Felimazole for Cats: Thyroid Medicine


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Frequently Asked Questions

Felimazole treats hyperthyroidism in cats, a condition in which too much thyroid hormone is produced. It will not cure the disease, but will usually control it if given for the rest of the cat's life.

Felimazole is for cats.

  • Controls the symptoms of hyperthyroidism for an improved quality of life

  • Treats hyperthyroidism without surgery or anesthesia

  • Available in a dosage size specifically for cats

Methimazole reduces the amount of thyroid hormone produced by the thyroid gland.

The generic form Felimazole is methimazole.

Felimazole tablets are given by mouth. Methimazole, the active ingredient, is very bitter. Felimazole comes as a coated tablet for ease of administration. Do not break or crush the tablet. Always follow the dosage instructions provided by your veterinarian. If you have difficulty giving the medication, contact your veterinarian. Wash hands with soap and water after giving the medication.

Blood levels of thyroid hormone will be checked by your veterinarian at regular intervals, and the dose increased or decreased as needed. The lowest effective dose should be used.

This medication should only be given to the pet for whom it was prescribed.

In most cases Felimazole will lower the level of thyroid hormone in hyperthyroid cats. It will generally take 1-3 weeks before the level of thyroid hormone produced by the body will start to decrease. Felimazole will not cure the disease, but will usually control it if given for the rest of the cat's life.

Felimazole comes in a coated tablet form.

Talk to your veterinarian about what tests and exams may be necessary while your pet is taking Felimazole. Also, discuss what type of outcome is expected. Have your veterinarian explain the other treatment options for hyperthyroidism in cats.

In addition to monitoring of thyroid hormone levels, laboratory tests to check liver and kidney function and blood cell counts may be performed before starting treatment and then regularly thereafter.

Tell your veterinarian if your pet has a blood disease, liver disease, or autoimmune disease; or may be pregnant, is nursing, or if you intend to breed your pet.

Notify your veterinarian of any other medications or supplements your pet is taking, and also if your pet has had any reactions to previous medications.

If you miss a dose, give it as soon as you remember. If it is almost time for the next dose, skip the one you missed and go back to the regular schedule. Do not give two doses at once.

Methimazole will not cure the disease, but will usually control it if given for the rest of the cat's life. Follow your veterinarian's dosage instructions carefully. Side effects are most common during the first 3 months of treatment.

Not for use in pets who are hypersensitive (allergic) to it. Use with extreme caution in pets with anemia, clotting disorders, bleeding, low white cell and platelet counts, or diseases of the immune system, liver, or kidney.

The manufacturer recommends that Felimazole should not be used in pregnant or lactating animals (females nursing their young).

Methimazole can cause birth defects. Pregnant women should be extremely cautious in handling this medication or the waste products from a cat receiving Felimazole.

Side effects tend to occur in the first three months of treatment. If you observe any of the effects listed below, contact your veterinarian.

May see changes in appetite, vomiting, diarrhea, skin lesions, lethargy, or abnormal vocalization. These usually occur within the first two weeks of treatment and may stop even with continuation of treatment.

Less common side effects include liver problems resulting in the above signs plus yellowing of the gums, skin, or eyes; itching of the face, resulting in scratching; bleeding tendencies; swollen lymph nodes; anemia; and low white blood cell counts. Your veterinarian will determine if these effects necessitate stopping the medication and treating with surgery or radioactive iodine therapy.

Treatment with methimazole may unmask hidden kidney disease.

If your pet is unusually tired, has a fever (temperature over 103°F), or shows signs of bruising or bleeding, contact your veterinarian immediately.

If your pet experiences an allergic reaction to the medication, signs may include facial swelling, hives, scratching, sudden onset of diarrhea, vomiting, shock, seizures, pale gums, cold limbs, or coma. If you observe any of these signs, contact your veterinarian immediately.

Store at room temperature in a tight, light-resistant, childproof container. Keep out of reach of children and pets.

Overdosing is more likely seen from chronic overdosing than with a single large dose. Signs may include extreme sedation, incoordination, loss of appetite, vomiting, and loose stool. If you know or suspect your pet has had an overdose, or if you observe any of these signs in your pet, contact your veterinarian immediately.

Consult your veterinarian before using Felimazole with any other medications, including vitamins and supplements, since interactions may occur. The drugs clomipramine, amitriptylline, omeprazole, and cyclophosamide may interact with methimazole and cause severe side effects.