Carbimazole: treating an overactive thyroid

Carbimazole blocks the thyroid gland from processing iodine, which reduces how much thyroid hormone is made. The NHS explains that this eases symptoms such as weight loss, mood swings and irritability. Doctors sometimes add a beta blocker for a racing heartbeat, anxiety or irritability while carbimazole takes effect.

The NHS says you may begin to feel better after one to three weeks but full control can take one to two months. Treatment also stops the weight loss caused by the overactive thyroid; once hormone levels are normal, the tablets do not themselves affect weight.

Tablets come in several strengths, and the NHS says adult starting doses are in a range that the doctor then adjusts. Early on it is taken two to three times a day, spaced out, and later often once daily when hormone levels settle. It can be taken with or without food. The NHS advises that if you forget a dose, take it as soon as you remember, even if the next dose is due, in which case both can be taken together. One extra dose is unlikely to harm; call NHS 111 if you take two or more extra doses.

Blood tests are done before and during treatment. The NHS says they are usually about every six weeks at first and about every three months once levels are stable, and more often during pregnancy or if symptoms cause concern. Dose is gradually reduced once control is reached. You may need carbimazole for twelve to eighteen months, and some people continue for years or long term. Do not stop it before surgery or tests unless told; the NHS says doctors may ask for a pause before radioactive iodine treatment.

Common effects usually appear in the first eight weeks and ease: nausea, dizziness, headaches, joint pain, rash or itching, and hair thinning that usually reverses over months. The NHS suggests simple meals and taking tablets with food for sickness, rising slowly if dizzy, and unperfumed moisturiser for itchy skin.

Uncommon but serious effects need you to stop the tablets and contact a doctor or NHS 111 straight away, according to the NHS: high temperature, sore throat, mouth ulcers, severe tiredness or flu-like illness (possible low white cell count), easy bruising or bleeding, yellow eyes or skin or dark urine, swollen glands, muscle pain or weakness, reduced urination or blood in urine, and sudden severe stomach pain (possible pancreatitis). Call 999 for swelling of the lips or throat or difficulty breathing.

• Fever, sore throat, mouth ulcers: seek urgent medical review before the next dose.

• Sudden severe stomach pain: urgent advice.

• Allergic swelling or breathing difficulty: call 999.

The NHS says most adults, children and babies can take carbimazole. Tell your doctor about previous allergy to carbimazole or propylthiouracil, severe pancreatitis after carbimazole, planned radioactive iodine treatment, liver problems, blood disorders, an enlarged thyroid, difficulty having regular blood tests, or pregnancy, breastfeeding or plans to conceive.

The NHS says carbimazole is not often recommended in pregnancy because it can very occasionally cause problems for the baby's development in early pregnancy, so patients typically switch to propylthiouracil. If you become pregnant while taking it, carry on and speak to a doctor immediately. It is usually fine during breastfeeding in small amounts for a healthy baby, with the baby watched for feeding problems, sleepiness, irritability, rashes, constipation or frequent infections. No evidence suggests it reduces fertility, and it does not affect contraception.

The NHS says alcohol is fine though it may worsen dizziness. A low-iodine diet may be suggested, limiting processed foods, cheese, eggs and iodised salt. Do not drive if dizzy. Carbimazole is prescription only. Suspected side effects can be reported through the Yellow Card scheme.