Hyperthyroidism questions love to test decision-making, not memorization: Which antithyroid drug is preferred here—and why? This post gives you a step-by-step flowchart for methimazole vs PTU, plus the highest-yield one-liners, toxicities, and USMLE traps.
Big picture: what these drugs do (USMLE one-liner)
Thioamides (methimazole, PTU) inhibit thyroid hormone synthesis by blocking thyroid peroxidase (TPO) → prevents iodination and coupling of tyrosine residues on thyroglobulin.
PTU also blocks peripheral conversion of (via 5’-deiodinase inhibition).
Mnemonic (visual):
- “Methimazole = Most of the time”
- “PTU = Pregnancy (1st trimester) + thyroid storm + Peripheral conversion block”
Step-by-step flowchart (shareable)
1) Are you treating hyperthyroidism/thyrotoxicosis?
- Yes → go to step 2
- No → thioamides not indicated
2) Is this thyroid storm (life-threatening thyrotoxicosis)?
Clues: fever, tachycardia/AFib, delirium, diarrhea, heart failure (often after surgery, infection, iodinated contrast)
➡️ Choose: PTU (first)
Because it blocks TPO + blocks conversion (fastest “T3-lowering” of thioamides).
Storm regimen (classic USMLE order):
- PTU
- β-blocker (e.g., propranolol; also decreases peripheral conversion)
- Iodide (SSKI/Lugol) after thioamide (prevents hormone release)
- Glucocorticoids (decrease conversion; treat relative adrenal insufficiency)
- Supportive care + treat trigger
If not storm → step 3
3) Is the patient pregnant?
- 1st trimester ➜ PTU preferred
- Why: methimazole has classic teratogenicity in early pregnancy
- 2nd/3rd trimester ➜ switch to methimazole
- Why: PTU has higher risk of severe hepatotoxicity
If not pregnant → step 4
4) Is the patient breastfeeding?
- Generally methimazole preferred (commonly used; compatible at typical doses)
- PTU is less favored long-term due to liver risk (but may still be used in select cases)
(USMLE angle: they won’t usually force nuanced lactation dosing—just know pregnancy trimester logic + PTU liver toxicity.)
5) Default choice for most nonpregnant patients
➡️ Methimazole
Reason: longer half-life, once-daily dosing, and less hepatotoxic than PTU.
Quick compare table (high-yield)
| Feature | Methimazole | PTU (propylthiouracil) |
|---|---|---|
| Mechanism | Inhibits TPO (organification + coupling) | Inhibits TPO + blocks |
| Best use | Most cases of Graves/hyperthyroidism | Thyroid storm, 1st trimester pregnancy |
| Dosing | Often once daily | More frequent dosing |
| Major toxicity to remember | Teratogenic (1st tri): aplastic cutis, choanal/esophageal atresia | Severe hepatotoxicity (black box vibe for USMLE) |
| Shared serious AE | Agranulocytosis (both) | Agranulocytosis (both) |
| Other classic AE | Rash, arthralgias | Can cause ANCA-positive vasculitis (more classically PTU) |
Toxicities & how they show up on exams
1) Agranulocytosis (both drugs) — the “stop immediately” toxicity
Presentation: fever, sore throat, infections, mouth ulcers
Action:
- Stop the drug immediately
- Check CBC with differential
- Manage infections; consider G-CSF if severe
USMLE trap: If a stem says “hyperthyroid patient on methimazole now has fever and sore throat,” the next step is discontinue + CBC, not “reassure.”
2) Hepatotoxicity (PTU > methimazole)
Presentation: fatigue, RUQ pain, jaundice, dark urine, elevated AST/ALT
Action: stop PTU; evaluate liver injury
Why it matters: This is the reason you switch PTU → methimazole after 1st trimester.
3) Teratogenicity (methimazole in 1st trimester)
Buzzwords: aplasia cutis, choanal atresia, esophageal atresia
Rule: PTU in 1st trimester, then methimazole afterward.
Mini “one-liners” you can paste into your notes
- Methimazole = first-line thioamide for most hyperthyroid patients (better dosing, less severe liver toxicity).
- PTU = preferred in thyroid storm because it also blocks conversion.
- Pregnancy: PTU in 1st trimester, methimazole in 2nd/3rd.
- Fever + sore throat on thioamide = agranulocytosis until proven otherwise → stop drug + CBC.
Rapid-fire USMLE-style checkpoints
- Graves disease: thioamides decrease synthesis; definitive therapy can be radioiodine or surgery depending on context.
- Iodide therapy: give after thioamide in thyroid storm to avoid providing substrate for new hormone synthesis (classic sequencing question).
- β-blockers: symptomatic control (tremor, palpitations); propranolol also decreases peripheral conversion.
Pocket mnemonic (final recap)
“M is for Most; P is for Pregnancy (first) + Periphery + Panic (storm).”
- Methimazole: most cases
- PTU: Pregnancy (1st trimester), Peripheral block, Panic-level thyroid storm