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Methimazole is a human drug, used for the medical management
for hyperthyroidism. Methimazole inhibits the synthesis of thyroid hormones
by interfering with metabolic steps involving iodine incorporation, and
the formation of iodothyronine. Methimazole has no effect on pre-existing circulating
or stored thyroid hormones and it has no effect on supplemented thyroid
hormones.
Methimazole is currently the drug of choice for the treatment of feline hyperthyroidism. It has largely replaced propylthiouracil for this purpose due to the lower incidence of adverse side effects. Treatment with methimazole is a form of medical management for hyperthyroidism; it does not cure the condition.
Cats being treated with methimazole should be closely
monitored for the first three months. They should receive a CBC, platelet
count, serum T4 every 2 to 3 weeks; liver function and ANA testing may
be performed as needed. Endogenous circulating T4 levels should be reduced
in 1-3 weeks. After the animal is stabilized on the medication, T4 levels
should be followed every 3 to 6 months.
• Adverse side effects due to methimazole use usually occur within the first three months of therapy.
• Common: transient GI disturbance including anorexia, vomiting
and depression. Methimazole is a very bitter medication. Different formulations
of the medication are available and giving the medication with food may
improve may improve palatability. Transient hematologic disturbances (eosinophilia,
leukopenia, lymphocytosis) are seen in about 15% of cats. These usually
occur within the first 2 months of therapy. Approximately 50% of animals
receiving methimazole for greater than 6 months develop a positive ANA.
These cats may require a dose reduction.
• Rare: Within the first few weeks of treatment, a small percentage
of cats self mutilate their face and neck through scratching. These animals
will probably need to discontinue treatment with Methimazole. The most serious side effects
include bleeding, thrombocytopenia, agranulocytosis, and hepatopathy.
These animals should discontinue treatment and will probably require supportive
therapy. Acquired myasthenia gravis has been reported.
• Methimazole should be used with caution and with extra monitoring in cats with liver disease, autoimmune disease or pre-existing blood abnormalities.
• Individual response to methimazole may vary. Regular monitoring of T4 levels is necessary in order to avoid drug-induced hypothyroidism.
• Methimazole readily crosses the placenta and is excreted in milk. Kittens out of queens receiving this drug may be born hypothyroid. They should go on milk replacement after receiving colostrum.
• No reported drug interactions in the cat. In humans, methimazole
is reported to potentiate the activity of some anti-coagulants.
• Overdose of Methimazole will increase the likelihood and seriousness of the above-mentioned side effects.
Dr.
Barbara Forney is a veterinary practitioner in Chester County, Pennsylvania.
She has a master's degree in animal science from the University of Delaware
and graduated from the University of Pennsylvania School of Veterinary Medicine
in 1982.
She began to develop her interest in client education and medical writing 1997. Recent publications include portions of The Pill Book Guide to Medication for Your Dog and Cat, and most recently Understanding Equine Medications published by the Bloodhorse.
Dr. Forney is an FEI veterinarian and an active member of the AAEP, AVMA, and AMWA.
You can purchase books by Dr. Forney at www.exclusivelyequine.com
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