Hyperthyroidism in Cats: Symptoms, Treatment and Diet

Medically reviewed by: Kinga Wojtczak, Doctor of Veterinary Medicine, specialist in canine and feline diseases

Every living organism, from the moment it comes into the world, is subject to the relentless process of aging and, with every day of its life, whether it likes it or not, inexorably draws closer to its biological end. Of course, in everyday life we tend not to think of ourselves in such a gloomy way; nevertheless, the progressive processes associated with the functioning and wear and tear of the body's organs go on all the time.

This is the natural order of things, although in truth everyone would like to stay beautiful, young and healthy for as long as possible, if not forever. While in humans aging takes place over a longer time span and certain signs become noticeable only after several decades, in our animals, because they live shorter lives, it progresses much faster. We have to accept that even the most active and energetic dog or cat will one day face old age and the geriatric diseases that come with it.

It is sometimes hard to accept that a 7–8-year-old dog or cat already counts as an older animal and therefore needs more care from us, more frequent tests and more check-ups at the veterinary clinic. Keep in mind that any disease detected early has a better chance of being fully cured, or at least of keeping the animal comfortable for longer and extending its life.

So what should you pay particular attention to as the owner of an older cat? There are many diseases that can appear in a senior cat. Among them, when it comes to endocrine disorders, the most common is an overactive thyroid, that is, hyperthyroidism. It is this condition that I will try to explain to you in the article below. At the same time, I would like to make owners of older cats aware of certain symptoms and of the need for a prompt visit to the veterinary clinic and possible medical intervention.

What the thyroid gland is and why it is so important

The thyroid gland (glandula thyroidea) is a paired endocrine gland located near the larynx.

It produces hormones that are extremely important for the proper functioning of the body:

• thyroxine (T4),
• triiodothyronine (T3),
• calcitonin.

The gland's parenchyma contains follicles that produce these two hormones (T3 and T4), as well as clear cells (C cells) that produce calcitonin and, to a lesser extent, serotonin and somatostatin. From the point of view of hyperthyroidism, we are interested in the hormones T3 and T4. These hormones are derived from the amino acid tyrosine, which is the starting material for their production. The whole process takes place inside the follicles of the thyroid gland, from which the hormones are released into the bloodstream as needed.

Importantly, it is thyroxine, or T4, that is mostly (as much as 99%) bound to a transport protein, unlike triiodothyronine, or T3, which is loosely bound and therefore quickly and easily available to the body when needed. This is why T3 is considered the biologically active form, even though there is actually much less of it in the bloodstream (the ratio of T4 to T3 is 50:1).

Most triiodothyronine is also formed in the peripheral blood itself, through deiodination of thyroxine. The small amount of thyroid hormones that are not bound to proteins but circulate in the blood is called free thyroxine (fT4) or free triiodothyronine (fT3). A specific property of the thyroid gland is its huge capacity to store iodine and to serve as the body's reservoir of this element. This allows the organ to keep working properly for many months, even when the diet contains no iodine at all.

As a side note, this property was put to use after the Chernobyl nuclear power plant accident, when children were given Lugol's solution containing iodine to saturate their thyroid glands with this element rather than with its radioactive isotope. Thyroid hormone production is tightly regulated by the pituitary gland, which produces the hormone TSH (thyrotropin); this in turn depends on the hypothalamic hormone TRH, or thyrotropin-releasing hormone, and the regulation works by negative feedback.

Thyroid hormones are extremely important for the body's metabolism, because they act on all of its tissues and organs and thus affect almost every biochemical process. They are also vital in fetal development, especially of the muscular and nervous systems. They regulate fat and carbohydrate metabolism, affect heart muscle function (positive inotropic and chronotropic effects) and also stimulate blood formation, that is, erythropoiesis.

They also affect the proper functioning of osteoclasts and osteoblasts, the cells responsible for bone development and bone metabolism. Working together with growth hormone, they are essential for the body's development, both somatic and sexual. Now that we know what role these hormones play in a cat's body, we can move on to describing the pathological situation in which there is far too much of them.

Causes of hyperthyroidism in cats

The main cause of hyperthyroidism in cats is benign neoplastic growths of the thyroid gland. We do not know what causes these growths, but based on the high levels of antinuclear antibodies (ANA) found in affected cats, we may suspect an autoimmune basis for this disease. In cats, these neoplastic growths are always hormonally active and cause a state of hyperthyroidism with all its consequences.

In a small number of cases, fewer than 5%, hyperthyroidism may be caused by malignant thyroid tumors, that is, hormonally active adenocarcinomas. Diet also plays a role in the development of thyroid tumors. Cats living in iodine-poor areas are more prone to thyroid enlargement and neoplastic processes. They more often develop adenocarcinomas as a result of chronic iodine deficiency, which occurs in areas where this element is washed out of the soil, for example by frequent flooding.

Scientists also suspect many more factors that may influence the development of the disease, although in many cases this has not been conclusively documented. One frequent suspicion, for example, is that not going outdoors, and the lifestyle that goes with it—eating ready-made canned food and using a litter box—has a strong influence and promotes the development of hyperthyroidism.

In cats, the thyroid gland consists of two lobes, and hyperthyroidism affects either one of them or both. So, as we can see, not everything about hyperthyroidism is understood yet, and it is sometimes hard to give a simple answer to the question: what causes this feline disease?

Which cats are most at risk?

The disease is much more common in middle-aged cats or older cats, that is, cats over 8 years of age (other sources mention 12–13-year-old cats). No breed or sex predisposition to hyperthyroidism has been found so far, although compared with other cats it occurs more often in Siamese or Himalayan cats. There is also a suspicion that indoor-only house cats that use a litter box and eat commercial cat food are more likely to develop this condition.

So we can clearly see that the disease does not lend itself to easy classification, and for now we can only be sure that it mostly occurs in middle-aged animals or seniors.

Symptoms of hyperthyroidism in cats

The thyroid is an extremely important secretory gland that produces hormones, so the symptoms of their excess must be serious and must under no circumstances be dismissed by the cat's owner. Hyperthyroidism speeds up the metabolic rate, which results in a negative protein balance (more protein is broken down in the body than is produced) and, as a consequence, an increased demand for oxygen.

Cats with hyperthyroidism usually have a higher body temperature, which results precisely from the increased basal metabolic rate, as well as polydipsia and polyuria (that is, they drink and pee more than is normal for the species).

The textbook symptoms of hyperthyroidism in cats are therefore:

• weight loss, reflecting the increased metabolic rate,
• polydipsia, that is, increased water intake,
• polyuria, that is, a larger volume of urine passed,
• polyphagia, that is, increased calorie intake,
• vomiting, diarrhea,
• tachycardia, that is, a faster heart rate,
• increased physical activity,
• hypertrophic cardiomyopathy.

Hyperthyroidism and thyroid hormone toxicity (thyrotoxicosis) lead to hypertrophy of the heart muscle and arterial hypertension, and thus cause cardiac signs. A characteristic symptom is a marked increase in appetite, although it is not always present (severe heart muscle failure or an advanced neoplastic process will cause anorexia), combined with progressive weight loss.

As the animal loses weight, it loses skin elasticity, which gives the odd impression that there is too much skin. Sick cats may show clear agitation and excessive physical activity, together with difficulty performing movements. Even slight physical effort may be accompanied by open-mouth breathing as a sign of dyspnea, or by dilated pupils.

Muscle tremors, dyspnea, exercise intolerance, together with neglected grooming, poor coat condition or muscle wasting make up the typical picture of a cat with hyperthyroidism. A characteristic sign associated with hyperthyroidism in cats is neuromyopathy, manifested by weakness or ventroflexion of the neck. The fatigue can be so severe that in some cases cats have to rest even after a small effort such as walking from one place to another. Fortunately, such extreme fatigue does not occur in every case.

Keep in mind that the symptoms will not be equally severe in every case: when the cause of hyperthyroidism is a benign thyroid growth, the symptoms will develop slowly and gradually, unlike thyroid cancer, which most often causes symptoms to appear suddenly and quickly. Let us also keep in mind congestive heart failure in advanced cases, as well as hypertension, and not attribute them to the aging process, but always try to determine the cause of this condition.

Hyperthyroidism leads to the development of hepatopathy, that is, liver enlargement, as a consequence of hypoxia of the liver and increased oxygen consumption in the digestive system. So we can see how varied the clinical signs of this disease can be and how difficult it can sometimes be to make a definite diagnosis and start effective treatment on their basis. What else can we find in a cat affected by hyperthyroidism?

The vast majority of sick animals will have an enlarged thyroid gland, which we can detect on palpation by feeling the cat's neck. A benign growth will show up as enlargement of both thyroid lobes, whereas a malignant tumor usually affects only one lobe.

In summary, we can see how serious and varied the symptoms accompanying hyperthyroidism are, which results from the role this gland plays in regulating metabolism, and how difficult it can sometimes be to make a definite diagnosis.

Diagnosing the disease

In the clinical examination, it is extremely important to know the signs of the disease, which, after all, do not have to appear in their full range in every case. As I mentioned earlier, hyperthyroidism occurs most often in older animals, so it should always be kept in mind when examining a middle-aged or older cat, that is, one over 8–10 years of age. We therefore start the diagnostic process with a medical history, supplemented by palpation of the thyroid area, where we can feel an enlargement of the gland.

Only then do we carry out laboratory blood tests, which may show:

• increased activity of liver enzymes: * AST, * ALT, * ALP, * GLDH, * AP,
• hyperglycemia, that is, an elevated blood glucose level,
• kidney failure.

The complete blood count usually shows an increased white blood cell count. These are, of course, nonspecific results, and the disease cannot be diagnosed conclusively on their basis. For this we need to carry out more precise, specific measurements of thyroid hormone levels, namely of the total thyroxine concentration in the blood, that is, T4. An increased serum T4 level together with other clinical signs clearly indicates hyperthyroidism.

A normal T4 level in a cat with clinical signs and an enlarged thyroid gland, however, does not rule out hyperthyroidism and requires the tests to be repeated in a few months. Measuring T4 is a cheap, simple and, most importantly, extremely reliable and dependable method of diagnosing hyperthyroidism in cats. Also keep in mind that measuring T3, fT3 and fT4 has little diagnostic value and is not fully reliable, so it is not recommended.

Remember that hormone tests must be done in specialized veterinary laboratories, not laboratories for human medicine, because only such results will have diagnostic value and be reliable. Other tests can also be performed, such as the TSH stimulation test, the T3 suppression test or measurement of serum TSH. An ultrasound examination of the thyroid gland is certainly helpful in the diagnosis: it determines the size, structure and position of the gland and shows whether a pathological process is going on in it.

X-rays are used to detect any metastases. Hyperthyroidism causes cardiac signs, so examining the heart can also reveal certain pathological processes. We can therefore use an ECG or a heart ultrasound (echocardiogram) to make a definite diagnosis. Thyrotoxicosis causes hypertrophy of the heart muscle and leads to its failure and destruction, which will be reflected in the echocardiographic examination.

Now that we have a diagnosis, let us move on to the therapeutic approach, that is, treatment.

Treatment of hyperthyroidism in cats

The treatment options we can use are:

• drug therapy,
• a surgical procedure,
• radiotherapy, which is less available in Poland.

In general, we aim to reduce the concentration of thyroid hormones and their effect on the body, and we achieve this by using one of the methods listed above.

Surgical removal of the thyroid gland

Thyroidectomy, that is, surgical removal of the thyroid gland, is considered an effective treatment that eliminates hyperthyroidism. Of course, it should be performed by a highly experienced surgeon, so that the parathyroid glands, located at the caudal pole of the thyroid lobes, or the nerves (mainly the recurrent laryngeal nerve) are not damaged during the procedure. When opting for surgery, we must keep in mind the risks involved, especially in the context of existing arterial hypertension, hypertrophic cardiomyopathy or kidney failure, and the difficulties with anesthesia.

To minimize the potential risk associated with the procedure itself, drug therapy can be given for 4–6 weeks before surgery. For this purpose, we use:

• sympatholytic drugs,
• antithyroid (thyrostatic) drugs,
• high doses of iodine (less available in Poland).

Removing the thyroid gland will, of course, cause a drop in hormone levels, hence the need to monitor them in the serum and provide supplementation. In the postoperative period, we should also monitor the calcium level to check whether the parathyroid glands have been damaged. In the case of thyroid cancer affecting one of the lobes, only the diseased lobe is removed, while the other, healthy one remains and produces thyroid hormones at the right physiological level.

Medication for hyperthyroidism in cats

The aim of drug therapy, by contrast, is to lower thyroid hormone levels without surgically removing the gland. It is also used to prepare the patient for surgery. Remember that continuous administration of the drugs does not lead to a permanent cure; it only suppresses the symptoms of the disease, which will return as soon as the drugs are stopped. The drug commonly used in this situation is methimazole (thiamazole), sold as Metizol, at a starting dose of 1.25–2.5 mg per cat twice a day (3–5 mg per cat 3 times a day). This treatment is highly effective, with a success rate of around 90%.

Sometimes, however, side effects occur in the form of red blood cell disorders, which can lead to hemorrhage, impaired liver function or kidney function, or gastrointestinal signs. We must therefore regularly carry out the appropriate blood tests to detect potential disorders quickly and prevent them. The side effects disappear once the drug is stopped, which is a major advantage.

Radiation therapy

Treatment with radioactive iodine, although highly effective, is not performed in Poland. Radioactive iodine isotopes given subcutaneously are taken up by thyroid cells and, by emitting β radiation, destroy the overactive cells. This therapy has a very high cure rate, but it involves a lot of inconvenience for owners. Cats treated this way must be isolated from their surroundings for a long period (at least a month) so that the radiation they emit does not pose a risk to the people around them.

This, of course, requires meeting a number of specific radiological requirements, which is why centers offering this type of treatment are not widely available. With this method, one must also be prepared for hypothyroidism developing as a complication, but treating it is usually not a major problem. In congestive heart failure, we use treatment that eliminates pulmonary edema (diuretics, e.g., furosemide) and specific cardiac treatment that supports the failing heart (digoxin, ACE inhibitors or vasodilators).

We should also not forget to treat arterial hypertension, which often accompanies hyperthyroidism. Ideally, of course, blood pressure should first be measured accurately, and only then should the appropriate medication be started (e.g., benazepril 0.25–0.5 mg/kg once a day, enalapril 0.5 mg/kg once a day). Left untreated, the disease leads to progressive damage to the heart muscle, heart failure, pulmonary edema, and kidney and liver failure; as a result, it poses a serious threat to life, and its symptoms considerably reduce the quality of life of what is already an older cat. That is why it is extremely important to diagnose it and start treating both the underlying cause and the symptoms as soon as possible.

The prognosis for hyperthyroidism is good in most cases. It is guarded only when other diseases are present, such as heart disease or kidney failure.

Diet for hyperthyroidism in cats

Specialized foods can be helpful in treating hyperthyroidism.

They contain the right levels of nutrients and thus support the recovery process:

• iodine,
• phosphorus,
• sodium,
• taurine,
• omega-3 fatty acids.

An example of a food for a cat with hyperthyroidism is Hill's Feline y/d, a food for thyroid problems. To put together a good diet, it is a good idea to consult a feline nutritionist.

Summary

To conclude, I want to emphasize once more that with hyperthyroidism in cats, the earliest possible diagnosis is extremely important. Since we do not fully know the factors that cause the disease, we are unable to prevent hyperthyroidism in cats. However, we should not give up too easily, because early diagnosis and treatment will effectively prevent the development of secondary pathological changes related to circulatory, kidney, liver or heart failure. Every owner of a middle-aged or older cat should take their pet for a routine blood test, including thyroid hormone levels, from time to time.

This will allow the disease to be detected earlier, and effective treatment will ensure a long life for our beloved pet—which I sincerely wish all cat owners.

FAQ