メチマゾール
Methimazole
1-methyl-1,3-dihydro-2H-imidazole-2-thione (Thiamazole)
別名: Tapazole · Felimazole · Northyx · Antitiroide · Danantizol · Favistan · Mercaptizol · Metibasol · Strumazol · Thacapzol · Thycapzol · Thyrozol · Tirodril · Unimazole · Thiafeline · Thyronorm · Thiamazole · Mercazolylum · Methylmercaptoimidazole · Thiamazolum · Tiamazol · Methimazole · Mercazolil
VetSheet 獣医チームが監修更新日 2026年4月5日エビデンスに基づく獣医学リファレンス
用量は獣医専門家向けの臨床参考資料です。必ず最新の医薬品情報および個別症例に基づいて確認してください。
動物種別用量
🌎 NA — North America🌍 EU — Europe
犬
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| As an investigative method to reduce nephrotoxicity associated with cisplatin therapy | 40 mg/kg IV over one minute prior to cisplatin. | IV | once | Prior to cisplatin | 🌎 NA |
| Hyperthyroidism | 2.5-5 mg/dog | PO | q12h | Long-term | 🌍 EU |
- As an investigative method to reduce nephrotoxicity associated with cisplatin therapy: No commercially available parenteral product in USA at time of writing.
- Hyperthyroidism: Dose depends on size.
猫
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| Hyperthyroidism | The starting dose is 2.5 mg administered every 12 hours. Following 3 weeks of treatment, the dose should be titrated to effect based on individual serum total T4 (TT4) levels and clinical response. Dose adjustments should be made in 2.5 mg increments. The maximum total dosage is 20 mg per day divided, not to exceed 10 mg as a single administration. | PO | q12h | Lifelong | 🌎 NA |
| Hyperthyroidism (cats with azotemia or for clients declining radioiodine) | 1.25-5 mg per cat twice daily (start at lower end. | PO | q12h | Lifelong | 🌎 NA |
| Hyperthyroidism (transdermal administration) | 2.5 mg to inner pinna q12h. | topical | q12h | Lifelong | 🌎 NA |
| Hyperthyroidism (gradual titration) | Initially, 2.5 mg (total dose) PO once a day for 2 weeks. If adverse reactions not noted... increase to 2.5 mg PO twice daily and the same parameters are checked in another 2 weeks. The dosage should then be increased every 2 weeks by 2.5 mg per day until serum T4 is between 13 and 26 nmol/L or adverse effects develop. | PO | q24h initially, then q12h | Lifelong | 🌎 NA |
| Hyperthyroidism (based on renal status) | If no signs of renal insufficiency/failure, begin at 5 mg (total dose) PO twice daily in cases with severely increased T4 levels. If renal insufficiency present (or not sure), start at 2.5 mg twice daily. If azotemia and overt renal failure, start at 1.25 mg twice a day. | PO | q12h | Lifelong | 🌎 NA |
| Hyperthyroidism | 2.5 mg/cat | PO | q12h | Long-term | 🌍 EU |
| Hyperthyroidism | Apply transdermal gel to pinna | Topical | q12h | Long-term | 🌍 EU |
- Hyperthyroidism (transdermal administration): Methimazole (50 mg/mL; 5 mg/0.1 mL) in PLO. Person applying should wear gloves or finger cots. Somewhat lower efficacy than PO (67% vs 82% euthyroid at 4 weeks). Lower incidence of GI effects.
- Hyperthyroidism (based on renal status): Monitor in 1-2 weeks (T4, CBC with platelet count, renal blood parameters, urinalysis).
- Hyperthyroidism: 2 to 3 weeks of treatment generally needed to establish euthyroidism. Adjust dose based on T4 monitoring.
- Hyperthyroidism: Used particularly in fractious cats or those with GI side effects. Wear gloves.
用量は獣医専門家向けの臨床参考資料です。必ず最新の医薬品情報および個別症例に基づいて確認してください。
概要
メチマゾールはチオイミダゾール誘導体の抗甲状腺薬であり、北米において猫の甲状腺機能亢進症に対する内科的治療のゴールドスタンダードとされています。
主な臨床的ポイント：
- 主な用途: 猫の甲状腺機能亢進症の管理。血中の甲状腺ホルモン濃度を効果的に低下させ、体重減少、多食、頻脈などの臨床症状を改善します。
- 代替投与経路: 経口投与が困難な猫に対しては、耳介内側に塗布する経皮吸収ゲル（PLOゲル）として調剤することが可能であり、良好な臨床効果（経口よりはやや劣る）を示し、胃腸の副作用を軽減できる可能性があります。
- 治験的用途: 犬において、シスプラチン誘発性腎毒性を予防するための静脈内注射として使用された報告があります。
- 臨床のポイント: メチマゾールは甲状腺機能亢進症を管理するものであり、完治させるものではありません。根治的治療（放射性ヨウ素治療や甲状腺摘出術）を行わない限り、生涯にわたる毎日の投薬が必要です。
作用機序
Methimazole acts directly within the thyroid gland to block the synthesis of thyroid hormones (T3 and T4).
- Mechanism: It actively inhibits the enzyme thyroid peroxidase (TPO).
- Pathway: Inhibition of TPO → prevents the oxidation of iodide to iodine → blocks the incorporation of iodine into tyrosyl residues of thyroglobulin → inhibits the coupling of iodinated tyrosyl residues to form iodothyronine (T3 and T4).
- Important Note: Methimazole has no effect on the release or activity of thyroid hormones that are already formed and stored, or those currently in systemic circulation. This accounts for the 1 to 3 week lag time often seen before serum T4 levels significantly decrease and clinical improvement is noted.
安全性・警告
禁忌
- Hypersensitivity to methimazole, carbimazole, or polyethylene glycol
- Autoimmune disease
- Primary liver disease
- Renal failure
- Hematologic disorders or coagulopathies
- Pregnant or lactating queens
- Pregnant queens
- Animals with known hypersensitivity to thiamazole or carbimazole
有害事象
- Vomiting
- Depression/Lethargy
- Transient eosinophilia, leukopenia, and lymphocytosis
- Self-induced facial excoriations (pruritus)
- Bleeding/Coagulopathies
- Agranulocytosis
- Positive direct antiglobulin test (DAT)
- Positive ANA (in chronic use >6 months)
- Acquired myasthenia gravis (rare)
- Inappetence / Anorexia
- Jaundice / Hepatopathy
- Cytopenias (thrombocytopenia, leukopenia, anemia)
- Immune-mediated diseases (e.g., myasthenia gravis)
- Dermatological changes (pruritus, facial excoriation, alopecia)
- Decreased GFR (unmasking of renal failure)
注意事項
Black Box / Handling Warning: Pregnant women, women who may become pregnant, and nursing mothers should wear gloves when handling tablets, litter, or bodily fluids of treated cats.
- Renal Unmasking: Restoring euthyroidism can decrease glomerular filtration rate (GFR) and unmask underlying chronic kidney disease in cats. Careful monitoring of renal values is essential.
- Hematologic & Hepatic Monitoring: Severe adverse effects like agranulocytosis and hepatotoxicity usually occur within the first 3 months. Routine bloodwork monitoring is critical during this phase.
- Nursing: Kittens should be placed on a milk replacer if the mother is receiving the drug, as it is excreted in milk and can cause fetal/neonatal hypothyroidism.
医薬品相互作用
Methimazole can reduce hepatic oxidation of benzimidazoles and increase blood levels.
A reduction in dose may be needed when the patient becomes euthyroid.
Potential for increased risk for hepatotoxicity; increased monitoring (LFT's) necessary.
Methimazole may decrease digoxin efficacy, but a reduction in dose may be needed when the patient becomes euthyroid.
Concurrent use of phenobarbital may reduce the clinical effectiveness of methimazole.
A reduction in dose may be needed when the patient becomes euthyroid.
Anticoagulants may be potentiated by methimazole. Increased monitoring of anticoagulant effect is warranted.
Reduce hepatic oxidation and may lead to increased circulating thiamazole concentrations
Thiamazole interferes with radioiodine uptake; must be discontinued prior to I-131 treatment
Concurrent use is not recommended as it confounds dietary management and iodine restriction
監視
- Baseline and every 2-3 weeks for the first 3 months: CBC, platelet count, Serum T4
- Liver function tests and ANA if indicated by symptomatology
- After stabilization (at least 3 months): T4 at 3-6 month intervals
- Renal parameters (BUN, Creatinine, USG) to monitor for unmasked kidney disease
- Cats receiving >10 mg/day require more frequent monitoring
- Serum total T4 (measure 4-6 hours post-pill)
- Haematology (CBC for cytopenias)
- Biochemistry (BUN, Creatinine, Liver enzymes)
- Monitor at 3, 6, 10, and 20 weeks after starting therapy, and every 3 months thereafter
薬物動態
半減期
吸収
In normal cats, oral bioavailability is highly variable (45-98%). There is usually a 1-3 week lag time between starting the drug and significant reductions in serum T4.
分布
Volume of distribution in cats is 0.12-0.84 L/kg. Methimazole actively concentrates in thyroid tissue. High levels cross the placenta and are detected in breast milk.
代謝
Undergoes hepatic metabolism.
消失
Eliminated primarily via the kidneys and feces after hepatic metabolism.
過剰投与
Acute toxicity from overdosage mirrors severe adverse effects: agranulocytosis, hepatopathy, and thrombocytopenia are the most serious concerns.
Treatment:
- Follow standard protocols for oral ingestion (empty stomach if not contraindicated, administer activated charcoal).
- Provide symptomatic and supportive care (e.g., IV fluids, hepatoprotectants, broad-spectrum antibiotics if neutropenic).
製品
製剤
- Oral tablets (sugar-coated, plain, scored)
- Transdermal gel (compounded)
- Intravenous injection (investigational/compounded)
- Oral: 1.25 mg tablet
- Oral: 2.5 mg tablet
- Oral: 5 mg tablet
- Oral: 5 mg/ml solution
- Transdermal gel (named patient basis/compounded)
動物用医薬品
- Methimazole Tablets (sugar-coated): 2.5 mg & 5 mg (Felimazole - Dechra)
- Felimazole 1.25 mg, 2.5 mg, 5 mg tablets
- Thiafeline 2.5 mg, 5 mg tablets
- Thyronorm 5 mg/ml oral solution
ヒト用医薬品
- Methimazole Tablets (plain & scored): 5 mg, 10 mg, 15 mg & 20 mg (Tapazole, Northyx, generic)
規制ステータス
POM-V classification.
POM-V classification.
規制データなし: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
保管・安定性
Store tablets in well-closed, light-resistant containers at room temperature; protect from moisture. Compounded transdermal PLO gels typically have a guaranteed stability of 2 weeks.
飼主向け情報
ペットのお薬について
メチマゾールは、猫の甲状腺機能亢進症（甲状腺の働きが活発になりすぎる病気）を管理するための毎日のお薬です。
- 作用の仕組み: 過剰な甲状腺ホルモンの生成をブロックし、猫の体重、心拍数、行動を正常に戻すのを助けます。
- 完治させる薬ではありません: この薬は病気をコントロールするものであり、完治させるものではありません。通常、生涯にわたる投薬が必要になります。
- 効果が出るまで: 症状の明らかな改善が見られるまで、1〜3週間かかることがあります。
投薬のアドバイス
- 錠剤は非常に苦い味がします。獣医師の指示がない限り、砕かないでください。食事に混ぜても猫が食べるのを拒否する可能性があります。
- 錠剤を飲ませるのが難しい場合は、猫の耳の毛のない部分に塗る経皮ゲルについて獣医師にご相談ください。
安全上の警告
飼い主様への注意: メチマゾールは人間の甲状腺機能や胎児の発育に影響を与える可能性があります。妊娠中の方、妊娠を希望されている方、授乳中の方は、錠剤、猫のトイレ砂、または治療中の猫の体液を扱う際には手袋を着用してください。
注意すべき症状
猫に以下の症状が見られた場合は、すぐに獣医師にご連絡ください：
- 嘔吐や食欲不振（特に最初の数週間）
- 極度の無気力や衰弱
- 顔や首を激しく掻く
- 原因不明のあざや出血
VetSheet医薬品リファレンスは獣医専門家の臨床意思決定支援を目的としており、専門的判断または製造業者の最新医薬品情報の代替にはなりません。