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Hyperthyroidism In Cats – Is Fish The Problem?

A photograph of a grey and white long haired cat dipping its left paw into a large bucket of water. It is outside in a garden with trees and grass behind it.

Hyperthyroidism is an endocrine disorder affecting the thyroid, a gland that is located on either side of the trachea. It is considered one of the most common endocrine disorders in cats, and one of the most diagnosed disorder in veterinary practices. This had lead to research into what has caused the rise and prevalence of this disease. Some studies have made to links towards diet and in particular the feeding of fish-based diets.

What Is Hyperthyroidism?

The thyroid is responsible for the production and release of the hormones triiodothyronine and thyroxine. When released in balance the hormones increase the metabolic rate of the body to enable it to break down nutrients more effectively, raise body temperature, increase pulse and heart rate, and promote growth in young animals. When an imbalance occurs that is when diseases such as hypothyroidism and hyperthyroidism occur. Hyperthyroidism is a result of the thyroid overproducing hormones, which speed-up the cat’s metabolism more than necessary. Signs that a cat may have developed hyperthyroidism include:

  • Increased appetite

  • Weight loss, especially combined with an increased appetite and higher food consumption

  • Increased drinking and urination

  • Poor coat quality

  • Aggressive or restless behaviour

  • Vomiting

  • Breathing difficulties

  • Muscle weakness

If you are concerned that your cat is exhibiting any of these symptoms, please consult your vet!

What Causes Hyperthyroidism In Cats?

Studies are yet to discover a definite cause for hyperthyroidism in cats, however, several theories have been put forward. A correlation has been made between hyperthyroidism and exposure to environmental chemicals such as pesticides and tobacco smoke, the wearing of flea collars, preference for indoor living, consuming high amounts of canned food, the inclusion of soya in commercial cat foods, and the amount of fish and iodine included in a cat’s diet.

Marine-sourced ingredients such as fish meat and seaweed contain naturally high levels of iodine. Iodine is a trace element required by cats specifically for the manufacturing of thyroid hormones, and also has anti-inflammatory, antibacterial, antiviral, and anti-fungal properties. However, excessive or deficient levels of iodine in our cats' bodies can inhibit the production of the thyroid hormones. It is important to note that cat foods vary greatly in iodine levels depending on the manufacturer and ingredients.

Foods containing marine fish, animal heart, or liver have been proven to be higher in iodine. Beef and poultry based foods have proven to be lower in iodine and as a result are less likely to contribute to hyperthyroidism in cats.

Another possible reason why the feeding of fish meat has been linked to hyperthyroidism in cats is the levels of pollutants fish are exposed to in the wild. Fish meat can contain detrimental levels of mercury, other heavy metals, and polybrominated diphenyl ethers (PBDEs). These are all known endocrine disruptors, meaning that they inhibit the thyroid gland’s hormone production.


Unfortunately, there is still no definite cause for hyperthyroidism in cats. However, steps can be taken to help reduce the risk of exposure to harmful iodine levels and chemicals. If possible, feed foods that contain a suitable level of iodine for your pet. The recommended dietary intake of iodine for a healthy 4kg adult cat is 300 micrograms (mg) per 1000 kcal.

If you are unsure of the iodine content of your cat’s food, contact the manufacturer directly, or drop us an email and we will do our best to help! If you feed your cat a fish based diet then ensure that you are feeding a high-quality complete food that has an appropriate level of iodine, and ensure that the food provides as much information as possible about the sources of the fish meat.

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