Propylthiouracil vs Methimazole
How to tell Propylthiouracil and Methimazole apart when a board question makes them look alike.
PTU first trimester or thyroid storm; methimazole usual nonpregnant default after early pregnancy risk window.
Side by side
Preferred thionamide in the first trimester and in thyroid storm because it also blocks peripheral T4-to-T3 conversion, but severe hepatotoxicity is the concern.
Usual preferred thionamide outside early pregnancy because it is generally easier to use, but early pregnancy teratogenicity and agranulocytosis are key risks.
PTU inhibits thyroid peroxidase and peripheral conversion of T4 to T3.
Methimazole inhibits thyroid peroxidase-mediated organification and coupling.
First-trimester Graves disease, thyroid storm, or hepatotoxicity warning points PTU.
Nonpregnant Graves disease or second/third trimester switch points methimazole; aplasia cutis/embryopathy is the early-pregnancy warning.
The trap
Both are thionamides that block thyroid peroxidase and reduce thyroid hormone synthesis, and both can cause agranulocytosis (so fever and sore throat warrant urgent CBC in either drug). Three deciding axes pick between them: (1) pregnancy trimester (PTU first trimester to avoid methimazole-associated aplasia cutis and methimazole embryopathy; switch to methimazole in second trimester to reduce PTU hepatotoxicity); (2) thyroid storm (PTU is preferred because it also blocks peripheral conversion of T4 to T3 via type 1 deiodinase); (3) routine maintenance (methimazole is the nonpregnant default because of once-daily dosing and lower hepatotoxicity risk; PTU is reserved for special situations). PTU box warning is liver failure; methimazole risks include teratogenicity in early pregnancy.
The repair move
For thionamides, sort by scenario first: first trimester, thyroid storm, or routine long-term Graves therapy.
Try a free question
CoreA pregnant patient in the first trimester needs antithyroid therapy. The clinician chooses the drug with lower early teratogenic concern but plans to switch later because of maternal hepatotoxicity risk. Which drug is chosen first?
Show answer and explanation
Answer: A. Propylthiouracil. PTU is preferred in the first trimester when an antithyroid drug is needed, but hepatotoxicity limits long-term preference.
Why the other choices are tempting
- B. Methimazole is generally preferred after the first trimester but is avoided early when possible because of embryopathy risk.
- C. Levothyroxine treats hypothyroidism, not hyperthyroidism.
- D. Radioiodine is contraindicated in pregnancy.
- E. Liothyronine is T3 replacement.
Flashcards
Tap a card to flip it.
What is the fastest separator for Propylthiouracil vs Methimazole?
PTU first trimester or thyroid storm; methimazole usual nonpregnant default after early pregnancy risk window.
What clue should make you choose Propylthiouracil?
Preferred thionamide in the first trimester and in thyroid storm because it also blocks peripheral T4-to-T3 conversion, but severe hepatotoxicity is the concern.
Quick answers
What is the fastest way to tell Propylthiouracil from Methimazole?
PTU first trimester or thyroid storm; methimazole usual nonpregnant default after early pregnancy risk window.
What is the key clue for Propylthiouracil?
Preferred thionamide in the first trimester and in thyroid storm because it also blocks peripheral T4-to-T3 conversion, but severe hepatotoxicity is the concern.
What is the key clue for Methimazole?
Usual preferred thionamide outside early pregnancy because it is generally easier to use, but early pregnancy teratogenicity and agranulocytosis are key risks.
What is the common trap with Propylthiouracil vs Methimazole?
Both are thionamides that block thyroid peroxidase and reduce thyroid hormone synthesis, and both can cause agranulocytosis (so fever and sore throat warrant urgent CBC in either drug). Three deciding axes pick between them: (1) pregnancy trimester (PTU first trimester to avoid methimazole-associated aplasia cutis and methimazole embryopathy; switch to methimazole in second trimester to reduce PTU hepatotoxicity); (2) thyroid storm (PTU is preferred because it also blocks peripheral conversion of T4 to T3 via type 1 deiodinase); (3) routine maintenance (methimazole is the nonpregnant default because of once-daily dosing and lower hepatotoxicity risk; PTU is reserved for special situations). PTU box warning is liver failure; methimazole risks include teratogenicity in early pregnancy.