Antithyroid medication: the basics

What antithyroid medicines do

When the thyroid is overactive - as in Graves' disease or a toxic nodular goitre - it produces more T4 and T3 than the body needs. Antithyroid drugs work by blocking the enzyme (thyroid peroxidase) that the thyroid uses to manufacture these hormones. They do not treat the underlying cause, but they bring hormone levels back towards normal while other decisions are made.

They take effect gradually - most people notice an improvement in symptoms over several weeks, and it may take two to three months for levels to fully normalise. Blood tests are used to monitor progress.

The three antithyroid drugs

Carbimazole is the main antithyroid drug used in the UK and is usually the first choice for adults with hyperthyroidism. It is taken as a tablet, typically once or twice daily. Read more in the carbimazole lexicon entry.

Methimazole is the active form of carbimazole (carbimazole is converted into methimazole in the body). It is used more widely in countries outside the UK and works in the same way.

Propylthiouracil (PTU) is generally reserved for specific situations in the UK, most importantly the first trimester of pregnancy - when carbimazole is avoided because of a small risk to the developing baby. PTU also has a small risk of liver toxicity, which is why it is not the first choice outside pregnancy. Read more in the propylthiouracil lexicon entry.

How long is a course?

This varies with the cause and the individual. For Graves' disease, a typical course of carbimazole aimed at achieving remission lasts 12 to 18 months. During this time, doses are adjusted based on blood tests. Some people go on to achieve lasting remission after stopping; others relapse and need to consider a more definitive treatment such as radioactive iodine or surgery. Your specialist team will review the options with you based on how your levels respond.

Common questions

Will I feel better quickly? Some symptoms - such as palpitations and anxiety - may ease within weeks as levels begin to fall. Full normalisation typically takes a few months.

Do I need regular blood tests? Yes. In the early months, blood tests are usually done every four to eight weeks to check levels and guide any dose adjustments. How often you need monitoring long-term is something your team will advise on.

Can I miss doses? Try not to. Missing doses or stopping suddenly is not recommended while your levels are being controlled. If you are struggling to take your medication consistently, speak to your pharmacist or clinician - there may be practical solutions.

What about pregnancy? If you are taking an antithyroid drug and become pregnant, or are planning a pregnancy, speak to your specialist as soon as possible. The choice between carbimazole and PTU - and at what dose - requires individual specialist input and cannot be generalised here.

An important safety warning: agranulocytosis

This is something every person taking carbimazole or PTU needs to know.

Rarely, antithyroid drugs can cause agranulocytosis - a serious drop in the white blood cells that fight infection. It is an uncommon side effect, but it can develop rapidly, even in people who have been taking the medicine without problems for months.

This is the safety instruction your prescribing team should have given you when you started. If you develop a sudden sore throat, fever, mouth ulcers, or signs of infection, you should:

• Stop taking your antithyroid medication immediately
• Seek urgent medical attention - go to A&E, call 999 or 111, or contact your specialist team
• Tell the team you are taking carbimazole or PTU so they can arrange an urgent blood count

This is not a sign to watch and wait. The vast majority of sore throats have nothing to do with agranulocytosis, but the consequences of missing it are serious, so urgent assessment is always the right call.

Your prescribing team should give you written information about this when you start antithyroid medication. If you did not receive it, ask at your next appointment.

Questions to ask your clinician

• How often will my levels be tested, and what am I aiming for?
• When would you consider whether I have gone into remission?
• What are my options if carbimazole doesn't bring my levels under control?
• What should I do if I feel my symptoms are getting worse between appointments?
• I've noticed [symptom] - is this something to raise with you now or at my next appointment?

The questions tool can help you prepare your list before an appointment. And if you are ever uncertain whether a symptom needs urgent attention, when to seek urgent help gives a broader guide.