Feline Hyperthyroidism: Which Medication Is Best?

Felimazole, Thiamacare, Thyronorm, Thiafeline & Vidalta Explained

Feline hyperthyroidism is one of the most common endocrine diseases affecting older cats in the UK. In most cases, it is caused by a benign enlargement of the thyroid gland known as adenomatous hyperplasia or a thyroid adenoma.

An adenoma is a non-cancerous growth arising from glandular tissue. In hyperthyroid cats, this growth causes the thyroid gland to become enlarged and overactive. The abnormal thyroid tissue produces excessive amounts of thyroid hormone (primarily T4), independent of the body’s normal regulatory controls.

Thyroid hormones regulate metabolic rate. When levels are persistently elevated, the body enters a hypermetabolic state. This increases heart rate, raises blood pressure, accelerates weight loss despite increased appetite, and places strain on the heart, kidneys and other organs over time.

In the majority of cats, the condition is benign. Thyroid carcinoma (malignant cancer) is uncommon, accounting for a small minority of cases.

If your cat has been diagnosed with hyperthyroidism, commonly prescribed medications include Felimazole tablets for cats and Felimazole oral solution for cats, Thiamacare oral liquid, Thyronorm oral solution, Thiafeline tablets and Vidalta tablets.

This guide explains the differences between methimazole (thiamazole), carbimazole, and the main licensed UK products used to treat feline hyperthyroidism.

Methimazole vs Thiamazole: Are They Different?

No. Methimazole and thiamazole are two names for the same active drug.

  • Methimazole – name commonly used in North America
  • Thiamazole – European naming convention

Chemically and pharmacologically they are identical. Both inhibit the enzyme thyroid peroxidase, reducing the production of new thyroid hormones. If your prescription uses one term and online information uses the other, it refers to the same medication.

Methimazole (Thiamazole) Products for Cats in the UK

Felimazole (Tablets and Oral Solution)

Felimazole tablets and oral solution for cats

Active ingredient: Methimazole (Thiamazole)

Forms available: Coated tablets and oral liquid solution

Dosing: Typically twice daily

Key points:

  • Licensed veterinary medicine for feline hyperthyroidism
  • Available in both tablet and liquid formulation
  • Allows flexibility depending on patient preference

Felimazole for cats

Thiafeline Tablets

Thiafeline tablets for cats

Active ingredient: Methimazole (Thiamazole)

Form: Tablet

Dosing: Usually twice daily

Key points:

  • Pharmacologically equivalent to Felimazole
  • Same mechanism of action
  • Choice often based on prescribing preference or availability

Thiafeline tablets for cats


Thiamacare Oral Liquid

Thiamacare oral liquid for cats

Active ingredient: Methimazole (Thiamazole)

Form: Oral liquid

Dosing: Typically twice daily

Benefits:

  • Flexible dose adjustment
  • Helpful for cats that resist tablets

Handling note: Methimazole can be absorbed through skin. Hands should be washed after administration.

Thiamacare oral liquid for cats


Thyronorm Oral Solution

Thyronorm oral solution for cats

Active ingredient: Methimazole (Thiamazole)

Form: Oral liquid

Dosing: Usually twice daily

Key points:

  • Alternative liquid methimazole formulation
  • Equivalent pharmacological effect when dosed appropriately

Thyronorm oral solution for cats


Carbimazole for Cats: Vidalta Explained

Vidalta Modified-Release Tablets

Vidalta tablets for cats

Active ingredient: Carbimazole

How it works: Converted in the body into methimazole (thiamazole)

Dosing: Usually once daily (modified-release)

Key differences:

  • Prodrug of methimazole
  • Not milligram-for-milligram interchangeable with methimazole products
  • Once-daily dosing may improve compliance

Vidalta tablets for cats


Monitoring and Safety

All hyperthyroidism medications require regular blood monitoring. Baseline testing typically includes total T4, full blood count and biochemistry.

Rechecks are commonly performed at:

  • 2–3 weeks after starting - Most cats begin to reach euthyroidism within 2–3 weeks of starting antithyroid medication, which is why a T4 recheck at this interval is recommended
  • 6 weeks
  • 3 months
  • Every 3–6 months once stable

Possible side effects include gastrointestinal upset, lethargy, facial itching, elevated liver enzymes and rare blood abnormalities.

Normalising thyroid levels can reveal underlying kidney disease due to reduced glomerular filtration rate. This reflects physiological change rather than direct kidney damage from the medication.