Propylthiouracil
A thionamide kept for early pregnancy, thyroid storm and people who cannot take thiamazole, because it can cause severe liver injury.
Also called PTU (abbreviation).
Drug card
- Class
- Thionamide
- How it works
- Blocking thyroid peroxidase, so the thyroid makes less T4 and T3
- Used for
- Hyperthyroidism when thiamazole is not tolerated
- Preparing for thyroid surgery or radioiodine
- Thyroid storm
- Known for
- Severe liver injury, even liver failure
- Agranulocytosis, usually in the first 3 months
- Its name
- -uracil means an uracil derivative (anticancer or antithyroid)
Study it as a flashcard, and it comes back before you forget it.
Mechanism of action
How does propylthiouracil work?
A thiouracil that interferes with peroxidase-driven oxidation of iodine in the thyroid, so less T4 and T3 is made. It also inhibits type I 5'-deiodinase, which converts T4 to the more active T3 in the tissues; this extra action is why it can help in thyroid storm.
Like other thionamides: They inhibit thyroid peroxidase, the enzyme that attaches iodine to tyrosine residues in thyroglobulin and couples them into T4 and T3, so less new hormone is made. Hormone already stored or circulating is not touched, so the full effect takes weeks. Propylthiouracil also blocks the conversion of T4 to T3 in the tissues.
Uses
What is propylthiouracil used for?
- Hyperthyroidism when thiamazole is not tolerated
- Preparing for thyroid surgery or radioiodine
- Thyroid storm
Side effects
Side effects of propylthiouracil
- Severe liver injury, even liver failureKnown forSerious
- Agranulocytosis, usually in the first 3 monthsKnown forSerious
- ANCA-associated vasculitisSerious
- Aplastic anemia and low plateletsSerious
- Stevens-Johnson syndrome and TENSerious
- Nephritis and glomerulonephritisSerious
- Underactive thyroid from over-treatment
- Rash and hives
- Nausea and vomiting
- Joint pain
- Loss or change of taste
- Hair loss
Known for marks what an exam expects you to know it for.
Watch for
Cautions and what sets it apart
- Boxed warning: severe liver injury and liver failure, sometimes fatal; routine liver tests do not reliably prevent it.
- Not recommended for children unless thiamazole is not tolerated and surgery or radioiodine are unsuitable.
- Pregnancy: may be preferred in or shortly before the first trimester; switching to another drug afterwards may be advisable.
- Sore throat or fever: check for agranulocytosis at once and stop the drug if it is confirmed.
Good to know: It also blocks T4-to-T3 conversion in the tissues, but its boxed warning for liver failure keeps it for early pregnancy, thyroid storm and thiamazole intolerance.
Remember it
A way to remember it
PTU for the first trimester and thyroid storm; carbimazole or thiamazole for everyone else, and any sore throat means a blood count.
Propylthiouracil carries a boxed warning for liver failure, so it is kept for early pregnancy (when carbimazole and thiamazole are linked to birth defects) and thyroid storm, where its block on T4-to-T3 conversion helps. All three can cause agranulocytosis.
Written for biofideX
Its name
-uracil means uracil derivative (anticancer or antithyroid)
Fluorouracil, an anticancer antimetabolite, and propylthiouracil, an antithyroid drug, share the ending. WHO splits them into -racil and -thiouracil.
- Every drug name stem
Its class
Thionamides
Other thionamides with a card:
Easily mixed up with thyroid hormones.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.
Thyroid, steroids and bones
Propylthiouracil is one of 9 drugs practiced together here: their classes, how they work, what they treat and what they're known for.
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