An antithyroid medicine
Propylthiouracil
An antithyroid medicine that lowers the production of thyroid hormones in an overactive thyroid.
What is Propylthiouracil?
Propylthiouracil is an antithyroid medicine used to treat an overactive thyroid gland (hyperthyroidism) by reducing how much thyroid hormone the body makes. It is often chosen in early pregnancy and in severe flares such as thyroid storm. Its most important safety point is to report a sore throat, fever or mouth ulcers urgently, as these can signal a rare but serious drop in white blood cells.
Education and reference only. This is a plain-language guide to Propylthiouracil — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Propylthiouracil is one of the two main antithyroid medicines used in the UK, alongside carbimazole. It treats an overactive thyroid (hyperthyroidism), for example in Graves' disease, by reducing the amount of thyroid hormone the gland produces. It is often reserved for particular situations — such as early pregnancy, severe thyroid overactivity (thyroid storm), or when carbimazole cannot be used — rather than being everyone's first choice. Treatment may be for a fixed course or to prepare for other treatments such as surgery or radioactive iodine.
How it works
Propylthiouracil blocks an enzyme the thyroid gland uses to build thyroid hormones, so the gland makes less of them. It also reduces the conversion in the body of the main thyroid hormone into its more active form, which can help bring symptoms under control quickly in severe overactivity. Because hormone already made and stored is still released for a while, it can take some weeks for thyroid levels and symptoms to settle.
Company & origin
Originated / developed by: Available as a generic medicine from several manufacturers..
One of the older antithyroid drugs, propylthiouracil has been used in the UK for many decades to treat overactive thyroid.
What it treats
Conditions Propylthiouracil is used for
Practical use
How to take Propylthiouracil
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it regularly each day, usually in divided doses, at about the same times.
- Keep taking it even once you feel better — your thyroid levels are checked to guide treatment.
- Seek urgent advice if you get a sore throat, fever or mouth ulcers and ask for a blood test.
- Report yellow skin or eyes, dark urine or severe tummy pain promptly, as these can signal a liver problem.
- If a dose is missed, take it when you remember unless the next is near, then skip it — do not double up.
Weighing it up
Advantages & disadvantages of Propylthiouracil
Advantages
- Effectively brings down an overactive thyroid and controls symptoms.
- Often preferred in early pregnancy and in severe overactivity such as thyroid storm.
- Can be used to prepare patients for surgery or radioactive iodine treatment.
Disadvantages
- Carries a rare but serious risk of a sharp drop in white blood cells (agranulocytosis).
- Has a higher risk of liver toxicity than carbimazole, so is not usually first choice outside pregnancy.
- Usually needs taking more than once a day and several weeks before the full effect is felt.
Practical use
Good to know
The single most important thing to know is to seek urgent medical advice if you develop a sore throat, fever, mouth ulcers or any sign of infection — these can rarely signal a serious fall in white blood cells (agranulocytosis), and a blood test is needed. Propylthiouracil can also affect the liver, so report yellowing of the skin or eyes, dark urine, severe tummy pain or unusual tiredness. It is taken regularly each day, and it can take a few weeks before you feel the full benefit.
Who should not take it / use with caution
- People who have previously had a severe blood or liver reaction to propylthiouracil.
- People with current serious liver disease, unless a specialist advises otherwise.
- Used with caution and specialist input in pregnancy and breastfeeding, and at the lowest effective amount.
Monitoring
- Thyroid blood tests to guide treatment
- Liver blood tests, especially if symptoms suggest a liver problem
- A full blood count if infection, sore throat or fever develops
Side effects
- Rash, itching, joint aches or mild stomach upset, especially early on.
- Rarely, a serious fall in white blood cells — sore throat, fever or mouth ulcers need urgent assessment.
- Rarely, liver problems — report yellowing skin or eyes, dark urine, pale stools or severe tummy pain.
Key interactions
- As thyroid levels fall, the dose of blood thinners such as warfarin may need adjusting.
- Thyroid changes can also affect medicines such as beta-blockers and digoxin.
- Tell your team about all your medicines so monitoring can be adjusted.
Available as: Tablets.
Answers
Propylthiouracil: frequently asked questions
What should I do if I get a sore throat or fever?
Seek urgent medical advice the same day and ask for a blood test. Although uncommon, propylthiouracil can rarely cause a serious drop in infection-fighting white blood cells, and a sore throat, fever or mouth ulcers can be the warning sign.
Why might I take propylthiouracil instead of carbimazole?
Propylthiouracil is often preferred in early pregnancy and in severe thyroid overactivity (thyroid storm), or when carbimazole has caused problems. Outside these situations, carbimazole is usually first choice because propylthiouracil carries a higher risk of liver effects.
How long until I feel better?
It can take a few weeks, because the thyroid still releases hormone it has already stored. Your symptoms and blood tests are reviewed to adjust treatment, and a beta-blocker is sometimes added short-term to ease symptoms in the meantime.
Can I take it in pregnancy?
It is one of the antithyroid medicines used in pregnancy, often particularly in the first three months, under specialist supervision. Never adjust or stop thyroid treatment in pregnancy without advice.
Will I be on it for life?
Not necessarily. It may be given as a course of one to two years, after which some people stay well, while others need further treatment such as radioactive iodine or surgery. Your specialist will discuss the best long-term plan.
The wider class
About Antithyroid drugs
Propylthiouracil belongs to the antithyroid drugs class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF: Propylthiouracil.
- NICE CKS: Overactive thyroid (hyperthyroidism).
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