Indications
Carbimazole is used to treat an overactive thyroid gland (hyperthyroidism), including Graves’ disease and toxic nodular goitre. It is also used to bring thyroid levels under control before thyroid surgery or radioactive iodine treatment.
How It Works
Carbimazole is changed in the body into Methimazole (Thiamazole), which blocks an enzyme the thyroid gland needs to make thyroid hormones. As hormone levels fall over several weeks, symptoms such as fast heartbeat, weight loss, sweating, shakiness and anxiety improve. It does not affect hormone already stored, so the full effect takes 4 to 8 weeks.
Dosage and Administration
This is a hormone treatment, and the dose is set by the doctor, as prescribed, based on thyroid blood tests. Adults commonly start with 20 mg to 60 mg daily, then the dose is lowered to a maintenance dose of about 5 mg to 15 mg daily; treatment often lasts 12 to 18 months. Sometimes a higher dose is combined with Levothyroxine (the block and replace method). Take at about the same time each day.
Interactions
- Warfarin’s effect may change as thyroid levels return to normal; INR checks are needed.
- Doses of Digoxin, Theophylline and beta blockers such as Propranolol may need adjustment as thyroid function changes.
- Prednisolone and Erythromycin may be affected; tell your doctor about all medicines.
Contraindications
- Allergy to Carbimazole or Thiamazole.
- Previous severe blood disorder (for example agranulocytosis) or inflammation of the pancreas caused by these medicines.
- Severe liver disease.
Side Effects
Common effects include nausea, headache, mild stomach upset, itching, rash, joint pain and altered taste. Hair thinning can occur. A rare but serious effect is a sudden drop in white blood cells (agranulocytosis), which lowers resistance to infection. Liver problems and pancreatitis are also rare.
Stop taking it and get urgent medical help, with a blood count, if you develop a sore throat, mouth ulcers, fever, chills or any sign of infection; also for yellowing of the skin or eyes, dark urine, severe abdominal pain, unusual bruising or bleeding, or a severe allergic reaction.
Pregnancy and Lactation
Carbimazole can harm the developing baby, especially in the first 3 months, and should be avoided early in pregnancy where possible; Propylthiouracil is often preferred then. Women who could become pregnant should use effective contraception and tell their doctor at once if they become pregnant. Low doses may be used during breastfeeding with monitoring of the baby’s thyroid, on medical advice.
Precautions and Warnings
Regular thyroid blood tests are needed. Know the warning signs of infection described above and act on them quickly. Tell your doctor about liver disease or a large goitre pressing on the windpipe. Do not stop treatment without medical advice.
Overdose
Overdose may cause nausea, vomiting, headache and, with longer use, an underactive thyroid. Seek medical advice.
Therapeutic Class
Antithyroid medicine (thionamide).
Storage
Store below 30°C in a dry place, away from light and out of the reach of children.
Prepared by the Oshud Alo team from standard drug references. For information only; always follow your doctor's advice.