DrVirgo
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10. A patient with Graves disease is scheduled for a subtoral tyhroidectomy. PTU is used to control her hyperthyriodism until surgery. The enzyme inhibited by this drug is involved in which of the following reacions?
A. Conversion of iodide to iodine at the apex of the follicular cell
B. Iodide uptake at the base of the follicular cell
C. Iodination of thyroglobulin in the colloid
D. Proteolysis of iodinated thyroglobulin into MIT and DIT
E. Reuptake of iodinated thyroglobulin from the follicular lumen
Please explain what is the enzyme mentioned in the question and the mechanism of the drug...
Thanks. 
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| suv
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high dose PTU inhibits 5 deiodinase and it inhibits iodination of tyrosyl residues on thyroglobulin
so ans is c 
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| nadiabarati
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C
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| DrVirgo
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Its not C.
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| nadiabarati
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PTU inhibits many steps in the synthesis of thyroid hormones, including the addition of iodide to thyroglobulin by the enzyme thyroperoxidase, a necessary step in the synthesis of thyroxine, and by inhibiting the enzyme 5'-deiodinase which converts T4 to T3.
Notably, PTU does not inhibit the action of the sodium-dependent iodide transporter located on follicular cells' basolateral membranes. Inhibition of this step requires competitive inhibitors such as perchlorate and thiocyanate
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| drpkaur
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why it's not C..
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| nadiabarati
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yes why not C?!!!
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| DrVirgo
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Answer is A.
PTU inhibits thytoid peroxidase, the enzyme that allows the iodide taken up at the base of thyroid follicular cell to be oxidized to iodine at the apex of the cell. Thyroid peroxidase also catalyzes the oxidative coupling of two DIT molecules into T4 and one DIT and noe MIT to T3.
as for Choice C: Iodination of thyroglobulin in the colloid into DIT and MIT does nto involve thyroid peroxidase
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| nadiabarati
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So it means that I must not trust online articles? These opposite information drive me bonkers....AHHHHHHHHHH
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| nadiabarati
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The explanation that I posted is the exact copy of an online article about PTU!!!
Please someone help...
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| DrVirgo
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This question was a bit confusing for me too...
As I know it from Kap:
PTU (at high doses) inhibits the peripheral convrersion of T4 to T3
But the other explanation that I wrote about answer A is from Q-Book.
If anyone can help us clear this up please help.
Thanks.
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| Chrales
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The clue to the question is, that PTU inhibits thyroperoxidase, an enzyme with different abilities:
It mediates the formation of iodine from iodide (thus Answer A is correct) and it induces the formation of T4, by coupling 2 Diiodotyrosine-molecules
and
T3, by coupling 1 monoiodotyrosine + 1 Diiodotyrosine
The latter function is NOT the mechansim mentioned in answer C, though close to it. So answer C is not correct
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| docarchana
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we can say in general that PTU inhibits all the steps in TH synthesis except the uptake of iodide (inhibited by anions) source - kaplan
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