甲巰咪唑
**甲巰咪唑 (Methimazole)** 是一種硫咪唑衍生物抗甲狀腺藥物，在北美被視為**貓甲狀腺機能亢進症的黃金標準內科療法**。 主要臨床重點： - **主要用途**：控制貓的甲狀腺機能亢進。它能有效降低循環中的甲狀腺激素濃度，改善體重減輕、多食和心搏過速等臨床症狀。 - **替代給藥途徑**：對於難以餵食口服藥的貓，可將甲巰咪唑配製成**經皮吸收凝膠 (PLO gel)** 塗抹於耳廓內側，具有良好的臨床療效（雖然略低於口服），且可能減少胃腸道副作用。 - **研究性用途**：在狗中，曾被用作靜脈注射以預防順鉑 (cisplatin) 引起的腎毒性。 - **臨床要點**：甲巰咪唑僅能控制而*無法治癒*甲狀腺機能亢進。除非進行根治性治療（放射性碘或甲狀腺切除術），否則需要終身每日服藥。
作用機制: 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.
各物種劑量
- 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
- Hyperthyroidism (cats with azotemia or for clients declining radioiodine) · 1.25-5 mg per cat twice daily (start at lower end. · PO · q12h · Lifelong
- Hyperthyroidism (transdermal administration) · 2.5 mg to inner pinna q12h. · topical · q12h · Lifelong · 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 (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
- 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 · Monitor in 1-2 weeks (T4, CBC with platelet count, renal blood parameters, urinalysis).
- Hyperthyroidism · 2.5 mg/cat · PO · q12h · Long-term · 2 to 3 weeks of treatment generally needed to establish euthyroidism. Adjust dose based on T4 monitoring.
給藥途徑
禁忌症
- Hypersensitivity to methimazole, carbimazole, or polyethylene glycol
- Autoimmune disease
- Primary liver disease
- Renal failure
- Hematologic disorders or coagulopathies
- Pregnant or lactating queens
- Pregnant queens
- Lactating queens
- Animals with known hypersensitivity to thiamazole or carbimazole
不良反應
- Vomiting
- Anorexia
- Depression/Lethargy
- Transient eosinophilia, leukopenia, and lymphocytosis
- Self-induced facial excoriations (pruritus)
- Bleeding/Coagulopathies
- Hepatopathy
- Thrombocytopenia
- 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)
藥物相互作用
- Benzimidazole antiparasitics · Methimazole can reduce hepatic oxidation of benzimidazoles and increase blood levels.
- Beta-blockers · A reduction in dose may be needed when the patient becomes euthyroid.
- Bupropion · Potential for increased risk for hepatotoxicity; increased monitoring (LFT's) necessary.
- Digoxin · Methimazole may decrease digoxin efficacy, but a reduction in dose may be needed when the patient becomes euthyroid.
- Phenobarbital · Concurrent use of phenobarbital may reduce the clinical effectiveness of methimazole. · moderate
- Theophylline · A reduction in dose may be needed when the patient becomes euthyroid.
- Warfarin · Anticoagulants may be potentiated by methimazole. Increased monitoring of anticoagulant effect is warranted.
- Benzimidazoles · Reduce hepatic oxidation and may lead to increased circulating thiamazole concentrations · moderate
- Iodine-131 (Radioiodine) · Thiamazole interferes with radioiodine uptake; must be discontinued prior to I-131 treatment · major
- Low-iodine prescription diets · Concurrent use is not recommended as it confounds dietary management and iodine restriction · moderate
監測
- 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
過量
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).
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