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  • Hyperthyroidism In Cats – Could Too Much Fish Be Part Of The Problem?

    Hyperthyroidism In Cats – Could Too Much Fish Be Part Of The Problem?

    Increasingly research is being conducted into the effects of commercial diets on our pets’ health. As pet food manufacturers utilise a wider variety of ingredients, it is becoming more important to be aware of what is in your pet’s food, where it has come from, and what long term effects it may have on your pet’s health. 

    Hyperthyroidism is an endocrine disorder affecting the thyroid, a gland that is located on either side of your pet’s trachea.  First documented in cats in 1979, there has since been a rapid increase in cats being diagnosed with this disease.1 Hyperthyroidism is now the most common endocrine disorder in cats and one of the most diagnosed disorders in veterinary practice.2 This has lead to research into what has caused this disease to become so prevalent in cats. Studies have made links to a number of potential causes, including links towards diet and in particular the feeding of some fish based diets. 

    The thyroid is responsible for the production and release of thyroid hormones, tri-iodothyronine (T3) and thyroxine (T4). 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 juvenile 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 your cat’s metabolism more than necessary. Signs that your cat may have developed hyperthyroidism include (3):

    •  Increased appetite
    •  Weight loss, especially if your cat has an increased appetite and is eating more food
    •  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.

    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,4and the amount of fish and iodine included in a cat’s diet.5 It is this last theory that has proved to be the most popular.

    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.6 It also has anti-inflammatory, antibacterial, antiviral, and anti-fungal properties.However, excessive or deficient levels of iodine in your cat’s body can inhibit the production of the thyroid hormones. 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.8 Beef and poultry based foods have proven to be lower in iodine and accordingly, are less likely to contribute to hyperthyroidism in cats.9

    The changes in iodine levels in a cats’ day-to-day diet are believed to be a factor that has contributed to the rise in hyperthyroidism, as the consistency of the thyroid’s production of hormones is compromised, causing spikes in the level of hormones released.10 However, one study found that decreasing the iodine content in the diet of a cat diagnosed with hyperthyroidism did not affect the high levels of thyroid hormones present in the blood, suggesting it is inconclusive as to whether iodine alone is the cause of hyperthyroidism.11

    Another reason why the feeding of fish meat has been linked to hyperthyroidism in cats is the levels of pollutants that can currently be found in fish.  This is caused by pollution contaminating the oceans. Fish meat can contain detrimental levels of mercury and other heavy metals, and polybrominated diphenyl ethers (PBDEs). These are all known endocrine disruptors, meaning that they inhibit the thyroid gland’s hormone production.12 PBDEs are flame retardant chemicals found in electrical products, furniture, textiles, and construction materials.13 These are known contaminants and are found in our water, air, and soil. They are present in animals throughout the entire food chain. The first case of PBDE contamination into the environment was documented in 1979, the same year as the first recorded case of hyperthyroidism in cats.14

    This connection led scientists to examine the properties of PBDEs. Studies found that PBDEs have a similar structure to thyroid hormones and when present in moderate amounts in the body they disrupt thyroid hormone metabolism, potentially causing thyroid diseases such as hyperthyroidism.15 PBDEs are found in both humans and animals, however, cats have been found to have 20 – 100 times higher levels than humans.16

    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. 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 identifies which oceans the fish in the food has come from. This will allow you to make a decision on whether the fish is coming from heavily polluted oceans with a significant prospect of the fish being contaminated with high levels of PBDEs.