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 neonate has a goitre, high TSH and low T4. A radioiodine uptake study is high, and giving perchlorate releases most of the trapped iodine from the gland. Which enzyme defect is the commonest cause of this dyshormonogenesis?

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

The answer and the reasoning

D. Thyroid peroxidase

A positive perchlorate discharge test means iodide is trapped but not organified, which places the defect at oxidation and incorporation. Lack of the peroxidase is the commonest cause of this dyshormonogenesis. A defect of the symporter would show poor uptake from the start.

Option by option

  • A. Iodotyrosine deiodinase

    Iodotyrosine deiodinase defects cause loss of iodide in urine with a different discharge pattern.

  • B. Thyroglobulin synthesis

    A thyroglobulin synthesis defect does not stop iodide organification, so it does not typically give a positive perchlorate discharge.

  • C. Sodium-iodide symporter

    Symporter defects show low uptake and no iodide concentration in the gland.

  • D. Thyroid peroxidase (correct)

    Correct. Iodide is trapped but not organified, the signature of a peroxidase defect.

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 D, Thyroid peroxidase. A positive perchlorate discharge test means iodide is trapped but not organified, which places the defect at oxidation and incorporation.

Why is "Iodotyrosine deiodinase" not the answer?

Iodotyrosine deiodinase defects cause loss of iodide in urine with a different discharge pattern.

Why is "Thyroglobulin synthesis" not the answer?

A thyroglobulin synthesis defect does not stop iodide organification, so it does not typically give a positive perchlorate discharge.

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

Symporter defects show low uptake and no iodide concentration in the gland.

What concept does this question test?

It tests Thyroid hormone synthesis: thyroid peroxidase in Biochemistry. Lack of the peroxidase is the commonest cause of this dyshormonogenesis. A defect of the symporter would show poor uptake from the start.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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.