Thyroid: hormone transport: 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 · Thyroid: hormone transport

Sample item

A 28-year-old woman on an oral contraceptive pill has a total thyroxine level above the reference range, with normal free thyroxine and normal TSH. She has no symptoms. What is the most likely explanation?

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

The answer and the reasoning

B. Oestrogen-induced rise in hepatic thyroxine-binding globulin

Oestrogen increases hepatic synthesis and sialylation of thyroxine-binding globulin, so total T4 rises because more is protein-bound. The free hormone, which is the active fraction, stays normal, so TSH is normal and the patient is euthyroid. The same happens in pregnancy.

Option by option

  • A. Increased peripheral conversion of T4 to T3

    Peripheral conversion affects T3 and does not explain a raised total T4.

  • B. Oestrogen-induced rise in hepatic thyroxine-binding globulin (correct)

    Correct. More binding protein raises total but not free T4.

  • C. Subclinical hyperthyroidism from autonomous nodules

    Hyperthyroidism would raise free T4 and suppress TSH.

  • D. Reduced renal clearance of thyroxine

    Renal clearance is not a significant route of thyroxine loss, and TSH would rise if free T4 changed.

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, Oestrogen-induced rise in hepatic thyroxine-binding globulin. Oestrogen increases hepatic synthesis and sialylation of thyroxine-binding globulin, so total T4 rises because more is protein-bound.

Why is "Increased peripheral conversion of T4 to T3" not the answer?

Peripheral conversion affects T3 and does not explain a raised total T4.

Why is "Subclinical hyperthyroidism from autonomous nodules" not the answer?

Hyperthyroidism would raise free T4 and suppress TSH.

Why is "Reduced renal clearance of thyroxine" not the answer?

Renal clearance is not a significant route of thyroxine loss, and TSH would rise if free T4 changed.

What concept does this question test?

It tests Thyroid: hormone transport in Physiology. The free hormone, which is the active fraction, stays normal, so TSH is normal and the patient is euthyroid. The same happens in pregnancy.

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 transport?

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.