Thyroid gland hormones Flashcards
What is formed when iodine is attached to tyrosine?
mono-iodotyrosine (MIT)
di-iodotyrosine (DIT)
What is T3 and T4 made of?
MIT + DIT = T3
DIT +DIT = T4
What if the order is reversed in T3?
DIT +MIT = reverse tri-iodothyronine, rT3
binds to the receptor but doesn’t activate it = antagonist at T3 receptors
What are the 2 sides of a follicular cell? Which way do they face?
basal (faces blood) and apical/luminal (faces lumen/colloid)
How does the body get iodine usually? How does it get into the follicular cell? What is this process called? Active or passive transport?
diet - seafood and salt
through sodium-iodide symporter through basal side
electroneutral (no net electric charge)
active uptake (trapping)
How does iodine move from the follicular cell to the colloid? What enzyme oxidises the iodine? What does this form?
moves across via pendrin
oxidation of iodide via THYROID PEROXIDASE (TPO)
What is thyroglobulin?
a large glycoprotein with many tyrosine molecules where thyroid hormones are synthesised
Where is thyroglobulin (Tg) made?
in follicular cells
How is thyroglobulin transported from the follicular cells to the colloid?
exocytosis
What happens in the organification process?
iodination of tyrosine molecules on Tg -> MIT and DIT
What enzyme catalyzes the steps of forming thyroid hormones in the follicle? What are the 3 steps?
thyroid perosidase (TPO)
- oxidation of iodide
- organification
- coupling of iodotyrosines -> T3,T4
What happens to the T3 and T4 once it’s been synthesised? How long can supplies of thyroid hormone last
stored of ioninated Tg in the colloid
several weeks
How are thyroid hormones released into the blood?
Follicular cells pinch off bits of colloid then lysosomes added to give proteases which clean off the hormone to leave T3 or T4
By what process is thyroglobulin/Thyroid hormones taken into the follicular cells?
Tg taken up by endocytosis
Fusion with lysosomes results in proteolysis and release of T3 and T4 into plasma
What can T3/T4 inhibit? What can TSH inhibit?
T3/T4 can inhibit hypothalamus and anterior pituitary
TSH inhibits hypothalamus
What is the implication of TSH as a trophic hormone? What steps does TSH stimulate?
TSH maintains integrity of thyroid gland - if in excess, increases size and numbers of thyroid follicular cells
acts on every step (of TH production and release)
What can cold stimulate in neonates? What about in adults?
TRH (thyroid hormones)
negligible in adults, unless chronic exposure
What does stress do in terms of thyroid hormone synthesis?
Inhibits
what are the 3 proteins by which T3 and T4 can be bound to?
Thyroxine-binding globulin (TBG)
Transthyretin
Albumin
T3/T4 only work when bound to a protein T/F?
FALSE - protein bound hormones cannot act biologically
How can we measure thyroid function?
Measurement of free T4
Of T3/T4 and rT3, rank them in order of shortest to longest half-life
rT3
T3
T4
What is made in greater amounts? T3 or T4? Which is more potent? Which is the prohormone?
greater amounts of T4 made
T3 is more potent
T4 is the prohormone (T3 is the active hormone)
T4 still has some action by itself T/F?
TRUE