METHIMAZOLE
Chemistry - A thioimidazole-derivative antithyroid drug, methimazole occurs as a white to palebuff crystalline powder, having a faint characteristic odor and a melting point of 144 -147°C. It isfreely soluble (1 gram in 5 ml) in water or alcohol.
Uses, Indications - Methimazole is considered by most clinicians to be the agent of choice whenusing drugs to treat feline hyperthyroidism. Propylthiouracil has significantly higher incidences ofadverse reactions when compared to methimazole.
L/kg). After oral dosing, the plasma elimination half-life ranges from 2.3 - 10.2 hours. There isusually a 1-3 week lag time between starting the drug and significant reductions in serum T4.
Methimazole apparently concentrates in thyroid tissue.
Contraindications/Precautions - Methimazole is contraindicated in patients who are hypersensitive to it. It should be used very cautiously in patients with a history of or concurrent hematologicabnormalities, liver disease or autoimmune disease.
High levels of methimazole cross the placenta and may induce hypothyroidism in kittens born ofqueens receiving the drug. Levels higher than those found in plasma are found in human breastmilk. It is suggested that kittens be placed on a milk replacer after receiving colostrum frommothers on methimazole.
Overdosage - Acute toxicity that may be seen with overdosage include those that are listed aboveunder Adverse Effects. Agranulocytosis, hepatopathy, and thrombocytopenias are perhaps the mostserious effects that may be seen. Treatment consists of following standard protocols in handling anoral ingestion (empty stomach if not contraindicated, administer charcoal, etc.) and to treatsymptomatically and supportively.
Storage, Stability, Compatibility
Methimazole tablets should be stored in well-closed, light-resistant containers at room temperature.Pharmacology - METHIMAZOLE
Methimazole interferes with iodine incorporation into tyrosyl residues of thyroglobulin, thereby inhibiting the synthesis of thyroid hormones. It also inhibits iodinated tyrosylresidues from coupling to form iodothyronine. Methimazole has no effect on the release or activityof thyroid hormones already formed or in the general circulation.Uses, Indications - Methimazole is considered by most clinicians to be the agent of choice whenusing drugs to treat feline hyperthyroidism. Propylthiouracil has significantly higher incidences ofadverse reactions when compared to methimazole.
Pharmacokinetics - METHIMAZOLE
Information on the pharmacokinetics of methimazole in cats is available(Trepanier, Peterson, and Aucoin 1989). These researchers reported that in normal cats, thebioavailability of the drug is highly variable (45-98%), as is the volume of distribution (0.12 - 0.84L/kg). After oral dosing, the plasma elimination half-life ranges from 2.3 - 10.2 hours. There isusually a 1-3 week lag time between starting the drug and significant reductions in serum T4.
Methimazole apparently concentrates in thyroid tissue.
Contraindications/Precautions - Methimazole is contraindicated in patients who are hypersensitive to it. It should be used very cautiously in patients with a history of or concurrent hematologicabnormalities, liver disease or autoimmune disease.
Adverse Effects, Warnings
Approximately 15-20% of cats treated with methimazole developadverse effects. In a study of 262 cats receiving methimazole (Peterson, Kintzer, and Hurvitz 1988), the following adverse effects (% incidence) were reported: anorexia (11.1%), vomiting (10.7%), lethargy (8.8%), excoriations (2.3%), bleeding (2.3%), hepatopathy (1.5%), thrombocytopenia(2.7%), agranulocytosis (1.5%), leukopenia (4.7%), eosinophilia (11.3%), lymphocytosis (7.2%), positive ANA (21.8%), positive direct antiglobulin test (1.9%). Clinical symptoms were noted in18.3% of the cats. The gastrointestinal adverse effects generally developed within the first month oftreatment and usually resolved even with continued therapy.High levels of methimazole cross the placenta and may induce hypothyroidism in kittens born ofqueens receiving the drug. Levels higher than those found in plasma are found in human breastmilk. It is suggested that kittens be placed on a milk replacer after receiving colostrum frommothers on methimazole.
Overdosage - Acute toxicity that may be seen with overdosage include those that are listed aboveunder Adverse Effects. Agranulocytosis, hepatopathy, and thrombocytopenias are perhaps the mostserious effects that may be seen. Treatment consists of following standard protocols in handling anoral ingestion (empty stomach if not contraindicated, administer charcoal, etc.) and to treatsymptomatically and supportively.