Thyroid hormone synthesis: thyroid peroxidase: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Thyroid hormone synthesis: thyroid peroxidase

Sample item

A woman with Graves disease is started on carbimazole. Inside the follicular cell, the drug mainly prevents iodide oxidation, iodination and coupling by inhibiting which enzyme?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Thyroid peroxidase

Thionamides, which include carbimazole, methimazole and propylthiouracil, inhibit the enzyme that organifies iodine and couples iodotyrosines. This lowers new hormone production, though stored hormone delays the clinical effect. Propylthiouracil also inhibits peripheral deiodination, and perchlorate blocks the symporter.

Option by option

  • A. Iodothyronine deiodinase

    Iodothyronine deiodinase is additionally inhibited by propylthiouracil, but it is not the main intracellular target.

  • B. Sodium-iodide symporter

    The sodium-iodide symporter is blocked by perchlorate and thiocyanate, not thionamides.

  • C. Thyroid peroxidase (correct)

    Correct. Thionamides block iodide oxidation, organification and coupling.

  • D. Thyroglobulin protease

    Thyroglobulin proteolysis is inhibited by lithium and large doses of iodide, not thionamides.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, Thyroid peroxidase. Thionamides, which include carbimazole, methimazole and propylthiouracil, inhibit the enzyme that organifies iodine and couples iodotyrosines.

Why is "Iodothyronine deiodinase" not the answer?

Iodothyronine deiodinase is additionally inhibited by propylthiouracil, but it is not the main intracellular target.

Why is "Sodium-iodide symporter" not the answer?

The sodium-iodide symporter is blocked by perchlorate and thiocyanate, not thionamides.

Why is "Thyroglobulin protease" not the answer?

Thyroglobulin proteolysis is inhibited by lithium and large doses of iodide, not thionamides.

What concept does this question test?

It tests Thyroid hormone synthesis: thyroid peroxidase in Biochemistry. This lowers new hormone production, though stored hormone delays the clinical effect. Propylthiouracil also inhibits peripheral deiodination, and perchlorate blocks the symporter.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Thyroid hormone synthesis: thyroid peroxidase?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.