Thyroid: peripheral conversion: practice question

A Physiology 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.

Try it

Physiology · Thyroid: peripheral conversion

Sample item

A 35-year-old woman in thyroid storm is given propylthiouracil rather than carbimazole. Besides inhibiting thyroid peroxidase, which additional action makes propylthiouracil useful in this setting?

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

The answer and the reasoning

A. Inhibition of peripheral conversion of thyroxine to triiodothyronine

Propylthiouracil blocks organification of iodine by thyroid peroxidase and also inhibits type 1 5'-deiodinase, which converts T4 to the more active T3 in peripheral tissues. This lowers T3 levels quickly in severe thyrotoxicosis. Carbimazole and methimazole act only on the thyroid peroxidase step.

Option by option

  • A. Inhibition of peripheral conversion of thyroxine to triiodothyronine (correct)

    Correct. Reduced peripheral T3 formation helps in thyroid storm.

  • B. Binding of circulating thyroid hormone to thyroxine-binding globulin

    Antithyroid drugs do not alter protein binding of thyroid hormone.

  • C. Blockade of the thyroid TSH receptor

    TSH receptor blockade is not a mechanism of antithyroid drugs.

  • D. Inhibition of iodide uptake by the sodium-iodide symporter

    Blocking iodide uptake is the action of perchlorate and thiocyanate.

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.

More Physiology questions

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is A, Inhibition of peripheral conversion of thyroxine to triiodothyronine. Propylthiouracil blocks organification of iodine by thyroid peroxidase and also inhibits type 1 5'-deiodinase, which converts T4 to the more active T3 in peripheral tissues.

Why is "Binding of circulating thyroid hormone to thyroxine-binding globulin" not the answer?

Antithyroid drugs do not alter protein binding of thyroid hormone.

Why is "Blockade of the thyroid TSH receptor" not the answer?

TSH receptor blockade is not a mechanism of antithyroid drugs.

Why is "Inhibition of iodide uptake by the sodium-iodide symporter" not the answer?

Blocking iodide uptake is the action of perchlorate and thiocyanate.

What concept does this question test?

It tests Thyroid: peripheral conversion in Physiology. This lowers T3 levels quickly in severe thyrotoxicosis. Carbimazole and methimazole act only on the thyroid peroxidase step.

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: peripheral conversion?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology 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.