admittomedicine

propylthiouracil

1 min read

the niche thionamide

A second-line antithyroid drug reserved for specific high-stakes situations due to its risk of severe hepatotoxicity. Preferred only in 1st Trimester Pregnancy, Thyroid Storm, and minor reactions to Methimazole.

  • Mechanism:
    1. Inhibits Thyroid Peroxidase (TPO) (blocks synthesis).
    2. Inhibits Type 1 Deiodinase: Blocks peripheral conversion of T4 T3 (rapidly lowers active hormone).
  • Dosing:
    • Hyperthyroidism: 100–300 mg PO divided TID (short half-life).
    • Thyroid Storm: Loading dose 500–1000 mg, then 250 mg q4h.
  • PK: Half-life: 1–2 hours (requires TID dosing). High protein binding (less placental transfer than MMI, though clinical relevance debated).

indications

  1. Hyperthyroidism in Pregnancy (Trimester 1): Reduced teratogenicity compared to Methimazole.
  2. Thyroid Storm: Benefit of blocking T4 T3 conversion.
  3. Methimazole Intolerance: Minor reactions (rash) only. Do not switch if agranulocytosis occurred.

adverse effects

  • Black Box Warning: Severe Liver Injury/Acute Liver Failure. Can be fatal. Monitor LFTs (though sudden onset makes monitoring imperfect).
  • Agranulocytosis: Similar risk to Methimazole.
  • ANCA Vasculitis: Associated with long-term use.

related pages: Methimazole, Thyroid Storm, Hyperthyroidism in Pregnancy

Backlinks