Thyroid Gland Driscoll Flashcards
What is this:
Located immediately below the larynx
on each side of and anterior to the
trachea
Normally weighs 15-20g in adults
thyroid gland
What make thryoid hormone?
foliicular eptihelial cells of the thyroid gland
What line the capillaries and provide blood supply to the follicles.
endothelial cells
(blank) involved in the production of calcitonin, a hormone involved in calcium metabolism
parafollicular or C cells
What all are in a thryoid gland?
Follicular cells endothelial cells parafollicular cells or C cells fibroblasts lymphocytes adipocytes
What is the morphology of the thyroid gland?
it is a bunch of closed follicle filled with colloid (thryoglobulin)
lined by epithelial cells
Thyroid epithelial cells are morphologically and functionally (blank)
polarized
Apical surfaces of the thyroid epithelial cells face the (blank).
Follicular lumen
Where is colloid stored?
in the follicular lumen
The basolaterlal surface of the thyroid eptihelial cell faces the (blank) and is exposed to the bloodstream.
interstitium
I accumulation and hormone biosynthesis is dependent on (blank)
cellular and tissue organization
What is this:
93% of total secretion (~80 µg/day)
Most is converted to T3 in the tissues
T4 (thyroxine) Tetraiodothryonine
What is this:
7% of total secretion (~5 µg/day)
Similar function as thyroxine but ~4x more potent
At lower levels in plasma than T4
triiodothyronine (T3)
What are the 5 factors involved in thyroid hormone synthesis?
TSH thryoglobulin Iodine Membrane Transporters Enzymes
(blank) plays an important role in the synthesis and storage of thyroidhormone
Thyroglobulin
(blank) is a large glycoprotein containing multiple tyrosine residues
thryoglobulin
Where is thyroglobulin synthesized?
Where is it secreted?
Where is it stored?
Thyroid follicular epithelial cells
through apical membrane into follicular lumen
Colloids
(blank) can be considered a scaffold upon which thryoid hormone synthesis take place
Thyroglobulin
Why is iodine important?
thryoid hormones need it
(blank) is the worlds most prevalent, yet easily preventable cause of brain damage.
iodine deficiency
What takes place in both the collid and follicular epithelial cells?
biosyntehsis of thryoid hormones
What is the first stage of thyroid hormone synthesis and what does it do?
What is it influenced by?
step 1: iodide trapping
transport of iodides from the blood into the thyroid cell and follicles (conc. iodide in cell)
TSH
What is the mechanism of iodide trapping?
Na/I symport (puts iodide into cell)
via Na/K Atpase
Transporters out of cell into follicle lumen via Pendrin (Cl/I countertransporter)
Iodide transcellular transport relies on the functional and morphologic (blank) of the cell
polarization
Where is iodide more concentrated, inside the thyroid cells or in the blood?
30X higher in thyroid cells
What is stage 2 of thryoid hormone synthesis and what does it do?
Formation and secretion of thryoglobulin
Synthesized thyroglobulin in ER and Golgi and putting it into follicular colloid.
Thryoglobuin is made up of a bunch of what and does what?
tyrosine (140)
Makes thryoid hormones
(blank) form within the thryoglobin molecule.
thryoid hormones
What is stage 3 of thryoid hormone synthesis and what does it do?
Oxidation of iodide and coupling of iodine w/ thyroglobulin
How does Stage 3 happen?
iodide gets oxidized by TPO (thyroperoxidase)
Oxidized iodide combines w tyrosine AAs from thyroglobulin molecule (organification of iodide).
20% of tyrosine is iodinated
After you get to stage 3 of thyroid hormone synthesis you are left with iodinated tyrosine,then what happens?
iodination and coupling of iodotyrosine residues to form T3 and T4
Explain the successive stages of iodination and coupling of iodotyrosine residues to form T3 and T4?
1) tyrosine iodized to monoiodotyrosine (MIT)
and then to DIT
2) DIT and DIT= T4
3) DIT and MIT= T3
What is Reverse T3 and is it important?
it is 3,3,5 triiodothyronine instead of T3 which is 3,5,3 triiodothyronine.
No it is not importnant
What does TPO (thyroperoxidase) do?
oxidates iodide
iodination of tyrosines on thryoglobulin (organification)
synthesis of T4 or T3 from two DIT
If (blank) is blocked or hereditarily absent: rate of formation of thyroid hormones falls to zero
TPO
Is goiter present in hypothyroidism or hyperthyroidism?
both
(blank) in the colloid acts as storage form of thyroid hormones
Thyroglobulin
Do thyroglobulins contain large amounts of T4 and T3?
no small amounts but enough to supply body for 2-3 months
Where are thyroid hormones located when in storage form?
in follicular colloid of thryoglobulin
If you dont have iodine for a few months are you screwed?
no, cuz of the storage of hormones in your follicular colloid of your thyroglobulin
What is the fourth stage of thyroid hormone synthesis?
endocytosis of thryoglobulin and release of thyroid hormones
Explain the mechanism of stage four
thryoglobulin enters follicular cell via endocytosis (pseudopodia)
lysosoe fuses with vesicle
thryoglobin is digested (by protease)
T3 and T4 are released into blood
Is thyroglobulin released into the blood?
nope, just T3 and T4
What is stage five of thyroid hormone synthesis?
recycling of iodine for reuse
How does iodine get recycled?
75% of MIT and DIT are not turned into Thyroid hormones, instead the unused MIT and DIT get cleavedby deiodinase in cytosol to make iodine which is reused
(blank) is an iodine deficiency due to failure of the recycling process
Congenital absence of deiodinase
(blank) provide a substantial reservoir of extrathyroidal hormone
Plasma Binding proteins
i.e they store thyroid hormones until they need to be used
In the blood stream what happens to T3 and T4?
combine immediately w/ plasma proteins synthesized by the liver
TBG (major)
Transthyretin
Albumin
If T3 and T4 doesnt bind to a plasma protein in the blood (1%) what happens?
it is free in the circ.
Are binding proteins saturated?
no and store 3X the amount secreted per day
Do acute changes in hormone secretion have a large effect on free hormone concentrations?
no
The binding capacity of plasma proteins for thyroid hormones is far from saturated, an even massive increase in secretion rate would have little effect on the conc. of hormone that is unbound.
T or F?
True
Changes in binding protein levels alters (Blank) concentrations.
thyroid hormones
How can you alter the binding-hormone capacity of an individual?
by disease or hormonal changes
If you have high estrogen levels (like in pregnancy), how will this effect your total thyroid hormone levels?
high estrogen-> high binding protein synthesis-> new equilibrium b/w free and bound hormone-> increased total thyroid hormones-> dynamic changes occur throughout life of individual
Does the plasma binding proteins for thyroid hormones have a high or low affinity?
What does this tell us?
high affinity
That thyroid hormones are released slowly to the tissues
Which has a longer half life, T3 or T4 and why?
T4 because binding proteins have a higher affinity for them
What does the thyroid hormone typically release and what happens to this when it gets to extrathryoidal tissues?
mostly is released as T4 but gets deiodinated via deiodinase to T3 in the extrathryoidal tissues
What is the most active hormone and the most used hormone by the tissues?
T3
The metabolism of T3 and T4 into active and inactive intermediates involves the action of 3 types of (blank)
deiodinases
HOw can thyroid hormones be excreted into bile?
can be conjugated to glucuronic acid