Thiamazole (Methimazole)

Thiamazole (Methimazole) In Dogs & Cats: Uses, Dosage and Side Effects

Overview

• Interferes with the synthesis of thyroid hormones by inhibiting peroxidase-catalysed reactions (blocks oxidation of iodide), the iodination of tyrosyl residues of thyroglobulin, and the coupling of mono- or di-iodotyrosines to form T3 and T4.
• Has no effect on iodine uptake, peripheral de-iodination of T4 to T3 or release of stored hormone.

Uses of Thiamazole (Methimazole)

• Control of thyroid hormone levels in cats with hyperthyroidism.
• Generally, 2 to 3 weeks of treatment are required to establish euthyroidism.
• Monitor therapy based on serum thyroxine concentrations (4–6 hours after dosing) and adjust dose accordingly for long-term management.
• Assess hematological and biochemical parameters, along with serum total T4 levels, after 3, 6, 10, and 20 weeks, then every 3 months thereafter, adjusting dosage as necessary.
• Transdermal thiamazole gels are also used, especially in fractious cats or those experiencing GI side effects from oral formulations. However, this route is not as reliable or safe for humans applying the gel.

Dose of Thiamazole (Methimazole) in Dogs and Cats

Dogs:

• Hyperthyroidism: 2.5–5 mg/dog p.o. q12h depending on size.

Cats:

• Hyperthyroidism: 2.5 mg/cat p.o. q12h.
• Apply transdermal gel to pinna.

Side Effects of Thiamazole (Methimazole) in Dogs and Cats

• Vomiting and inappetence/anorexia may occur, typically transiently.
• Rare but reported adverse effects include jaundice, cytopenias, immune-mediated diseases (such as myasthenia gravis), and dermatological changes (like pruritus, alopecia, and self-induced trauma); withdrawal of treatment may be necessary.
• Treatment of hyperthyroidism can reduce glomerular filtration rate, leading to elevated serum urea and creatinine values, and occasionally revealing occult renal failure.
• Animals reacting adversely to carbimazole are likely to react similarly to thiamazole.

Contraindications of Thiamazole (Methimazole) in Dogs and Cats

• Do not use in pregnant or lactating queens.

Some Notes:

• Phenobarbital may reduce clinical efficacy.
• Benzimidazole drugs reduce hepatic oxidation and may lead to increased circulating drug concentrations.
• Thiamazole should be discontinued before iodine-131 treatment.
• Do not use with low iodine prescription diets.