Hyperthyroidism is a commonly-diagnosed glandular condition in cats. The condition is most frequently caused by a benign (non-cancerous) tumor on the thyroid called a thyroid adenome. In only 2 % of the cases, this tumor is cancerous. The tumor produces an abundance of thyroxine (a thyroid hormone) that is released into circulation. This results in an overall increase in metabolism and affects many organ systems.
Causes of feline hyperthyroidism
Hyperthyroidism in cats was first described in 1979. Since then, its prevalence has increased dramatically in the feline population (Peterson, 2012). The exact causes of enlargement of the thyroid gland and subsequent hyperthyroidism in cats are unknown. However, because there has been such a steep rise in the number of cases of this condition in cats since it was first described, environmental factors have been suspected.
There have been several studies that have linked increased levels of hyperthyroidism in cats with being fed high amounts of canned cat food, especially those made of fish, seafood, or liver. The rise of hyperthyroidism would seem to coincide with the increase in feeding commercial cat food diets to pet cats.
Some of the suspected goitrogens (substances that may contribute to thyroid dysfunction) in commercial feline diets include:
- BPA (bisphenol A). This is a chemical that has been used since the 1960s to make certain plastics. Some cat food cans are lined with plastic that may contain BPA. The smaller the can, the more surface area of the food comes in contact with the plastic liner, and the more BPA may leach into the food. It is difficult to know how much BPA, if any, is in cat food cans today; the amounts vary and change quite often. BPA has been hypothesized to be one of the reasons canned cat foods seem to be linked with higher rates of hyperthyroidism in cats.
- Too much or too little iodine in some cat food products may be a contributing factor to the development of hyperthyroidism. Humans are quite sensitive to iodine, and thyroid problems are intricately connected with not getting enough or getting too much. The specific type of thyroid problem depends on multiple factors. Cats may be just as sensitive to iodine excess or deficiency. Because ocean fish are high in iodine, excessive iodine triggering hyperthyroidism would be one way to explain why some studies have found higher rates of the disease in cats fed high amounts of fish-based canned cat foods.
- Selenium is a mineral that some scientists think may be involved in hyperthyroidism in cats. Both selenium deficiency and excess may result in increased thyroid function through different mechanisms. However, studies haven’t shown definitively that selenium levels are associated with hyperthyroidism in cats.
- Isoflavones found in soy protein, are known to cause thyroid disruption in rats. Isoflavones are commonly used in cat foods as a protein source. It isn’t known whether they could cause or contribute to hyperthyroidism in cats, but there are those who believe they do.
- PBDEs (polybrominated diphenyl ethers) are chemicals that are used as flame retardants. In recent years, some scientists have theorized that PBDEs may contribute to the development of hyperthyroidism in cats when they are ingested. Part of the reason this is suspected is because the discovery and subsequent rise in hyperthyroid cases in cats coincided with the introduction and rise in use of PBDEs. A 2011 study showed higher PBDE levels in the homes of hyperthyroid cats than those of cats with normal thyroid function (Donna A. Menschinga, 2012). This indicates a possible link to PBDE levels and hyperthyroidism in cats. Another study indicated that cats don’t metabolize PBDEs in the same way as humans or rats do (Jessica Norrgran, 2012). If this is the case, these chemicals may circulate longer in cats’ bodies, allowing them to cause thyroid dysfunction. PBDEs mainly gain entry into cats’ bodies through ingestion, and they can be ingested in the following ways:
- PBDEs are present in house dust, and cats can ingest it while grooming themselves. These chemicals end up in house dust because of their presence in household items. PBDEs are not stable in these items, but are “shed” from them over time.
- PBDEs are used in some garments, blankets, upholstery, mattresses, plastics, and electronics. Cats’ bedding may contain them, and cats can lick them off of themselves while grooming.
- PBDEs are also found in cat food. Two types have been found in high amounts in fish and seafood-based canned foods, and other types are found in high levels in dry cat foods.
Signs of feline hyperthyroidism
The most common signs seen in cats with hyperthyroidism are:
- Weight loss in spite of ravenous appetite
- Increased water drinking and urination
- Inappropriate urination
- Hyperactivity and nervousness
- Poor hair coat
- Increased vocalizations
- Vomiting and diarrhea
This condition commonly affects the heart causing a more rapid heart rate, a heart murmur, and high blood pressure. Some cats may not show any of the above clinical signs but instead show opposite signs such as depression and sluggishness.
Hyperthyroidism is generally a disease of older cats, with the majority of diagnoses being made in felines that are over 10 years old.
Diagnosis of feline hyperthyroidism
Diagnosis is made by taking a medical history, doing a physical examination, palpation of an enlarged thyroid gland and it is confirmed with specialized blood tests. Because hyperthyroidism tends to occur in older cats, many of these cats have concurrent illnesses such as kidney disease, diabetes or heart disease. Other diagnostic tests such as cardiac or kidney ultrasound may be required to determine your cat’s overall health status.
Treatment of hyperthyroidism in cats
Several treatment options for hyperthyroidism exist and there are advantages and disadvantages for each.
- Surgery (thyroidectomy) to remove the affected, enlarged thyroid gland is used in some instances. However, it is delicate surgery and may result in disturbance or removal of the parathyroid glands, which are not visible and are often adhered closely to the thyroid glands. Hypoparathyroidism, or decreased parathyroid hormone levels, may result from the removal of these glands, and that condition can be life-threatening. Thyroid tissue often grows in areas other than the thyroid glands, most notably inside the chest, so it’s not always possible to remove all of the affected tissue, and the hyperthyroidism may return. Cats with hyperthyroidism frequently have heart disease, making anesthesia dangerous for these pets.
- Medication is the most common treatment used for feline hyperthyroidism. The medication methimazole decreases thyroid levels in the blood and controls the cat’s hyperthyroidism. This is a life-long treatment, not a cure. Blood work will need to be run routinely and adjustments in the medication dosage will need to be done as needed to maintain good thyroid hormone levels. Some cats can’t tolerate the medication and develop life-threatening anemia, so other treatment options must be sought for them.
- Radioactive iodine therapy is considered the gold standard treatment for cats with hyperthyroidism. Radioactive iodine is given intravenously and kills abnormal thyroid tissue wherever it is in the body, sparing normal tissue. Therapy may need to be repeated if further thyroid tissue becomes abnormal. Cats that receive this therapy also must spend time in a specialized veterinary hospital and care must be taken with cleaning up their waste, which will be radioactive for a short period of time.
- Therapeutic diet is a fairly new treatment for hyperthyroidism in cats. Hills Prescription Diets provides y/d, a diet that is extremely low in iodine. When fed exclusively to a hyperthyroid cat, Hills claims that the food can result in normal thyroid levels.
- Alternative therapies such as herbal treatments like Pet Wellbeing’s Thyroid Support Gold – Cat Hyperthyroidism Support can be used in some cats to help encourage normal thyroid and heart health (because the heart is affected by hyperthyroidism).
Research is still being done into hyperthyroidism in cats and what the environmental and nutritional triggers might be. Many in the veterinary and scientific communities do feel that the evidence of a link between commercial cat food diets and hyperthyroidism is strong. Many more believe that the cause is likely multifactorial, requiring a conglomeration of these and other triggers. For instance, Siamese and Himalayan cats are less likely to develop hyperthyroidism than other breeds, indicating that a genetic component may be involved.