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Methimazole

Methimazole

Lauren A. Trepanier , DVM, PhD, DACVIM, DACVCP, University of Wisconsin–Madison

Pharmacology & Medications

|May 2015|Peer Reviewed

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Methimazole is a popular antithyroid drug used for treating hyperthyroid cats in the U.S., particularly when radioiodine is not readily available or is cost prohibitive.

Overview

  • Methimazole is approved for use in animals (Felimazole; dechra-us.com) and humans.
  • Methimazole compounded with pluronic lecithin organogel (PLO) is one of few veterinary drugs with demonstrated efficacy when administered transdermally.
    • Concentration: 50 mg/mL
    • Starting dose: 2.5 mg/cat q12h

Toxicities

  • Dose-dependent
    • In 10%–20% of cats treated with oral methimazole, mild-to-moderate vomiting, diarrhea, and decreased appetite typically developed during the first 4 weeks of treatment.1,2
      • GI signs are significantly less common in cats receiving transdermal treatment than in those receiving oral methimazole.1
    • Cats often develop mild increases in BUN and creatinine with treatment to the euthyroid state.3
      • Low urine specific gravity and high serum thyroxine (T4) concentrations may increase risk for posttreatment development of azotemia,4 although cats with highly concentrated urine can still be at risk.5
      • Serum T4 concentrations should be targeted to the mid-normal range, as overtreatment to low serum T4 can worsen azotemia and lead to shortened survival times.6
  • Idiosyncratic
    • Acute, apparently nondose-dependent (ie, idiosyncratic) toxicities can develop at 1–4 weeks of treatment and typically include
      • Facial excoriation around the neck and pinnae, blood dyscrasia (eg, neutropenia, thrombocytopenia), and new hepatopathy
      • Leukopenia resulting from only lymphopenia does not indicate methimazole discontinuation.
    • Idiosyncratic hepatopathy is typically a mixed pattern (ie, with elevations in both hepatocellular and cholestatic enzymes) and may involve hyperbilirubinemia.
      • Liver enzyme activity should be compared with values obtained before treatment, as many hyperthyroid cats have elevated ALT and/or ALP at diagnosis.7
        • These should resolve with treatment.
    • Cats may develop myasthenia gravis, characterized by neuromuscular weakness and positive acetylcholine receptor autoantibodies during the first few months of treatment8,9; however, this is rare.

Management of Adverse Events

Methimazole is one of few veterinary drugs with demonstrated efficacy in a compounded formulation for transdermal administration.

  • For simple GI upset without biochemistry abnormalities, discontinue methimazole until signs resolve.
    • Restart at a 50% dose reduction or switch to transdermal methimazole.1
  • Idiosyncratic toxicities fail to respond to dose reduction.
    • Discontinue methimazole and schedule alternative treatment (eg, radioiodine, Hill’s Prescription Diet y/d Feline Thyroid Health [hillsvet.com], thyroidectomy).
      • For facial excoriation and if pruritus is severe, consider short-term antiinflammatory doses of prednisolone.
      • For blood dyscrasia, evaluate for fever or bruising.
        • Neutropenia and thrombocytopenia will typically resolve after drug discontinuation without additional intervention.10
        • In cases of severe neutropenia (ie, <1000–1500 µL), antibiotics (eg, amoxicillin–clavulanate) may be indicated.
        • Recheck CBC 1 week after discontinuation.
      • For hepatopathy, consider short-term treatment with glutathione precursor (eg, S-adenosylmethionine [SAMe])
        • Recheck liver enzyme activity 1–2 weeks after discontinuation.
      • For myasthenia gravis, consider pyridostigmine treatment.
        • Follow clinical response and acetylcholine receptor antibody titers after discontinuation.

Monitoring

  • Clinical monitoring by owners is important, as toxicities can develop between routine rechecks.
  • Rechecks at 2 and 4 weeks after treatment initiation should be sufficient to determine euthyroidism and presence of toxicity.
    • Along with monitoring serum T4 concentrations and general clinical status, cats should be monitored for
      • New azotemia via BUN, creatinine, and urine specific gravity
      • Idiosyncratic toxicity via CBC and liver enzyme activities
  • Once euthyroid state reached, routinely (q3–6mo) check renal values, blood pressure, and serum T4 concentrations.

References and Author Information

For global readers, a calculator to convert laboratory values, dosages, and other measurements to SI units can be found here.

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