T3
where is type 3 deiodinase enzyme found
placenta, foetal tissues, brain except from hypothalamus)
what does type 3 deiodinase enzyme do
inactivated T3 and T4
where is the thyroid gland normally located
C5/6-T1
along the thyroglossal duct
what does the thyroid develop from, how does it descend
evagination of the pharyngeal epithelium that descends from the foramen caecum to the normal location
what are the abnormalities thatch occur during the descent of the thyroid, what do they result in
failure to descend = lingual thyroid
excessive descent = retrosternal location in the mediastinum
thyroglossal duct cyst
what is the thyroid composed of
follicles surround by flat/cuboidal follicular epithelial cells
what makes up the thyroid follicles
dense, pink material that contains thyroglobulin
occasionally there are some C cells
what are C cells, what do they secrete
C cells = parafollicular cells
they secrete calcitonin
how are T3 and T4 synthesised
iodine is take up by the follicle cells where it attaches to tyrosine residues on thyroglobulin to form MIT and DIT. these then couple to form T3 and T4
attachment if iodine to thyroglobulin residues are inhibited by what drugs
carbimazole
propylthiouracil
how does T3 & T4 travel in the blood, why?
they are hydrophilic so must bind to plasma proteins to travel in the blood
give examples of plasma binding proteins and their percentages
thyroxine binding globulin (TBG) = 70%
thyroxine binding prealbumin (TBPA) = 20%
albumin = 5%
what hormone does thyroxine binding globulin (TBG) bind to more strongly
T4
when do thyroxine binding globulin (TBG) levels naturally increase
pregnancy
on the contraceptive pill
what is the effect of thyroid hormones on basal metabolic rate, how does this happen
INCREASES basal metabolic rate by:
increasing size + number of mitochondria
increasing O2 use and rate of ATP hydrolysis
increasing synthesis of respiratory chain enzymes
what is the effect of thyroid hormones on thermogenesis
INCREASE
what is the effect of thyroid hormones on carbohydrate metabolism
INCREASES:
stimulation of glycogenolysis and gluconeogenesis = increase blood glucose
insulin-dependent glucose uptake
what is the effect of thyroid hormones on lipid metabolism
mobilise fats from adipose tissue
increases fatty axis oxidation in tissues
what is the effect of thyroid hormones on protein metabolism
INCREASES protein synthesis
what is the effect of thyroid hormones on permissive symoathomimetic action
increase responsiveness to adrenaline and noradrenaline by increasing the number of receptors
cardiovascular responsiveness is increased
