Mechanism of Action
Indications
- For the treatment of hyperthyroidism and thyrotoxicosis. It is also used to prepare patients for thyroidectomy.
- Medical therapy for hyperthyroidism typically involves either titrating the dose of carbimazole until the patient becomes euthyroid or maintaining a high dose of carbimazole to suppress endogenous thyroid production and then replacing thyroid hormone with levothyroxine (“block and replace”). Treatment is usually given for 18–24 months followed by a trial withdrawal.[3]
- The onset of the anti-thyroid effect is rapid but the onset of clinical effects on thyroid hormone levels in the blood is much slower. This is because the large store of pre-formed T3 and T4 in the thyroid gland and bound to thyroid binding globulin (99% bound) has to be depleted before any beneficial clinical effect occurs.
Contraindications
- Known hypersensitivity to Carbimazole or any component of the formulation.
- During lactation (unless the potential benefit outweighs the risk).
- History of severe reactions to other thionamides.
- Agranulocytosis (a severe reduction in a specific type of white blood cell) secondary to Carbimazole or other thionamides.
- Patients with bone marrow suppression.
- Severe liver disorders.
- Concurrent use with radioiodine (increases the risk of hypothyroidism).
- Neonates (due to the potential for a rare but serious liver disorder).
- History of aplastic anemia (bone marrow failure) with other antithyroid drugs.
- In patients scheduled for iodine 131 therapy.
Dosages:
Dosage varies based on the severity of hyperthyroidism, patient response, and individualized treatment plans. Generally:
- Initial dose: 20-60mg daily in divided doses.
- Maintenance dose: 5-20mg daily.
Side Effects
- Nausea
- Headache
- Joint pain
- Skin rash
- Pruritus (itching)
- Urticaria (hives)
- Alopecia (hair loss)
- Agranulocytosis (rare but serious)
- Thrombocytopenia (reduced platelet count)
- Leucopenia (reduced white blood cell count)
- Hepatitis (inflammation of the liver)
- Jaundice (yellowing of the skin and eyes)
- Nephritis (inflammation of the kidneys)
- Taste disturbances
- Abdominal pain
- Vomiting
- Fever
- Swollen lymph nodes
- Arthralgia (joint pain)
- Neutropenia (reduced neutrophil count)
- Myalgia (muscle pain)
- Paresthesia (tingling or prickling sensation)
- Vertigo
- Drowsiness
- Edema (swelling)
- Sialadenitis (inflammation of the salivary glands)
- Vasculitis (inflammation of blood vessels)
- Peripheral neuropathy (nerve damage)
- Loss of sense of smell
- Anemia
Drug Interactions:
Here are 30 potential drug interactions:
- Other antithyroid drugs
- Beta-blockers (e.g., propranolol)
- Warfarin (increased anticoagulant effect)
- Digoxin (possible increased serum levels)
- Theophylline (elevated serum levels in hyperthyroidism)
- Erythromycin (increased effect of antithyroid drugs)
- Phenobarbital (decreased effect of antithyroid drugs)
- Corticosteroids (risk of hypokalemia)
- Amiodarone (contains iodine, may alter thyroid function)
- Diazoxide (increased effect)
- Chlorpropamide (increased effect)
- Insulin (dosage may need adjustment)
- Furosemide (high dose, increased risk of hypotension)
- Lithium (can impair thyroid function)
- Cytotoxic drugs (increased bone marrow suppression risk)
- Anticoagulants (enhanced effect)
- Adrenergic antagonists
- Anion exchange resins
- Histamine H2 antagonists
- Phenytoin
- Antidepressants (TCAs)
- Potassium iodide
- Radiocontrast agents
- Glucocorticoids (long-term therapy)
- Antipsychotics
- Rifampicin
- Bile acid sequestrants
- Beta-adrenergic agonists
- Antihypertensive agents
- Sulfonylureas
Pregnancy and Lactation
Use during Lactation
Carbimazole is not approved for marketing in the United States by the U.S. Food and Drug Administration, but is available in other countries. Doses of carbimazole of 30 mg daily or 50 mg weekly have not adversely affected the few breastfed infants studied. Carbimazole is a prodrug for methimazole which has been studied extensively during breastfeeding; maternal methimazole therapy does not affect thyroid function or intellectual development in breastfed infants with doses up to 20 mg daily. Some experts now recommend that methimazole should be considered the antithyroid drug of choice in nursing mothers.
The American Thyroid Association recommends only monitoring infants for appropriate growth and development during routine pediatric health and wellness evaluations and routine assessment of serum thyroid function in the child is not recommended. Rare idiosyncratic reactions (e.g., agranulocytosis) might occur, and the infant should be watched for signs of infection. Monitoring of the infant’s complete blood count and differential is advisable if there is a suspicion of a drug-induced blood dyscrasia.
Breastfed Infants
Eleven mothers were taking oral carbimazole in dosages ranging from 5 to 20 mg daily during pregnancy and 5 to 15 mg daily during breastfeeding (extent not stated). None of the 12 infants, including one set of twins, had a serum thyroxine (T4) concentration below the lower limit of normal on day 4 of life. Thyrotropin (TSH) concentrations were normal in all infants when measured at various times over the first 21 days postpartum.
Four women were receiving 10 to 20 mg of carbimazole daily. Infant blood samples were obtained on days 4, 7, 10, and 42 and at 3 and 6 months postpartum. Thyroid function was normal in 3 infants. In one infant, the TSH was elevated for the first 10 days of life.
A mother with twins began taking carbimazole 30 mg daily 2 months postpartum. The dosage was lowered as she became euthyroid. The infants were breastfed (extent not stated) and clinical and laboratory examinations were performed over the following 4 months. There was no evidence of alterations in thyroid function.
Fifteen mothers received carbimazole 10 to 20 mg daily for 12 to 40 weeks during pregnancy and 9 continued to take the drug during 2 to 26 weeks of lactation. Their infants were monitored for up to 18 months. Infant thyroid function was within nornml over this period, the range of mean values for individual infants being TSH 1.4 to 5.9 millliunits/L, free T3 6.2 to 9.3 pmol/L and T4 104 to 189 nmol/L. Physical examination at intervals for 2 to 18 months was normal in all infants who received carbimazole and Griffiths mental development scales were normal in all 6 infants who were assessed at 18 months.
A mother was taking carbimazole 50 mg once weekly during pregnancy and postpartum. Her infant was exclusively breastfed for the first 84 days of life and had clinical and laboratory examinations performed over the first 4 months of life. Although the infant’s tone and deep tendon reflexes were slightly increased and she was easily irritable, serum thyroid hormone levels were normal as was her growth. No symptoms or signs of hypothyroidism were observed.
FAQ
Common questions about carbimazole
How does carbimazole work?
When you have an overactive thyroid, also known as hyperthyroidism or thyrotoxicosis, your thyroid gland produces too much of the thyroid hormones. When these hormone levels are too high, you can have mood swings and weight loss.
Your thyroid gland uses a chemical called iodine to produce these hormones. Carbimazole blocks the way your body processes iodine and reduces the amount of thyroid hormones produced. This can then help your symptoms.
How long does carbimazole take to work?
Carbimazole starts working straight away, but it can take 1 to 2 months before your symptoms improve and you start to feel better.
How long will I take carbimazole for?
Once your thyroid hormone levels are under control, your dose of carbimazole may be gradually lowered and then stopped. Depending on your condition, you may need to take carbimazole for 12 to 18 months.
Some people need to continue taking the medicine for several years, possibly for the rest of their life.
However, if carbimazole is not working for you, your doctor may suggest alternatives such as radioactive iodine treatment or surgery to remove part or all of your thyroid.
Is carbimazole safe to take for a long time?
Yes, it’s safe to take carbimazole for a long time, even many years.
It’s important to have regular blood tests during treatment, to make sure your dose is not too high or too low. Your doctor will make sure you are on the lowest dose that works for you.
Your doctor will be able to explain the risks and benefits of taking carbimazole, compared with the risks of not having treatment for your overactive thyroid.
What will happen if I stop taking carbimazole?
Do not stop taking carbimazole without speaking to your doctor first.
If you stop taking your medicine, your thyroid hormone levels will rise and you will start to feel ill again. This may not happen straight away. It takes time for your thyroid hormone levels to become too high and for your symptoms to come back.
Are there other medicines for an overactive thyroid?
The main treatments for an overactive thyroid are:
- medicines such as carbimazole and propylthiouracil
- a combination of medicines such as carbimazole (for overactive thyroid) and levothyroxine (for underactive thyroid) to help find the right hormone balance (block and replace regime)
- radioactive iodine treatment
- surgery
You will usually be referred to a specialist in hormonal conditions (endocrinologist). They will be able to help you manage your condition and discuss the best treatment option for you.
Can I drink alcohol with it?
Yes, you can drink alcohol while taking carbimazole. Alcohol does not affect how this medicine works.
However, if carbimazole makes you feel dizzy then it’s best to avoid alcohol as it can make this side effect worse.
Is there any food or drink I need to avoid?
Carbimazole is not usually affected by what you eat or drink.
However, to keep your thyroid healthy, your doctor may suggest a low-iodine diet. This is because the thyroid gland needs iodine to make the thyroid hormones. Eating foods that are high in iodine can make your symptoms worse. Your doctor may recommend avoiding processed foods, cheese, eggs, and salt with added iodine (iodised salt).
If carbimazole makes you feel dizzy, it’s best to avoid alcohol, coffee, and other drinks containing caffeine as these can also make the dizziness worse.
Do I need to avoid people with viruses while I’m taking it?
Generally, there is no problem with being around other people, including someone with a virus, when taking carbimazole.
However, there’s one uncommon side effect of carbimazole, which may lower the number of white cells in your blood. This can make you more likely to get an infection.
Tell your doctor if you think you are getting a sore throat or if you have a high temperature.
Will it make me lose or put on weight?
One of the symptoms of an overactive thyroid gland is weight loss. Once you start taking carbimazole, you will stop losing weight as your hormone levels stabilise.
Once you are on the right dose of carbimazole, and your thyroid hormone levels have returned to normal, the treatment will no longer affect your weight.
Do I need to stop taking carbimazole before surgery or before tests?
No, carry on taking carbimazole, following the instructions that come with your medicine.
If you need to make any changes, a doctor will talk to you about it before the surgery.
Will it affect my contraception?
It’s important to use reliable contraception whilst taking carbimazole. You can discuss contraception with your doctor, sexual health or family planning clinic.
Carbimazole will not affect any contraception, including the combined pill and emergency contraception.
If you are vomiting or have severe diarrhoea for more than 24 hours, your contraceptive pills may not protect you from pregnancy. Look at the leaflet that comes with your contraceptive pills to find out what to do.
Will it affect my fertility?
There’s no clear evidence to suggest that taking carbimazole will reduce fertility in either men or women.
However, speak to your doctor if you’re trying to get pregnant as it’s important not to get pregnant while you’re taking carbimazole. Use contraception until your medicine has been reviewed.
Can I drive or ride a bike?
For most people, carbimazole does not affect their ability to drive or ride a bike.
However if carbimazole makes you feel dizzy or unable to concentrate, do not drive a car, cycle, use tools or operate machinery until you feel OK again.
What else can I do for an overactive thyroid?
In addition to taking carbimazole, there are some lifestyle changes that can help with your symptoms. These include weight management, exercise and dealing with stress.
Taking carbimazole will stop the weight loss caused by an overactive thyroid. Your doctor may give you a meal plan to help you gain or lose weight while taking this medicine.
They may also recommend a low iodine diet. This is because the thyroid gland needs iodine to make thyroid hormones. Eating foods that are high in iodine can make your symptoms worse. This may mean avoiding processed foods, cheese, eggs and salt with added iodine (iodised salt).
Regular exercise and managing stress can help with other symptoms such as anxiety, irritability, difficulty sleeping and hyperactivity.
Dr. MD Harun Ar Rashid, FCPS, MD, PhD, is a highly respected medical specialist celebrated for his exceptional clinical expertise and unwavering commitment to patient care. With advanced qualifications including FCPS, MD, and PhD, he integrates cutting-edge research with a compassionate approach to medicine, ensuring that every patient receives personalized and effective treatment. His extensive training and hands-on experience enable him to diagnose complex conditions accurately and develop innovative treatment strategies tailored to individual needs. In addition to his clinical practice, Dr. Harun Ar Rashid is dedicated to medical education and community outreach, often participating in initiatives that promote health awareness and advance medical knowledge. His career is a testament to the high standards represented by his credentials, and he continues to contribute significantly to his field, driving improvements in both patient outcomes and healthcare practices.
