Propylthiouracil
Brand names: PTU
Class: 🦋 Thyroid & Antithyroid
When a vignette puts you on a hyperthyroid pregnant patient in early pregnancy, propylthiouracil is the answer. PTU inhibits thyroid peroxidase and additionally blocks peripheral conversion of T4 to T3 — useful in thyroid storm. The trimester rule is the high-yield AANP point: PTU is preferred in the first trimester to avoid methimazole embryopathy, then switch back to methimazole in the second and third trimesters because PTU carries a higher risk of severe hepatotoxicity and acute liver failure. Other indications are thyroid storm (where the rapid T4-to-T3 block matters) and methimazole intolerance. Both PTU and methimazole carry agranulocytosis warnings — fever and sore throat means stop and check a CBC.
✅ Indications
Hyperthyroidism in 1st-trimester pregnancy, thyroid storm, methimazole intolerance.
⚙️ Mechanism of Action
Thioamide — inhibits thyroid peroxidase AND peripheral T4→T3 conversion.
📏 Dosing
50–150 mg PO TID.
🚫 Contraindications
Severe hepatic disease, 2nd-3rd trimester pregnancy (switch to methimazole).
⚠️ Adverse Effects
Hepatotoxicity (BLACK BOX — hepatic failure), agranulocytosis, ANCA vasculitis, rash.
🔬 Monitoring
LFTs, CBC, TSH/free T4.
💎 Board Pearls
- 🤰 ONLY used in 1st-trimester pregnancy (avoids aplasia cutis risk of methimazole).
- 🔥 Preferred in THYROID STORM (blocks both synthesis AND T4→T3 conversion).
- ⚠️ BLACK BOX hepatic failure — monitor LFTs.
Practice Questions
A 42-year-old woman with untreated Graves disease is diagnosed with thyroid storm (temperature 40.1°C, heart rate 158 bpm, delirium, vomiting) after a recent viral illness and is started on a beta-blocker, iodine (given one hour after antithyroid therapy), and IV hydrocortisone. Which antithyroid agent is PREFERRED as part of her initial regimen?
Related Drugs in This Class
- Levothyroxine — Synthroid, Levoxyl, Tirosint
- Methimazole — Tapazole
Sources
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