Side Effects of Thyroid Medication in Cats

If you’re a cat owner who has recently started treating your feline friend for hyperthyroidism, you’ve probably been introduced to the world of cat thyroid medications. One such widely used drug is methimazole. As effective as these medications may be, it’s vital to understand the potential side effects they may pose.

Understanding Feline Hyperthyroidism

Before diving into the side effects, let’s briefly touch on hyperthyroidism. It’s a condition resulting from an overactive thyroid gland, leading to an excessive production of thyroid hormones. Common symptoms include weight loss, increased appetite, excessive thirst and urination, nervousness, and hyperactivity. This condition often affects older cats, but with proper treatment, they can lead a comfortable life.

Methimazole: A Key Player in Hyperthyroidism Management

Methimazole is a prevalent medication used to treat feline hyperthyroidism. By reducing the production of thyroid hormones, methimazole helps manage the symptoms and improves the quality of life for affected cats. However, as with any medication, it can come with potential side effects.

Unpacking the Side Effects of Methimazole

Gastrointestinal Distress

One of the most common side effects associated with methimazole is gastrointestinal upset. Cats may experience vomiting, diarrhea, and reduced appetite, usually within the first month of starting the medication. Monitoring your cat’s eating habits and litter box activity can help you spot these issues early.


Your once vibrant and playful feline friend may exhibit signs of lethargy while on methimazole. This is a standard response to the medication as the body adjusts to lower thyroid hormone levels. However, prolonged lethargy should warrant a vet’s attention.

Facial Itching and Self-Excoriation

Some cats may develop facial itching and self-excoriation, an uncommon but notable side effect of methimazole. This typically manifests as scratching or rubbing their face and neck excessively, potentially leading to skin damage.

Blood Dyscrasias and Hepatopathy

Less frequently, methimazole may cause blood dyscrasias, which can affect granulocytes and platelets. Hepatopathy or liver disease may also occur. Regular blood tests and liver function monitoring are essential to promptly identify and address these potential complications.

Frequently Asked Questions

1. How Long Can a Cat Stay on Thyroid Medication?

The duration for which a cat remains on thyroid medication is largely individual and depends on the nature of their hyperthyroidism. Some cats may need lifelong medication. Methimazole is designed for long-term use, and many cats live healthy lives for years while taking it. Regular vet visits are necessary to monitor your cat’s response and adjust the dosage if required.

2. What Happens If I Stop Giving My Cat Thyroid Medication?

Abrupt discontinuation of your cat’s thyroid medication can cause a rebound of hyperthyroid symptoms, which may be more severe than before. Symptoms like rapid weight loss, increased appetite, and excessive thirst and urination may return. Additionally, untreated hyperthyroidism can lead to severe complications such as hypertension and heart disease. Therefore, it’s crucial never to stop the medication without consulting your vet.

3. How Do I Know If My Cat’s Thyroid Medication is Working?

Determining whether the thyroid medication is working involves monitoring your cat for changes in hyperthyroid symptoms and regular veterinary check-ups. Decreased appetite, weight stabilization, reduced thirst and urination, and a return to normal behavior may suggest that the medication is working. However, blood tests are the most accurate way to assess the medication’s effectiveness by measuring your cat’s thyroid hormone levels.

4. Is Thyroid Medication Toxic to Cats?

Thyroid medication like methimazole is generally safe for cats when used under veterinary supervision. However, as with any drug, there’s a potential for side effects, some of which may be severe. Regular monitoring and dosage adjustments can help mitigate these risks. If your cat exhibits any unusual behaviors or signs of illness while on the medication, contact your vet immediately.

5. Can I Give My Cat Methimazole Once a Day?

The dosage and frequency of methimazole administration depend on the severity of your cat’s hyperthyroidism and their response to the medication. Some cats may do well with once-daily dosing, while others may require the dosage to be split into two administrations per day. Your vet will guide you on the best regimen for your feline friend.

6. What are the Symptoms of Too Much Methimazole in Cats?

Overdosing on methimazole can lead to symptoms of hypothyroidism since the medication’s role is to decrease the production of thyroid hormones. These symptoms may include lethargy, hair loss, weight gain, and cold intolerance. If your cat exhibits these signs or any other unusual behaviors, it’s important to reach out to your veterinarian promptly.

7. How is Methimazole Administered to Cats?

Methimazole is typically administered orally in either pill or liquid form. Some cats may resist oral administration, so a transdermal gel applied to the skin of the inner ear is also available. Your vet can guide you on the best method for your cat, considering their preferences and your comfort with administering the medication.

8. Does Methimazole Cure Hyperthyroidism in Cats?

Methimazole does not cure hyperthyroidism but manages the condition by reducing the production of thyroid hormones. This helps to control the symptoms and improve your cat’s quality of life. A cure for hyperthyroidism would involve addressing the underlying cause, such as removing the affected thyroid gland or treating with radioactive iodine, but these approaches come with their own sets of benefits and risks.

9. Can Methimazole be Used Concurrently with Other Medications in Cats?

Generally, methimazole can be used alongside other medications. However, it may interact with certain drugs such as beta-blockers and digoxin, altering their effects. Therefore, it’s vital to inform your vet about all the medications your cat is currently taking, including over-the-counter drugs and supplements.

10. Can Hyperthyroidism Return After Stopping Methimazole Treatment?

Yes, hyperthyroidism can return after stopping methimazole treatment since the medication manages the condition rather than curing it. Methimazole suppresses the overproduction of thyroid hormones but doesn’t eliminate the root cause, such as an overactive thyroid gland. Thus, if the medication is discontinued, the symptoms of hyperthyroidism may reappear.

11. Can Hyperthyroidism in Cats Cause Kidney Problems?

Yes, hyperthyroidism can mask kidney problems in cats because the increased metabolic rate associated with the condition can improve kidney function. Once a cat starts on thyroid medication, this metabolic rate decreases, potentially unmasking underlying kidney disease. This is why vets often perform kidney function tests before and after starting thyroid medication.

12. Are There Any Natural Remedies for Hyperthyroidism in Cats?

While some natural supplements are marketed as remedies for feline hyperthyroidism, their efficacy and safety are not as well-studied as conventional treatments like methimazole. Any potential natural remedy should not replace veterinary-prescribed treatment but can potentially be used as a supportive measure. Always consult your vet before starting your cat on any new supplement or treatment.

13. How Does a Vet Test for Hyperthyroidism in Cats?

A diagnosis of hyperthyroidism is typically made based on a combination of clinical signs and blood tests. The blood tests measure the levels of thyroid hormones (T3 and T4) in the blood. In some cases, additional tests like thyroid scans or ultrasounds may be used to visualize the thyroid gland.

14. Can a Cat Overdose on Methimazole?

Yes, like any medication, it is possible for a cat to overdose on methimazole. This typically happens if the cat is given too high a dose. Signs of overdose may include vomiting, loss of appetite, bleeding tendencies, and signs of hypothyroidism like lethargy and cold intolerance. If you suspect your cat has overdosed on methimazole, contact your vet immediately.

15. Does Methimazole Cause Weight Gain in Cats?

Methimazole itself does not cause weight gain. However, as the medication controls the overactive thyroid and the cat’s metabolic rate returns to normal, your cat may start to regain the weight lost due to hyperthyroidism. This is usually a healthy weight gain. However, significant or rapid weight gain should be reported to your vet.

16. Is Methimazole the Only Treatment for Feline Hyperthyroidism?

While methimazole is a commonly used medication for feline hyperthyroidism, it isn’t the only treatment available. Surgical removal of the thyroid gland (thyroidectomy) or radioactive iodine therapy can also be options. However, these treatments might have their own set of benefits and risks. Your vet can discuss the best course of treatment for your cat based on their health and specific needs.

17. Can Methimazole Be Given Once a Day to Cats?

Methimazole is usually given twice a day to maintain steady levels of the drug in the bloodstream. However, the exact dosage and frequency depend on your cat’s individual case and your vet’s recommendation. Always follow your vet’s advice regarding the medication schedule.

18. What Happens If I Miss a Dose of My Cat’s Thyroid Medication?

If you miss a dose of your cat’s thyroid medication, give it as soon as you remember. However, if it’s close to the time for the next dose, skip the missed dose and continue with the regular schedule. Never double up on a dose. If you frequently miss doses, discuss this with your vet to explore solutions.

19. Can Thyroid Medication for Cats Be Used in Dogs?

While both cats and dogs can have thyroid problems, their conditions are typically opposite. Cats often develop hyperthyroidism (overactive thyroid), while dogs often develop hypothyroidism (underactive thyroid). Therefore, a medication that works for feline hyperthyroidism like methimazole could have harmful effects on a dog. Never use your cat’s medication for your dog without consulting your vet.

20. How Long Will My Cat Need to Take Methimazole?

In most cases, if methimazole is chosen as the treatment route for your cat’s hyperthyroidism, it will likely need to be a lifelong therapy. This is because methimazole manages the symptoms of hyperthyroidism but doesn’t cure the condition. Always follow your vet’s instructions regarding the duration of medication use.

Leave a Reply

Your email address will not be published. Required fields are marked *

Back to Top