thyroid hormones Flashcards
(35 cards)
what cells secrete T3+4
thyroid follicular cells
what cells secrete calcitonin
parafollicular C cells
where is thyroglobulin produced and what does contain
in the thyroid follicular cells and released via exocytosis.
iodide -enough for 90 days worth of thyroid hormones
where is Thyrotropin releasing hormone stored
hypothalamus
where is thyroid stimulating hormone stored
anterior pituitary
consequence of high t3 on TSH
low TSH = thyroid regression and reduced activity
low t3 on TSH
high TSH = thyroid enlargement and increased activity
what stimulates thyroid follicular cells to produce t3&4
TSH in the bloodstream
follicular cells have csr (GPC receptor) for TSH which activated adenylate cyclase to produce cAMP which activates functions
how is iodide transported from the blood to the cells
via the sodium iodide symporter
2 sodium cations for each iodide anion
how is iodide transported from the cells to the lumen
via the pendrin transporter which exchanges chloride ions for iodide ions
what happens to iodide when in the lumen
it is oxidised to atomic iodine
what enzyme covalently binds iodine to tyrosine residues in thyroglobulin
thyroid peroxidase
which molecules make up t4
2 molecules of di-iodotyrosine
which molecules make up t3
one DIT then one MIT
which molecules make up reverse T3
MIT THEN DIT = inactive
how is T3&4 released from the thyroglobulin backbone
subject to proteolysis via lysosomes in cells
example of 2 hormones that are used to treat hypothyroidism
levothyroxine sodium t4
liothyronine sodium t3
example of 2 drugs that are used to treat hyperthyroidism
carbimazole
propylthiouracil
how can liver disease affect t thyroxine (t4) delivery
t4 is transported in the serum with specific binding protein which are synthesised in the liver. liver disease will decrease t4 transport to peripheral tissues.
why is it difficult to transport thyroxine (t4)
it is insoluble in serum = hydrophobic
why is t4 more abundant than t3
the binding ration of t4;t3 is
20;1 . transthyretin does not bind t3.
which protein best binds t4
thyroxine binding globulin
advantages of thyroxine finding globulin
prevents loss of t4 in urine
which hormone is more active
t3 is biologically active. t4 is relatively inactive an so must be converted to t3
