Antithyroid drugs – Propylthiouracil MCQs With Answer

Introduction: Antithyroid drugs are essential in managing hyperthyroidism, and Propylthiouracil (PTU) is a key agent covered in the B. Pharm curriculum. This concise guide explains PTU’s mechanism of action—inhibiting thyroid peroxidase and peripheral T4-to-T3 conversion—its pharmacokinetics, common and serious adverse effects such as hepatotoxicity and agranulocytosis, dosing principles, monitoring parameters, and important drug interactions. Understanding PTU’s indications, contraindications (including pregnancy considerations), and differences from methimazole prepares students for clinical decision-making and safe dispensing. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the primary mechanism of action of propylthiouracil (PTU)?

• Stimulating thyroid hormone secretion
• Inhibiting thyroid peroxidase and blocking iodination of tyrosyl residues
• Increasing peripheral conversion of T4 to T3
• Blocking thyroid hormone uptake by peripheral tissues

Correct Answer: Inhibiting thyroid peroxidase and blocking iodination of tyrosyl residues

Q2. In addition to inhibiting thyroid peroxidase, what other action does PTU have?

• Enhances TSH release
• Blocks peripheral conversion of T4 to T3
• Acts as a thyroid hormone receptor agonist
• Increases iodide uptake by the thyroid

Correct Answer: Blocks peripheral conversion of T4 to T3

Q3. Which adverse effect is most classically associated with PTU and requires immediate drug discontinuation?

• Mild rash
• Agranulocytosis
• Weight gain
• Hypertension

Correct Answer: Agranulocytosis

Q4. Which laboratory test is essential for baseline monitoring before and during PTU therapy?

• Lipid profile
• Complete blood count (CBC)
• Fasting glucose
• Serum electrolytes

Correct Answer: Complete blood count (CBC)

Q5. PTU is preferred over methimazole in which clinical situation?

• First trimester of pregnancy
• Long-term maintenance in all patients
• When rapid control of hyperthyroidism is unnecessary
• In patients with hepatic impairment

Correct Answer: First trimester of pregnancy

Q6. What serious hepatic adverse effect has been associated with PTU?

• Cholestatic jaundice only
• Severe fulminant hepatic failure
• Asymptomatic hyperbilirubinemia that never progresses
• Neonatal hepatitis when used postpartum

Correct Answer: Severe fulminant hepatic failure

Q7. Which pharmacokinetic property applies to PTU?

• Long plasma half-life of 7–10 days
• Extensive enterohepatic recycling with high oral bioavailability
• Shorter half-life than methimazole, requiring multiple daily dosing
• Primarily excreted unchanged in urine

Correct Answer: Shorter half-life than methimazole, requiring multiple daily dosing

Q8. Which monitoring parameter helps detect early hepatotoxicity in a patient on PTU?

• Serum creatinine
• Liver function tests (ALT, AST)
• Thyroid ultrasound
• Serum potassium

Correct Answer: Liver function tests (ALT, AST)

Q9. Which drug interaction is important to consider with PTU therapy?

• PTU increases warfarin metabolism leading to decreased INR
• PTU enhances the anticoagulant effect of warfarin by reducing vitamin K–dependent clotting factors
• PTU has no interaction with anticoagulants
• PTU potentiates insulin action causing hypoglycemia

Correct Answer: PTU enhances the anticoagulant effect of warfarin by reducing vitamin K–dependent clotting factors

Q10. What is a major advantage of PTU in thyroid storm management?

• It increases synthesis of T4
• It rapidly decreases peripheral conversion of T4 to T3
• It has a single dose daily dosing
• It permanently destroys thyroid tissue

Correct Answer: It rapidly decreases peripheral conversion of T4 to T3

Q11. Which symptom should prompt immediate CBC testing in a patient on PTU?

• Mild nausea
• Sudden fever and sore throat
• Transient headache
• Occasional insomnia

Correct Answer: Sudden fever and sore throat

Q12. How is PTU primarily administered in clinical practice?

• Intravenous continuous infusion only
• Oral tablet formulation
• Transdermal patch
• Inhalational aerosol

Correct Answer: Oral tablet formulation

Q13. Which is a typical starting total daily dose range for PTU in moderate hyperthyroidism (adult)?

• 50–150 mg once weekly
• 100–300 mg divided into multiple daily doses
• 1000 mg single dose
• 5–10 mg daily

Correct Answer: 100–300 mg divided into multiple daily doses

Q14. Which of the following is TRUE regarding PTU use during lactation?

• PTU is absolutely contraindicated during breastfeeding
• Low-dose PTU is often considered compatible with breastfeeding with monitoring
• PTU concentrates in breast milk causing neonatal hypothyroidism invariably
• PTU must be switched to methimazole in breastfeeding mothers

Correct Answer: Low-dose PTU is often considered compatible with breastfeeding with monitoring

Q15. Which immunologic adverse effect can occur with PTU therapy?

• Drug-induced lupus-like syndrome
• Type I hypersensitivity causing bronchospasm within seconds
• Immediate anaphylactic shock in all patients
• No immunologic effects are known

Correct Answer: Drug-induced lupus-like syndrome

Q16. Compared with methimazole, PTU has which characteristic?

• Longer duration of action allowing once-daily dosing
• Greater risk of severe hepatic injury
• No risk of teratogenicity in pregnancy
• More potent inhibition of thyroid peroxidase with fewer side effects

Correct Answer: Greater risk of severe hepatic injury

Q17. What is the recommended action if a patient on PTU develops agranulocytosis?

• Reduce the dose and continue therapy
• Immediately discontinue PTU and provide supportive care
• Switch to higher dose PTU to overcome resistance
• Ignore unless symptoms worsen

Correct Answer: Immediately discontinue PTU and provide supportive care

Q18. Which patient population requires special caution when prescribing PTU due to hepatotoxicity risk?

• Young adults aged 20–30 with no comorbidities
• Children and adolescents
• Patients with pre-existing liver disease
• Patients with controlled hypertension

Correct Answer: Patients with pre-existing liver disease

Q19. Why is PTU used in thyroid storm instead of radioactive iodine?

• Radioactive iodine acts faster than PTU
• PTU works quickly to block hormone synthesis and peripheral conversion
• Radioactive iodine is the first-line immediate therapy
• PTU permanently cures thyroid storm in one dose

Correct Answer: PTU works quickly to block hormone synthesis and peripheral conversion

Q20. Which effect of PTU contributes to reduction of circulating active thyroid hormone?

• Induction of hepatic CYP enzymes to increase T3 formation
• Inhibition of deiodinase-mediated T4 to T3 conversion
• Stimulating peripheral tissues to metabolize T3 faster
• Blocking thyroid hormone receptors in peripheral tissues

Correct Answer: Inhibition of deiodinase-mediated T4 to T3 conversion

Q21. Which test indicates therapeutic efficacy of PTU in treating hyperthyroidism?

• Rise in T3 and T4 levels
• Decrease in serum T3 and T4 and rise in TSH toward normal
• Immediate normalization of all thyroid antibodies
• Increase in radioactive iodine uptake

Correct Answer: Decrease in serum T3 and T4 and rise in TSH toward normal

Q22. Which adverse dermatologic reaction can occur with PTU?

• Petechial hemorrhages only
• Mild rash and urticaria
• Severe acneiform eruptions universally
• Photosensitivity leading to vitiligo

Correct Answer: Mild rash and urticaria

Q23. PTU’s onset of action on thyroid hormone synthesis is best characterized as:

• Immediate biochemical normalization within minutes
• Delayed clinical effect; biochemical changes in days to weeks
• Complete lack of biochemical effect
• Only psychological benefit without hormonal changes

Correct Answer: Delayed clinical effect; biochemical changes in days to weeks

Q24. Which of the following is a contraindication to PTU therapy?

• Mild transient liver enzyme elevation without symptoms
• History of severe PTU-induced hepatotoxicity
• Controlled hypothyroidism
• Nonpregnant adult with Graves’ disease

Correct Answer: History of severe PTU-induced hepatotoxicity

Q25. How does PTU differ pharmacologically from iodide therapy in hyperthyroidism?

• PTU blocks hormone synthesis and peripheral conversion; iodide acutely inhibits hormone release
• Iodide reverses agranulocytosis caused by PTU
• Both have identical mechanisms and adverse effects
• PTU stimulates thyroid hormone release while iodide blocks it

Correct Answer: PTU blocks hormone synthesis and peripheral conversion; iodide acutely inhibits hormone release

Q26. For a pregnant patient in second trimester with hyperthyroidism, which antithyroid drug is commonly preferred?

• Methimazole throughout pregnancy
• Propylthiouracil in first trimester, then consider switching to methimazole in second trimester
• Radioactive iodine during pregnancy
• No antithyroid therapy is ever used in pregnancy

Correct Answer: Propylthiouracil in first trimester, then consider switching to methimazole in second trimester

Q27. What counseling point is important for patients starting PTU?

• Skip monitoring; side effects are rare
• Immediately report fever, sore throat, or jaundice
• Increase dosage without consulting pharmacist if feeling unwell
• PTU should be taken only once weekly

Correct Answer: Immediately report fever, sore throat, or jaundice

Q28. Which mechanism explains PTU-induced agranulocytosis?

• Direct irreversible binding to thyroid tissue
• Immune-mediated destruction or toxic suppression of bone marrow granulopoiesis
• Excessive stimulation of neutrophil production causing exhaustion
• Enhanced granulocyte function leading to neutrophilia

Correct Answer: Immune-mediated destruction or toxic suppression of bone marrow granulopoiesis

Q29. When switching a patient from methimazole to PTU, which consideration is important?

• PTU has a longer dosing interval so compliance improves automatically
• Monitor liver enzymes more closely after switching because PTU has higher hepatotoxic risk
• There is no need for any monitoring change
• Stop all monitoring because cross-tolerance prevents adverse effects

Correct Answer: Monitor liver enzymes more closely after switching because PTU has higher hepatotoxic risk

Q30. Which statement about long-term remission after antithyroid drug therapy is TRUE?

• PTU guarantees permanent cure of Graves’ disease in most patients
• Relapse is possible; some patients may require definitive therapy such as radioactive iodine or surgery
• No monitoring is needed after achieving euthyroid status
• Long-term PTU carries no risk and is preferable for lifelong therapy in all cases

Correct Answer: Relapse is possible; some patients may require definitive therapy such as radioactive iodine or surgery

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com