What is carbimazole?

Carbimazole is the most commonly used drug in the UK to treat an overactive thyroid gland (hyperthyroidism).

Hyperthyroidism occurs when the thyroid gland in your neck makes too much of the hormone thyroxine, which controls your body’s metabolism.

Common symptoms include:

• sweating
• feeling too warm
• weight loss
• restlessness
• diarrhoea
• palpitations

Carbimazole works by stopping your thyroid gland from making thyroxine.

How long does it take to work?

It usually takes at least 2 weeks before you notice improvement in your symptoms.

If you have significant symptoms, your doctor may also prescribe beta blockers (for example, propranolol). These can be stopped once symptoms improve, but carbimazole should be continued.

You will have regular blood tests at clinic visits to monitor how well the treatment is working.

How long will I need treatment?

This depends on the cause of your hyperthyroidism:

• Graves’ disease (an immune condition): usually 18–24 months
• Overactive thyroid nodules (adenomas): treatment is often long-term

Stopping carbimazole too early can cause the condition to return.

Dosage

Carbimazole tablets are available in 5mg and 20mg strengths.

• Most people start on around 40mg daily (this may vary)
• The dose is gradually reduced as your thyroid levels improve

Side effects

Most people do not experience side effects.

Possible side effects include:

• rash or itching
• nausea
• indigestion
• headache

If these are persistent or troublesome, speak to your GP or hospital team.

Serious side effects (rare)

A rare but important side effect is a drop in white blood cells.

Warning signs include:

• severe sore throat
• mouth ulcers
• fever

If you develop these symptoms:

• stop taking carbimazole immediately
• contact your GP for a blood test

If the blood test is normal, it is safe to restart the medication.

Rarely, inflammation of the pancreas can occur. Symptoms include:

• abdominal pain or tenderness
• fever

If these occur:

• stop carbimazole
• seek urgent medical advice

Pregnancy and breastfeeding

• Carbimazole is not recommended in early pregnancy
• You may be switched to propylthiouracil
• Low doses may be used if breastfeeding

Your doctor will advise on the safest option.

Women of childbearing age should use effective contraception.

Other medicines

Carbimazole is generally safe with most medicines, but always check with your doctor.

What is radioiodine treatment?

Radioiodine treatment uses radioactive iodine to treat hyperthyroidism.

The treatment works by:

• destroying overactive thyroid tissue
• reducing the production of thyroxine

Only a small amount of radioactivity is needed because the thyroid gland absorbs most of the iodine.

How is radioiodine given?

Radioiodine is given as a capsule that you swallow with water.

The capsule is handled in a tube or cup to minimise direct contact.

Preparing for treatment

You may need to stop certain medications before treatment.

• Stop antithyroid tablets as instructed in your appointment letter
• Stop any iodine-containing tablets or supplements at least 1 week before treatment

It is important to follow these instructions carefully.

How long does it take to work?

• Treatment can take a few weeks to several months to take effect
• Around 80–90% of people are cured with one dose
• If needed, treatment can be repeated after 6 months

Safety

Radioiodine treatment has an excellent safety record and has been used since the 1940s.

Pregnancy and fertility

• You must not have treatment if you are pregnant or breastfeeding
• Avoid pregnancy for 6 months after treatment
• Men should not father a child for 4 months

Fertility is not affected for men or women long-term.

After treatment: radiation precautions

After treatment, your body will contain some radioactivity for a short time.

You will be given specific instructions, which may include:

General contact

• Keep a safe distance (about 1 metre) from others for prolonged periods
• Avoid close contact for more than 30 minutes per day

Children

• Avoid close contact with children:
  • under 3 years: 25 days
  • 3–5 years: 20 days
  • over 5 years: 14 days

Adults

• Avoid close close contact for 14 days

Sleeping

• Sleep in a separate bed from your partner for 14 days

Work and daily activities

• Take at least 5 days off work
• 14 days off if your job involves close contact with adults or food preparation
• 25 days off if working closely with young children or pregnant women

You can continue most day-to-day activities such as shopping and cooking.

Avoid places of entertainment for 14 days.

Travel

• You may travel home by public transport if the journey is less than 1 hour
• If travelling by car with someone else, sit diagonally opposite them
• Avoid long journeys:
  • no journeys longer than 1 hour during early days
  • avoid journeys over 3 hours during the second week

Hygiene advice

• Drink plenty of fluids
• Use the toilet frequently
• Flush twice after use
• Men should sit down to urinate
• Do not share towels

In the first 3–5 days:

• Wash crockery and cutlery thoroughly
• Take extra care with hand hygiene, especially when preparing food

Follow-up

You will need regular blood tests after treatment to monitor your thyroid function.

Side effects and results

Most people have no side effects.

Some people may have symptoms of an overactive thyroid (such as palpitations or sweating) 5–10 days after treatment.

Your doctor may:

• prescribe beta blockers
• restart antithyroid tablets temporarily

Over time, the thyroid may become underactive (hypothyroidism).

If this happens:

• you will be treated with thyroxine tablets
• these replace the hormone your body no longer produces

Results

• Around 9 in 10 people are cured with one treatment
• Up to 7 in 10 may develop an underactive thyroid over time
• This is easily treated with thyroxine

Important advice

• You will be given a treatment card — carry this with you
• It may be needed when travelling, as airport scanners can detect prior treatment