Vitamins and minerals: Wolff-Chaikoff effect: 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 · Vitamins and minerals: Wolff-Chaikoff effect

Sample item

A patient with Graves disease is given a large dose of Lugol iodine before thyroidectomy and thyroid hormone synthesis falls within days. Which effect explains this?

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

The answer and the reasoning

B. Excess iodide transiently blocks organification in the gland

A large iodide load inhibits organification of iodine in the thyroid, the Wolff-Chaikoff effect. The block is transient and normal glands escape from it, while diseased glands may not. This is why iodide is used briefly before thyroid surgery.

Option by option

  • A. Excess iodide stimulates secretion of thyrotropin

    Iodide has no direct stimulating effect on TSH secretion, which rises only secondarily if hormone levels fall.

  • B. Excess iodide transiently blocks organification in the gland (correct)

    Correct. High intrathyroidal iodide briefly suppresses hormone synthesis by blocking organification.

  • C. Excess iodide destroys the follicular cells

    Iodide does not kill follicular cells at these doses.

  • D. Excess iodide converts thyroxine to reverse T3 inside the gland

    Intrathyroidal conversion to reverse T3 is not the mechanism of this effect.

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 B, Excess iodide transiently blocks organification in the gland. A large iodide load inhibits organification of iodine in the thyroid, the Wolff-Chaikoff effect.

Why is "Excess iodide stimulates secretion of thyrotropin" not the answer?

Iodide has no direct stimulating effect on TSH secretion, which rises only secondarily if hormone levels fall.

Why is "Excess iodide destroys the follicular cells" not the answer?

Iodide does not kill follicular cells at these doses.

Why is "Excess iodide converts thyroxine to reverse T3 inside the gland" not the answer?

Intrathyroidal conversion to reverse T3 is not the mechanism of this effect.

What concept does this question test?

It tests Vitamins and minerals: Wolff-Chaikoff effect in Biochemistry. The block is transient and normal glands escape from it, while diseased glands may not. This is why iodide is used briefly before thyroid surgery.

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 Vitamins and minerals: Wolff-Chaikoff effect?

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.