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PTU Thyroid Storm — FRCA Primary MCQ

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HardPhysiology - Endocrine & MetabolicPTU Thyroid StormFRCA Primary

A patient with thyroid storm is given propylthiouracil. Temperature and tachycardia remain high during the first hour. Which statement best explains why this does not mean the drug has failed?

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Reveal the answer and explanation

Correct answer: A — It inhibits new synthesis and peripheral T4-to-T3 conversion

The best answer is “It inhibits new synthesis and peripheral T4-to-T3 conversion”. Propylthiouracil inhibits thyroid peroxidase and peripheral de-iodination of T4 to T3, but it does not neutralise circulating hormone or prevent release of hormone already formed. Clinical control therefore also requires supportive care and treatment of adrenergic effects and the precipitant. Its mechanism is neither follicular destruction, beta blockade nor isolated pituitary suppression.

Reference: Propylthiouracil 100 mg tablets, UK SmPC: https://www.medicines.org.uk/emc/product/14253/smpc; Royal College of Anaesthetists, Primary FRCA syllabus v2.2: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf