Metimazol
Methimazole
1-methyl-1,3-dihydro-2H-imidazole-2-thione (Thiamazole)
También conocido como: Tapazole · Felimazole · Northyx · Antitiroide · Danantizol · Favistan · Mercaptizol · Metibasol · Strumazol · Thacapzol · Thycapzol · Thyrozol · Tirodril · Unimazole · Thiafeline · Thyronorm · Thiamazole · Mercazolylum · Methylmercaptoimidazole · Thiamazolum · Tiamazol · Methimazole · Mercazolil
Revisado por el equipo veterinario de VetSheetActualizado 5 abr 2026Referencia veterinaria basada en la evidencia
Las dosis son una referencia clínica para profesionales veterinarios autorizados. Confirme siempre con el prospecto actual y cada paciente individual.
Dosis por especie
🌎 NA — North America🌍 EU — Europe
Perros
| Indicación | Dosis | Vía | Frecuencia | Duración | Región |
|---|---|---|---|---|---|
| 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.
Gatos
| Indicación | Dosis | Vía | Frecuencia | Duración | Región |
|---|---|---|---|---|---|
| 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.
Las dosis son una referencia clínica para profesionales veterinarios autorizados. Confirme siempre con el prospecto actual y cada paciente individual.
Resumen
Metimazol es un fármaco antitiroideo derivado del tioimidazol y se considera el estándar de oro en el tratamiento médico del hipertiroidismo felino en Norteamérica.
Puntos clínicos clave:
- Uso principal: Manejo del hipertiroidismo en gatos. Reduce eficazmente los niveles circulantes de hormona tiroidea, resolviendo signos clínicos como pérdida de peso, polifagia y taquicardia.
- Vías alternativas: Para gatos difíciles de medicar por vía oral, el metimazol puede formularse como un gel transdérmico PLO para aplicar en la cara interna del pabellón auricular, el cual ha demostrado buena eficacia clínica (aunque ligeramente inferior a la oral) con potencialmente menos efectos secundarios gastrointestinales.
- Uso en investigación: En perros, se ha utilizado profilácticamente mediante inyección intravenosa para prevenir la nefrotoxicidad inducida por cisplatino.
- Perla clínica: El metimazol controla pero no cura el hipertiroidismo. Se requiere terapia diaria de por vida a menos que se opte por un tratamiento definitivo (yodo radiactivo o tiroidectomía).
Mecanismo de acción
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.
Seguridad y advertencias
Contraindicaciones
- 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
Efectos adversos
- 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)
Precauciones
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.
Interacciones farmacológicas
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
Monitorización
- 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
Farmacocinética
Vida media
Absorción
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.
Distribución
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.
Metabolismo
Undergoes hepatic metabolism.
Eliminación
Eliminated primarily via the kidneys and feces after hepatic metabolism.
Sobredosis
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).
Productos disponibles
Formulaciones
- 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)
Veterinario
- 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
Etiquetado para humanos
- Methimazole Tablets (plain & scored): 5 mg, 10 mg, 15 mg & 20 mg (Tapazole, Northyx, generic)
Estado regulador
POM-V classification.
POM-V classification.
Sin datos reguladores para: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Almacenamiento y estabilidad
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.
Información para el cliente
Entendiendo la medicación de su mascota
Metimazol es un medicamento diario utilizado para controlar el hipertiroidismo (una glándula tiroides hiperactiva) en gatos.
- Cómo funciona: Bloquea la producción de exceso de hormonas tiroideas, ayudando a su gato a recuperar un peso, frecuencia cardíaca y comportamiento normales.
- No es una cura: Este medicamento controla la enfermedad pero no la cura. Es probable que su gato necesite este medicamento de por vida.
- La paciencia es clave: Puede tomar de 1 a 3 semanas ver una mejoría notable en los síntomas de su gato.
Consejos de administración
- Las pastillas tienen un sabor muy amargo. No las triture a menos que se le indique, ya que su gato podría rechazar la comida con la que se mezcla.
- Si tiene dificultades para administrar la pastilla, pregunte a su veterinario sobre un gel transdérmico que se puede frotar en la parte sin pelo de la oreja de su gato.
Advertencias de seguridad
Seguridad humana: El metimazol puede afectar la función tiroidea humana y el desarrollo fetal. Las mujeres embarazadas, las mujeres que intentan concebir y las madres lactantes deben usar guantes al manipular el medicamento, la caja de arena del gato o los fluidos corporales.
Qué observar
Contacte a su veterinario inmediatamente si su gato experimenta:
- Vómitos o pérdida de apetito (especialmente en las primeras semanas)
- Letargo extremo o debilidad
- Rascado intenso en la cara o el cuello
- Hematomas o sangrado inexplicables
La referencia de medicamentos de VetSheet está destinada a profesionales veterinarios autorizados como ayuda para la toma de decisiones clínicas, no como sustituto del criterio profesional ni del prospecto actual del fabricante.