Indications
Methimazole (also called Thiamazole) is an antithyroid medicine 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
The thyroid gland uses iodine to make thyroid hormones. Methimazole blocks an enzyme (thyroid peroxidase) that the gland needs for this, so less new hormone is made. It does not affect hormone already stored, so improvement usually begins after 1 to 3 weeks and full effect may take 1 to 2 months.
Dosage and Administration
The dose is set by the doctor according to how overactive the thyroid is and is adjusted using regular blood tests. It is usually started at a higher dose and then reduced to a lower maintenance dose, often continued for 12 to 18 months. Take it as prescribed at the same time each day, with or without food. Do not stop or change the dose yourself.
Interactions
Methimazole can change the effect of Warfarin, so clotting tests should be checked. As thyroid levels fall, doses of Digoxin, Theophylline and beta blockers (such as Propranolol) may need adjusting. Iodine-containing products, including some contrast dyes and Amiodarone, can affect treatment. Tell your doctor about all medicines.
Contraindications
Do not use if you are allergic to Methimazole or Carbimazole, or if you have had serious blood disorders (such as agranulocytosis) or severe liver problems from it before.
Side Effects
Common effects include rash, itching, joint pain, nausea, altered taste, headache and hair thinning. Serious but rare side effects are a sharp drop in white blood cells (agranulocytosis), which lowers the ability to fight infection, liver damage, and inflammation of blood vessels.
Get urgent medical help and have a blood count checked at once if you develop sore throat, mouth ulcers, fever or other signs of infection. Also seek help for yellowing of the skin or eyes, dark urine, pale stools, severe tiredness, unusual bleeding or bruising, or a severe allergic reaction.
Pregnancy and Lactation
Methimazole can cause birth defects if used in the first 3 months of pregnancy, so another medicine (Propylthiouracil) is usually preferred early in pregnancy; the doctor may switch back later. Tell your doctor at once if you plan or suspect pregnancy. Low doses are generally considered compatible with breastfeeding, with the baby’s thyroid checked if advised.
Precautions and Warnings
Regular thyroid blood tests are needed; too high a dose can make the thyroid underactive. Tell your doctor about liver disease. Report any signs of infection promptly. People aged over 40 or on higher doses may have greater risk of blood problems.
Overdose
An overdose may cause nausea, vomiting, stomach pain, headache, fever, joint pain and, with longer use, an underactive thyroid or blood disorders. Seek medical advice.
Therapeutic Class
Antithyroid agent (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.