Thyroid drugs: levothyroxine interactions: practice question

A Pharmacology 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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Pharmacology · Thyroid drugs: levothyroxine interactions

Sample item

A 40-year-old woman with hypothyroidism has been stable on levothyroxine 100 micrograms for a year. After starting ferrous sulphate and calcium carbonate tablets her TSH rises to 12 mIU/L. What is the best explanation and advice?

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

The answer and the reasoning

D. Reduced gut absorption through binding; separate the doses by at least 4 hours

Iron, calcium, aluminium antacids and sucralfate bind levothyroxine in the gut and reduce its absorption, so TSH rises. Levothyroxine should be taken on an empty stomach and separated from these agents by about 4 hours. Enzyme inducers such as rifampicin or phenytoin and oestrogen-induced TBG rise are other causes of increased requirement.

Option by option

  • A. Increased renal excretion of thyroxine; give it with food

    Iron and calcium do not increase renal loss, and food lowers levothyroxine absorption.

  • B. Increased thyroxine-binding globulin; stop the supplements

    TBG rises with oestrogen or pregnancy, not with mineral supplements.

  • C. Increased hepatic metabolism of thyroxine; double the dose permanently

    Enzyme induction occurs with rifampicin or phenytoin, not with iron or calcium.

  • D. Reduced gut absorption through binding; separate the doses by at least 4 hours (correct)

    Correct. Chelation in the gut lowers absorption and is solved by separating doses.

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 Pharmacology question

What is the answer to this Pharmacology question?

The best answer is D, Reduced gut absorption through binding; separate the doses by at least 4 hours. Iron, calcium, aluminium antacids and sucralfate bind levothyroxine in the gut and reduce its absorption, so TSH rises.

Why is "Increased renal excretion of thyroxine; give it with food" not the answer?

Iron and calcium do not increase renal loss, and food lowers levothyroxine absorption.

Why is "Increased thyroxine-binding globulin; stop the supplements" not the answer?

TBG rises with oestrogen or pregnancy, not with mineral supplements.

Why is "Increased hepatic metabolism of thyroxine; double the dose permanently" not the answer?

Enzyme induction occurs with rifampicin or phenytoin, not with iron or calcium.

What concept does this question test?

It tests Thyroid drugs: levothyroxine interactions in Pharmacology. Levothyroxine should be taken on an empty stomach and separated from these agents by about 4 hours. Enzyme inducers such as rifampicin or phenytoin and oestrogen-induced TBG rise are other causes of increased requirement.

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 drugs: levothyroxine interactions?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology 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.