Methimazole for Hyperthyroidism in Cats: What to Expect
Methimazole (Felimazole) is a medication that can treat hyperthyroidism in cats. Learn how methimazole works, the side effects, and how it’s given to cats.
methimazole
Used for Graves' Disease, Hyperthyroidism
Used for Graves' Disease, Hyperthyroidism
Methimazole is an antithyroid medication. It's used to treat adults and children with hyperthyroidism (high thyroid hormone levels). This medication comes as a tablet. At the start, you usually take your dose by mouth 3 times a day. Then, your prescriber can work with you to find a dosing plan that works for you. Side effects can include rash, nausea, drowsiness, and headache. Although the brand name Tapazole is no longer sold, generic methimazole tablets are still available.
What is Methimazole?
What is Methimazole used for?
- Hyperthyroidism related to:
- Graves’ disease
- Overactive thyroid nodules that can't be treated with thyroid surgery or radioactive iodine
- Preparation for thyroid surgery or radioactive iodine treatment
How Methimazole works
Methimazole is an antithyroid medication. With hyperthyroidism, your thyroid makes too much thyroid hormones. These hormones play a role in controlling your body temperature, energy, and how your organs work.
Methimazole works by stopping your thyroid from making too much thyroid hormones. This helps bring your hormone levels down. It can help with symptoms like muscle weakness, trouble tolerating heat, a fast or uneven heartbeat, and an enlarged thyroid. It can also help improve your energy.
When does Methimazole start working?Time passed since treatment started:
Drug facts
| Common Brands | Tapazole |
|---|---|
| Drug Class | Antithyroid agent |
| Controlled Substance Classification | Not a controlled medication |
| Generic Status | Lower-cost generic available |
| Availability | Prescription only |
Side effects of Methimazole
The following side effects may get better over time as your body gets used to the medication. Let your healthcare provider know immediately if you continue to experience these symptoms or if they worsen over time.
Common Side Effects
- Nausea (5-10%)
- Vomiting (5-10%)
- Stomach upset (5-10%)
- Rash or hives (5-10%)
- Itchiness (5-10%)
Less Common Side Effects
- Drowsiness
- Headache
- Muscle pain
- Joint pain
- Loss of taste
- Unusual hair loss
- Vertigo (feeling like your surroundings are spinning)
- Numbness or tingling
- Low white blood cells
- Anemia (low red blood cells)
- Low platelets
Methimazole serious side effects
Contact your healthcare provider immediately if you experience any of the following.
- Very low white blood cells: fever, fatigue, chills, sore throat
- Inflamed blood vessels (vasculitis): rash, nerve pain, chest pain, stomach pain
- Unusual bleeding or bruising: bleeding or bruising that doesn’t go away, black stools, blood in urine
- Liver problems: pain in the upper right side of your stomach, loss of appetite, yellowing of your skin or the whites of your eyes (jaundice)
When do Methimazole possible side effects begin and end?
Time passed since treatment started:
H
D
W
M
Raised, itchy rash
Nausea
Stomach upset
Risk of low white blood cells
Seek immediate medical care if you experience this side effect.
If a line continues to the right, you'll likely keep experiencing this side effect once it starts.
The following Methimazole side effects have also been reported
Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.
Check with your doctor immediately if any of the following side effects occur:
Incidence not known
Black, tarry stools
bleeding gums
bleeding under the skin
bloody or cloudy urine
burning, crawling, itching, numbness, prickling, "pins and needles", or tingling feelings
chills
dark urine
difficulty in breathing
dizziness or lightheadedness
drowsiness
feeling of constant movement of self or surroundings
general feeling of discomfort, illness, or weakness
headache
hoarseness
itching, skin rash
light-colored stools
loss of appetite and weight
loss of consciousness
lower back or side pain
painful or difficult urination
pinpoint red spots on the skin
redness, soreness, or itching skin
sensation of spinning
soreness of the muscles
sores, ulcers, or white spots on the lips or in the mouth
sores, welts, or blisters
stomach pain
swelling of the face, feet, or lower legs
swollen or painful glands
swollen salivary glands
swollen, painful, or tender lymph glands in the neck, armpit, or groin
tightness in the chest
unusual bleeding or bruising
unusual tiredness or weakness
unusual weight gain
upper right abdominal pain
yellow eyes or skin
Some side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:
Incidence not known
Abnormal loss of hair
changes in skin coloring
difficulty in moving
joint pain or swelling
loss of taste
muscle pain, cramps, or stiffness
pain or discomfort in the chest, upper stomach, or throat
Other side effects not listed may also occur in some patients. If you notice any other effects, check with your healthcare professional.
Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.
Get your GoodRx coupon
$14.29
GoodRx discount
$12.05
Pros and cons of Methimazole
Pros
- Maintenance dose usually taken once a day
- Might work better at improving thyroid hormone levels than propylthiouracil (PTU)
- Less risk of serious liver damage than other antithyroid medications
Cons
- Need to get blood tests often to check thyroid hormone levels for dose adjustments
- Might raise your risk of infection and easy bleeding
- Not safe to take during the first trimester of pregnancy
Pharmacist tips for Methimazole
- When you first start methimazole, you usually take it 3 times a day. Once your thyroid hormone levels become more stable, you might be able to take it once a day. Your prescriber can help you choose a schedule that works best for you.
- If needed, you can cut methimazole tablets in half. This can help if your dose is a half tablet or if you have trouble swallowing pills whole. The tablets are scored (grooved) in the middle to help guide you and make it easier to split. Use a pill splitter to cut tablets safely. Ask your prescriber if you have more questions.
- It can sometimes take up to several weeks to see your thyroid hormone levels improve after starting methimazole. And how fast your symptoms get better depends on how severe your condition is. Try to be patient if you don’t feel better right away after starting. Keep taking methimazole as prescribed. Talk to your prescriber if you have concerns about how well it's working.
- Methimazole can sometimes upset your stomach. If this happens, be sure to stay hydrated. Eating smaller portions and taking methimazole with food might help calm your stomach.
- Tell your care team if you feel tired, depressed, constipated, cold when others don't, or have dry skin. These can be signs of low thyroid hormone levels (hypothyroidism). Methimazole can sometimes lower your hormone levels too much. Your prescriber will check your levels with a blood test and adjust your dose if needed.
- Methimazole can sometimes lower your platelets and make it easier for you to bleed or bruise. Be careful and avoid activities that could make you bruise or get hurt. Get medical help right away if you notice blood in your urine or stool, black stools, or any unusual bleeding or bruising. These can be signs of more serious bleeding.
- If possible, limit or avoid eating foods high in iodine. Examples include some dairy products, iodized salt, fish, and seaweed. Your thyroid uses iodine to make thyroid hormones. So having too much iodine in the body can increase thyroid hormone production. This does the opposite of what methimazole is trying to do (lessen how much thyroid hormone there is).
- How long you take methimazole depends on why you need it and how your body responds to it. Most adults with Graves' disease take it for 12 to 18 months. Your care team will check your hormone levels with blood tests. This helps them decide when it's safe to try stopping or if you should keep taking the medication.
- Get your blood tests done on time during treatment. You'll need regular tests to check your thyroid hormone levels. This helps your prescriber see how well methimazole is working and if your dose needs to change. You might also need tests to check your blood cell counts, liver, and how well your blood clots. These tests help keep you safe.
- If you get pregnant or are trying to get pregnant: Tell your prescriber right away. Methimazole isn’t recommended during the first trimester since it can harm your baby. But it can be used with care in the second and third trimesters. Your prescriber can make sure your treatment plan is safe during pregnancy.
Risks and warnings for Methimazole
Methimazole can cause some serious health issues. This risk may be even higher for certain groups. If this worries you, talk to your doctor or pharmacist about other options.
Harm to an unborn baby during the first trimester of pregnancy
- Risk factors: Currently pregnant or thinking about becoming pregnant
Methimazole can harm an unborn baby if taken during the first trimester of pregnancy. In rare cases, it can cause birth defects.
Because of this risk, methimazole isn’t recommended for use early in pregnancy. If you’re pregnant or planning to become pregnant, talk to your prescriber right away. They can suggest safer options. If you and your prescriber decide to continue with methimazole, you should take the lowest dose possible.
Very low white blood cell count
Methimazole can lower your white blood cell counts, which can sometimes be life-threatening. Having a very low white blood cell count can put you at risk of getting sick more easily.
Look out for a sore throat, fever, or any other signs of infection while taking methimazole. If this happens, call your care team right away. They can check if your white blood cell count is low. They can also check if you have an infection and help treat it if needed.
Liver damage
Some people taking methimazole have had liver damage. The risk appears to be lower than with other antithyroid medications, especially in children.
Still, watch for symptoms of liver problems while taking methimazole. This can include yellowing of your skin or the whites of your eyes, pain in the upper right side of your stomach, nausea, and loss of appetite. If this happens, tell your care team right away. Your prescriber might order blood work to check your liver. If the problem is serious, you might need to stop the medication.
Hypothyroidism (low thyroid hormone levels)
Methimazole works by stopping your thyroid gland from making thyroid hormones. Sometimes, this can make your thyroid hormone levels too low.
Low thyroid hormone levels, or hypothyroidism, can cause symptoms like fatigue, dry skin, trouble tolerating cold, or constipation. Tell your care team if you have these symptoms. They can check your thyroid hormone levels with a blood test. If they're too low, your prescriber can adjust your methimazole dose so your levels get back in the normal range.
Thickening of blood vessels (vasculitis)
Although very rare, some people taking methimazole have had inflamed blood vessels. This can lead to serious complications. In some cases, this problem got better after stopping the medication. But in other more serious cases, people needed treatment, like steroids.
Symptoms can depend on which blood vessels are affected. But they can include rashes, chest pain, trouble breathing, stomach pain, or discolored urine. If you notice any unusual symptoms while taking methimazole, tell your prescriber. They can help see what might be going on.
Methimazole dosage
The average cost for 60 tablets of 5mg of Methimazole is $12.05 with a free GoodRx coupon. This is 15.68% off the average retail price of $14.29.
Interactions between Methimazole and other drugs
Methimazole may interact with certain medications or supplements. Always let your doctor and pharmacist know about any other medications or supplements (including prescribed and over-the-counter medications, vitamins, and dietary or herbal supplements) that you are currently taking. The list below does not include all possible drug interactions with Methimazole. Please note that only the generic name of each medication is listed below.
Although certain medicines should not be used together at all, in other cases two different medicines may be used together even if an interaction might occur. In these cases, your doctor may want to change the dose, or other precautions may be necessary. When you are taking this medicine, it is especially important that your healthcare professional know if you are taking any of the medicines listed below. The following interactions have been selected on the basis of their potential significance and are not necessarily all-inclusive.
Using this medicine with any of the following medicines is usually not recommended, but may be required in some cases. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.
- Bupropion
- Tegafur
Using this medicine with any of the following medicines may cause an increased risk of certain side effects, but using both drugs may be the best treatment for you. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.
- Acenocoumarol
- Anisindione
- Dicumarol
- Phenindione
- Phenprocoumon
- Warfarin
How much does Methimazole cost?
To access savings, use a GoodRx coupon and pay just a fraction of the retail price.
Methimazole (generic)
$12.05
What are alternatives to Methimazole?
There are a number of medications that your doctor can prescribe in place of Methimazole. Compare a few possible alternatives below.
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