Hyperthyroidism is a disease that is increasingly diagnosed in our cats and that usually affects older cats. As the production of hormones is involved, its effects will harm the entire body.
But with proper treatment, the life expectancy of our cat does not have to vary.
What is feline hyperthyroidism?
In hyperthyroidism, the thyroid gland produces excess hormones, generally due to a benign tumor. In a small percentage of cases the tumor will be malignant. It is more common in elderly cats , especially from the age of eight. It is the most frequent endocrine pathology at these ages.
Hyperthyroidism is the most common endocrine disorder in cats. It is produced by uncontrolled secretion of T4 by the thyroid gland, in most cases due to nodular hyperplasia or a benign adenoma . The elevation of this hormone in turn produces the suppression of the secretion of TSH and TRH, causing atrophy of healthy tissue, but not that of altered tissue, which perpetuates the problem.
The etiology of the disease has not been fully clarified, but it is known to be multi-factorial , with iodine concentration in the diet being a fundamental factor. Diets rich in iodine or sudden variations in its concentration in the diet of the same cat seem to contribute to the development of hyperthyroidism. The age (cats over six years) and the presence of goitrogenic substances (substances that may decrease thyroid hormone synthesis, thereby causing the generated pituitary more TSH) in the diet as food additive based diets from fish or liver , or from packaging components, are other factors that seem to be involved.
The thyroid gland is bilobed and is located in the neck. It secretes hormones that act on tissues throughout the body, so their failure can affect different organs.
These hormones are thyroxine or T4 and triiodothyronine or T3 . Iodine is an essential component of these hormones. Thyroxine controls energy production, which is needed to maintain normal basal metabolic rate, and is also involved in growth.
At what age do cats have hyperthyroidism?
Although there are cases described in young cats, it is a disease associated with cats older than 7 years. In fact, it is the most frequent hormonal disease in senior cats, so annual checkups are recommended to detect this disease early.
In hyperthyroidism, there is an increase in the synthesis and secretion of thyroxine, which increases the metabolic rate & lead to two heart-related complications: hypertension and thyrotoxic cardiomyopathy. The cause of the tumor is unknown, but immunological, environmental, infectious, nutritional or genetic factors are considered. The latter have determined a lower incidence of the disease in Siamese and Himalayan cats .
What symptoms does hyperthyroidism give the cat?
If we think that having an excess of thyroid hormones increases metabolism and that means that a hyperthyroid cat spends more energy, thereby increasing its appetite to try to compensate, although it is not enough, and therefore they lose weight.
That is why the most typical signs are weight loss associated with a good appetite or even a voracious appetite (there are cats that suddenly start stealing food when they have never done so before).
Our cat can drink and urinate more and in many cases our cat’s activity increases.
Hyperthyroid cats are sometimes more irritable (even aggressive), asking and meowing more often.
These signs at the beginning can be subtle or even give the feeling that our cat is healthy or “younger”.
Other symptoms it can give:
- Ugly looking hair, little care.
- Diarrhea and / or vomiting.
- Low tolerance for stressful situations: they can sometimes gasp (a very rare thing in cats).
- In some cases, especially in the more advanced ones, there may be loss of appetite and until they are depressed.
- Due to the excessive effort made by the heart to compensate for the increased metabolism, arrhythmias appear and if the disease is uncontrolled, the heart is altered and heart failure with the risk of death may appear.
- If there is hypertension, our cat may go blind as the most apparent symptom, although there may also be damage to the brain and kidneys, mainly.
The symptoms of hyperthyroidism are highly variable and, in addition, nonspecific, which means that there will be multiple diseases that manifest with this same clinical picture.
For this reason, it is necessary for the vet to carry out diagnostic tests, such as blood and urine tests. In general, a cat with this disease will show symptoms like the following:
- Hyperactivity, although there are cats that will be lethargic.
- Polyphagia, that is, an increase in food intake. Other cats, on the contrary, will present anorexia.
- Weight loss, despite eating more .
- Polydipsia (increase in water intake).
- Polyuria (increased urine output).
- Diarrhea .
- Mantle in poor condition due to lack of grooming.
- Dry hair and fine skin.
- Alopecic areas.
- Slight hyperthermia.
- Tachycardia, murmur, or arrhythmias.
- Accelerated breathing.- Respiratory problems.
- Excessive vocalizations.
- Cervical ventroflexion.
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How to diagnose hyperthyroidism in cats
The veterinarian can arrive at the diagnosis based on the symptoms, palpation of the thyroid gland, which is located in the neck but is not always touched, and a blood test that reveals the amount of thyroid hormones, preferably thyroxine or T4. The fact that its values are in normal ranges does not rule out the diagnosis because mild or hidden hyperthyroidism may occur.
In these cases, if there is a strong suspicion that the cat has hyperthyroidism, other tests will be performed, such as suppression with T3 or the response to another hormone, HRT. Even so, sometimes it can be complex to reach the diagnosis and it is necessary to repeat the tests. After confirmation, an ultrasound will give us more information about the state of the gland.
Concurrent diseases with hyperthyroidism
The excess in the production of thyroid hormones can concur with other diseases such as the following, which, in addition, usually delay the diagnosis of hyperthyroidism because in their course they will decrease the concentration of T4 :
- Hypertrophic cardiomyopathy, which can be the cause of murmurs, tachycardia, arrhythmia, etc.
- Kidney failure , very common in older cats.
- Gastrointestinal disorders, with anorexia, polyphagia, weight loss, increased frequency of defecation and the volume of feces, vomiting, diarrhea, etc.
What is the treatment for hyperthyroidism in a cat?
This disease presents the following treatment options from which the veterinarian must choose taking into account the circumstances of each cat:
- Thyroidectomy: is the removal of the thyroid or affected tissue. It is an intervention of intermediate difficulty, invasive and of moderate costs.
- Medication: In the long term, which can affect the cost, it inhibits the synthesis of thyroid hormone, but does not prevent the gland from increasing in size, so the cat must be checked every 4-6 months. In addition, it requires adjusting the dose. It can be administered in pills or gel.
- Radiation: Radioactive iodine is used to destroy over-functioning gland tissue. Healthy tissue is not affected. The drawbacks are that the cat must remain hospitalized for several days after the treatment, it can only be done in one of the few authorized centers because it is necessary to be able to handle and eliminate the waste and, in addition, it is expensive.
It is also necessary to take into account the concurrence with other diseases such as those we have mentioned, since it is what will determine the priorities in the treatment. This can produce side effects such as anorexia, vomiting, lethargy, etc.
What does the treatment consist of?
Treatment is aimed at controlling or stopping excess production of thyroid hormones. Cats generally respond well. In fact, they have a good prognosis, especially if they are recognized early and treated correctly. In the case that hyperthyroidism is within 2% caused by a malignant tumor, the treatment will be more complicated.
There are four types of treatment, and the choice of one of them will depend on the specific case of each animal. In the event that your cat has been diagnosed with hyperthyroidism, your veterinarian will be able to inform you of the advantages and disadvantages of each therapeutic plan for your pet.
Oral medications are available to control hyperthyroidism in the short or long term. The most widely used are methimazole and carbimazole, also used in the treatment of this disease in humans. Its function is to reduce the production and release of thyroid hormones.
The treatment is for life, and periodic analyzes are necessary to verify that the therapy is effective, as well as to readjust the dose or verify that the liver function has not been affected.
Although rare, there may be some minor side effects such as loss of appetite, vomiting, or diarrhea.
Another option to control the hyperthyroidism of our cat is to administer a specific diet low in iodine. The thyroid needs iodine to make its hormones. If there is a controlled concentration of this element in the diet, it will only serve to produce normal levels of these hormones.
There are specific veterinary prescription feeds with these characteristics. This therapy has been successful in many cats, as long as they are only fed this diet and are not given any other type of food.
In this case, a permanent cure is sought. Surgery involves a thyroidectomy, which is the removal of the affected thyroid tissue.
As these are geriatric patients, it must be borne in mind that anesthetic risks multiply. To reduce them, it is recommended to stabilize the cat the previous weeks using medical treatment. If you have other diseases, they must be taken into account before considering surgery or treated correctly to reduce complications.
The prognosis is generally very good, although there may be post-surgical problems (for example, incomplete removal or subsequent hypothyroidism).
Radioactive iodine therapy
The administration of radioactive iodine is a curative treatment for hyperthyroidism in 95% of cats. The therapy consists of a subcutaneous injection of iodine (I-131), which destroys thyroid cells that produce excess hormones. Radiation does not harm other tissues or the healthy part of the thyroid. After treatment, the feline must remain isolated for a time until the radioactivity drops.
How long does a cat with hyperthyroidism live?
If the chosen treatment is successful, the prognosis is good. In cases where concurrent diseases have appeared, however, the prognosis is reserved. Age also worsens the prognosis.