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Cat Hyperthyroidism

Veterinarian Reviewed on May 25, 2012 by Dr. Janice Huntingford

Cat Hyperthyroidism (Feline Hyperthyroidism)

Signs and Symptoms

Feline hyperthyroidism is an endocrine disorder that involves the production of too much thyroid hormone by the thyroid gland. This condition tends to affect middle-aged and older cats, and both males and females are equally at risk of developing this disorder. Hyperthyroidism affects every organ and cell in a cat’s body, and thus can cause a variety of different symptoms and warning signs. While a cat suffering from hyperthyroidism often will not display all of the symptoms indicative of this disorder, most felines will exhibit at least a couple of the classic warning signs.

If you are concerned that your cat may be suffering from hyperthyroidism, there are a number of symptoms and behaviors that you can watch for. Many cats with this disorder lose weight despite having a ravenous appetite. Increased thirst is also common. Behavioral changes may also occur, including hyperactivity, nervousness, and irritability. However, in some cases a cat will be weak and lethargic rather than hyperactive, or lethargy will follow a period of hyperactivity. The coat of a cat suffering from this condition may take on an unkempt appearance and the animal’s nails may become thickened. Some felines also suffer from vomiting and diarrhea. Two other symptoms frequently exhibited by cats with hyperthyroidism are rapid breathing and a rapid heartbeat. Often, this rapid and strong heartbeat can easily be felt by placing your hand over the cat’s chest.

Since cat hyperthyroidism has such an impact on so many organs and physiological functions, it can become extremely dangerous if left untreated, even fatal. Therefore, if your cat is exhibiting any of the above signs or symptoms, it’s vital to contact your veterinarian and have your pet examined as soon as possible. In many cases, treatment is very successful so the sooner your cat’s condition is identified, the sooner its health, comfort, and happiness can be restored.


If your cat is displaying troubling symptoms or behaviors, a veterinarian will follow a number of steps in order to arrive at a diagnosis of feline hyperthyroidism. To begin with, the practitioner will go over your pet’s medical history. At this point, as a pet owner, it’s important to supply as much information as you can about the warning signs and behaviors that you have observed at home. Next, the veterinary doctor will conduct a thorough physical exam, checking for the presence of any clinical signs. In particular, the doctor will feel your cat’s neck, checking for enlarged glands. In addition, heart rate and blood pressure will be monitored. When cat hyperthyroidism is suspected, certain tests will be performed in order to confirm whether or not this endocrine disorder is in fact the problem.

Blood tests will provide a general picture of what is going on with your pet. Such tests may reveal elevated levels of red blood cells and leukocytes along with low levels of lymphocytes and eosinophils. This is indicative of a stress response. Also, some cats with hyperthyroidism will have elevated levels of ALT, a liver enzyme, in their bloodstream. Other substances may be present in elevated levels as well, such as other enzymes, the chemical creatinine, phosphorus, and the bile pigment bilirubin. Some of these higher than normal levels are triggered by physiological complications that arise from hyperthyroidism. Furthermore, blood tests can also provide information on the condition of organs such as the heart, which is important when it comes to treating the feline patient.

In addition to the more general blood tests, thyroid tests can also be performed. Most typically, this involves checking the levels of the thyroid hormone thyroxin (T4). High levels of this hormone are very indicative of hyperthyroidism. In the majority of cats suffering from this endocrine disorder T4 levels will be so high that there will be no question that hyperthyroidism is the problem. However, on some occasions a cat’s hormone levels will fall within the upper levels of the normal range. In such cases, a further test known as a Free T4 (FT4) will be performed. In most cases, this second thyroid test is enough to confirm a diagnosis of hyperthyroidism.

In some circumstances, ultrasound or other imaging techniques may also be employed as diagnostic tools. Such methods allow a veterinarian to view the structure of the thyroid gland and will reveal physical abnormalities such as tumors. However, blood tests and thyroid tests are the methods most frequently employed to reach a diagnosis of cat hyperthyroidism.


If your cat is suffering from hyperthyroidism, it may be helpful to have a basic understanding of this endocrine disorder. This may help you to understand the symptoms and physiological effects that your pet is experiencing, as well as how certain treatments work and why they are necessary.

A cat’s thyroid is a butterfly-shaped gland located in the animal’s neck. This gland is responsible for producing the hormone thyroxin (T4), which plays a significant role in regulating a body’s metabolic rate. In other words, thyroxin determines the rate at which cells consume fuel to make energy. Hyperthyroidism is a disorder which involves the overproduction of thyroxin by the thyroid gland. When this occurs, a cat’s metabolic rate increases. Every cell in a cat’s body burns fuel for energy and, thus, every cell is affected by an elevated metabolic rate. This is why all organs and physiological systems are affected by hyperthyroidism, causing classic symptoms such as weight loss, hyperactivity, increased blood pressure, and an elevated heart rate.

When this disorder is left untreated, a cat’s heart will eventually begin to grow in size, due to the increased stress placed upon it. This is a condition referred to as myocardial hypertrophy. Since a heart affected in this way can wear out more quickly than a normal heart, hyperthyroidism can become very dangerous if left unchecked. Negative impacts on other organs and physiological functions also make this a dangerous disorder when left untreated.


There are certain factors that may increase a cat’s risk of developing hyperthyroidism. For instance, older cats are more likely to suffer from this disorder than are younger cats. Furthermore, it’s possible that environmental and dietary factors also play a role in the development of this condition in felines. Although potential environmental and dietary influences have not been firmly identified as contributors to this disorder, investigations continue to be conducted with respect to the possible effect of dietary iodine on the development of feline hyperthyroidism.

However, despite the mystery that still surrounds the issue of possible contributing factors, it is known that hyperthyroidism results from the presence of a tumor on this gland. In the majority of cases, such tumors are benign. In fact, less than two percent of these tumors tend to be malignant. Yet, even though most thyroid tumors are not cancerous, they still cause overproduction of thyroid hormones and, thus, cause hyperthyroidism. As a result, benign tumors are responsible for nearly all cases of feline hyperthyroidism, although the factors that may contribute to the development of such tumors remain, for the most part, unknown.

The treatment required for your cat’s hyperthyroidism will depend upon your pet’s individual health and circumstances. A variety of tests may be conducted prior to selecting a course of treatment in order to determine the condition of various organs and physiological systems. Once this is done, a treatment plan appropriate for your cat can then be selected.

There are three conventional forms of treatment that are typically used to treat cat hyperthyroidism — surgery, medication, and radiation. Surgery performed on a cat’s thyroid gland is a delicate procedure and can potentially cause damage to the parathyroid glands. When this occurs, the feline patient will then require daily calcium supplements. Since safer methods of treatment are now available, surgery is typically only employed in cases where the thyroid tumor is malignant, which occurs very rarely. Medication and radiation, on the other hand, are far more common treatments for feline hyperthyroidism. Medication involves the daily administration of a prescription hyperthyroid drug that can be given as an oral tablet, in liquid form, or as a transdermal gel rubbed into the cat’s ears. Radiation treatment, or radioiodine treatment, is a very effective method of treatment that involves an injection of radioactive iodine. The radioactive iodine then kills all abnormal thyroid cells and tissues without harming other organs.

In addition to these conventional treatments, homeopathic remedies are also available to help control feline hyperthyroidism and reduce symptoms. These natural products contain ingredients with a variety of beneficial properties that can increase your cat’s health and comfort. One such ingredient is lemon balm, which blocks thyroid stimulating hormone receptors, thereby hampering thyroid activity. Lemon balm also helps to reduce hyperactivity and tension, as does bugleweed. Other natural substances, such as Siberian ginseng and gotu kola help to balance hormones and provide relief from symptoms of hyperthyroidism. As a result, natural remedies have a number of different properties that can be very beneficial for cats suffering from this endocrine disorder. However, since every cat’s circumstances and needs are unique, be sure to consult with a veterinarian so that together you can develop and implement the most appropriate and effective treatment plan for your pet.

Read also: Cat Antioxidants
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Our Expert

Dr. Janice Huntingford
Janice Huntingford, DVM, has been in veterinary practice for over 30 years and has founded two veterinary clinics since receiving her Doctor of Veterinary Medicine at the Ontario Veterinary College, University of Guelph. She has studied extensively in both conventional and holistic modalities. Ask Dr. Jan

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