Exam 3- Physiology of the Thyroid Gland and Thyroid Hormones Flashcards
thyroid hormone synthesis starts with synthesis of _______________
thyroglobulin
tetraiodothyronin (thyroxine; T4) is produced from __________
tyrosine
what do thyroid follicular cells do to iodide?
trap and concentrate it via Na/I symporter in basal membrane
how does iodide get into the colloid?
chloride/iodide transporter in apical membrane
what does thyroperoxidase (TPO) do?
oxidizes I- (iodide) to iodine
iodinates tyrosine molecules
produces monoiodotyrosine (MIT) and diiodotyrosine (DIT), thyroid hormone precursors
after making MIT and DIT, what does thyroperoxidase do?
cleaves and couples MIT or DIT to acceptor DIT residues
thyroperoxidase actions result in a mixture of ___, ___, ___, and ___ attached to the thyroglobulin backbone
MIT
DIT
T3
T4
how do the final thyroid hormones get into the blood?
diffuse through basal membrane after being cleaved
what mediates synthesis and secretion of thyroid hormone?
thyroid stimulating hormone: receptors on basal membranes, G protein mediated increase in cAMP
what does thyroid stimulating hormone stimulate?
I- uptake
thyroglobulin synthesis
endocytosis of colloid
what is most of the hormone released from the thyroid gland?
thyroxine (T4)
from where is most of the T3 in serum derived?
deiodination of T4 in tissues
is T3 or T4 more metabolically active?
T3
why is thyroid hormone bound?
prolongs the circulating half life and controls the blood hormone levels
what proteins bind thyroid hormone?
thyroid hormone-binding globulin (TBG)
albumin
plasma lipoproteins
how does estrogen affect thyroid hormone-binding globulin (TBG)?
increases: increases total T4
how do cortisol and NSAIDS affect thyroid hormone levels?
compete for binding sites: lower total levels because cause it to be cleared more
what regulates thyroid hormone?
hypothalamus and central nervous system
how does heat affect TRH?
decreased by heat
increased by cold
true/false: excess iodide exerts negative feedback on thyroid hormone synthesis
true
do deiodinases differ by tissue?
yes
what does thyroid hormone (T3) bind to in the nucleus?
heterodimer with retinoid X receptor
what is thyroid hormone essential for in development?
brain
synapse formation
myelination
what does thyroid hormone do with erythropoiesis?
stimulates it
what cell organelles are necessary for making thyroglobulin?
rER makes backbone
Golgi glycosylates and packages
what stimulates the synthesis of thyroglobulin?
thyroid stimulating hormone
what percent of the body’s iodide is in the thyroid?
> 90%
where is coupling done?
thyroglobulin backbone
what are goitrogens?
substances that disrupt production of thyroid hormones
what is used to treat hyperthyroid cats?
methimazole
what percent of the body’s T3 is produced by the thyroid?
20%
what is the sequence for control of thyroid hormone?
thyrotropin releasing hormone from hypothalamus
thyroid stimulating hormone from thyrotropes in anterior pituitary
increased T4 release from thyroid
negative feedback from T4 to both hypothalamus and anterior pituitary
how does cortisol affect thyroid stimulating hormone?
inhibits its secretion
does iodide exert negative feedback on thyroid hormone?
yes: iodination thyroglobulin, thyroid hormone synthesis, and sensitivity to TSH
what is the thyroid nuclear receptor specific for?
T3
where is deiodinase 1 (D1) specific to?
liver, kidneys, thyroid, pituitary
what deiodinase is the main source of rT3?
D3
what deiodinase is specific to the brain?
D2
astroglia
what substance is essential to iodinases?
selenium
when is deiodinase 3 (D3) more active?
sustained catabolism periods (starvation, illness, hibernation)
fetus
what does thyroid receptor do in the absence of hormone?
binds to co-repressor proteins that silence gene expression
what is the thyroid receptor?
heterodimer with retinoid X receptor
what does thyroid hormone do to the daily energy use?
increases it
free fatty acids mobilized and used
Na/K ATPase stimulated
increased body heat
what does thyroid hormone synergize with in a young animal?
growth hormone and IGF-1
how does thyroid hormone stimulate carbohydrate metabolism?
facilitates gluconeogenesis: enhances effects cortisol, epinephrine, growth hormone, glucagon
facilitates glycogenolysis
enhances absorption carbohydrates in intestine
enhances insulin-dependent entry into cells
what does thyroid hormone do with fat metabolsm?
increases lipolysis
increases cholesterol synthesis, excretion in bile, and lipoprotein catabolism
increases triglyceride turnover and chylomicron clearance
what does thyroid hormone do with the beta:alpha-adrenergic receptor ratio?
increases beta-adrenergic receptors
