Reproduction: hCG and thyroid stimulation: 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 · Reproduction: hCG and thyroid stimulation

Sample item

A 26-year-old woman at 10 weeks of amenorrhoea has uterine size larger than dates, vaginal bleeding, a very high hCG, tremor, suppressed TSH and raised free T4. How does hCG cause thyrotoxicosis?

AIt induces antibodies against the TSH receptorBIt stimulates pituitary TSH releaseCIt converts T4 to T3 in the liverDAt very high concentrations it weakly activates the TSH receptor because of structural similarity

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

The answer and the reasoning

D. At very high concentrations it weakly activates the TSH receptor because of structural similarity

hCG, LH, FSH and TSH share an identical alpha subunit, and the beta subunits of hCG and TSH are similar. In hydatidiform mole hCG levels are very high and thyroid stimulation results; TSH is suppressed by negative feedback. Evacuation of the mole cures the hyperthyroidism.

Option by option

A. It induces antibodies against the TSH receptor
Thyrotropin receptor antibodies are the mechanism of Graves disease, not molar thyrotoxicosis.
B. It stimulates pituitary TSH release
TSH is suppressed in this case, so pituitary TSH release does not explain it.
C. It converts T4 to T3 in the liver
hCG has no role in peripheral deiodination.
D. At very high concentrations it weakly activates the TSH receptor because of structural similarity (correct)
Correct. hCG is a weak agonist at the TSH receptor.

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.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is D, At very high concentrations it weakly activates the TSH receptor because of structural similarity. hCG, LH, FSH and TSH share an identical alpha subunit, and the beta subunits of hCG and TSH are similar.

Why is "It induces antibodies against the TSH receptor" not the answer?

Thyrotropin receptor antibodies are the mechanism of Graves disease, not molar thyrotoxicosis.

Why is "It stimulates pituitary TSH release" not the answer?

TSH is suppressed in this case, so pituitary TSH release does not explain it.

Why is "It converts T4 to T3 in the liver" not the answer?

hCG has no role in peripheral deiodination.

What concept does this question test?

It tests Reproduction: hCG and thyroid stimulation in Physiology. In hydatidiform mole hCG levels are very high and thyroid stimulation results; TSH is suppressed by negative feedback. Evacuation of the mole cures the hyperthyroidism.

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 Reproduction: hCG and thyroid stimulation?

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.