Endocrinology: thyroid hormone metabolism: 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.

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Physiology · Endocrinology: thyroid hormone metabolism

Sample item

Most circulating triiodothyronine, the active thyroid hormone, arises from which process?

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

The answer and the reasoning

B. Peripheral deiodination of thyroxine

The thyroid secretes mainly T4. About 80 percent of T3 is formed in peripheral tissues by 5-prime deiodinases, so T4 acts as a prohormone. Reverse T3 is formed by a different deiodination.

Option by option

  • A. Hepatic synthesis from tyrosine

    The liver does not make thyroid hormone from tyrosine.

  • B. Peripheral deiodination of thyroxine (correct)

    Correct. Tissue deiodinases convert T4 to active T3.

  • C. Direct secretion by thyroid follicles

    The thyroid secretes a minority of T3 directly.

  • D. Pituitary conversion of TSH

    TSH is a pituitary hormone and is not converted to T3.

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 Physiology question

What is the answer to this Physiology question?

The best answer is B, Peripheral deiodination of thyroxine. The thyroid secretes mainly T4.

Why is "Hepatic synthesis from tyrosine" not the answer?

The liver does not make thyroid hormone from tyrosine.

Why is "Direct secretion by thyroid follicles" not the answer?

The thyroid secretes a minority of T3 directly.

Why is "Pituitary conversion of TSH" not the answer?

TSH is a pituitary hormone and is not converted to T3.

What concept does this question test?

It tests Endocrinology: thyroid hormone metabolism in Physiology. About 80 percent of T3 is formed in peripheral tissues by 5-prime deiodinases, so T4 acts as a prohormone. Reverse T3 is formed by a different deiodination.

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 Endocrinology: thyroid hormone metabolism?

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.