HYPERTHYROIDISM IN FEMALE CAT
Hyperthyroidism is one of the most common endocrine conditions affecting cats, especially older cats over the age of 10 years. Some veterinarians estimate that about 5 to 10% of middle- to old-aged cats will develop hyperthyroidism, and, due to factors that may include environmental exposures, that number is on the rise.
Thyroid hormones normally regulate many of the body’s metabolic processes. As circulating levels of thyroid hormones rise higher and higher in cats with hyperthyroidism, a progressive increase in their metabolic rate develops: this leads to a multitude of changes throughout the entire body, including loss of weight and muscle wasting. High thyroid hormone concentrations also interact with and stimulate the central nervous system, which commonly leads to increased activity or restlessness, as well as other behavior problems.
Clinical signs associated with hyperthyroidism can be quite dramatic and cats can become seriously ill with this condition. Untreated, hyperthyroidism in cats can lead to heart or kidney failure and can be fatal.
However, this disorder is a very treatable disease and most hyperthyroid cats will make a complete recovery.
What is hyperthyroidism?
Hyperthyroidism (overactive thyroid glands) is a very common disorder of older cats.It is caused by an increase in production of thyroid hormones from the thyroid glands, which are situated in the neck.Thyroid hormones are responsible for regulating many body processes and when too much hormone is produced the clinical signs can be quite dramatic, and cats can become seriously ill. Thyroid hormones also help control the body’s metabolic rate and cats with hyperthyroidism tend to burn up energy too rapidly and typically suffer weight loss despite having an increased appetite and increased food intake.Fortunately, the vast majority of cats that develop hyperthyroidism can be treated very successfully and most cats will make a complete recovery.
What causes hyperthyroidism?
In the vast majority of cases hyperthyroidism in cats is caused by a benign (non-cancerous) change. Cats have two thyroid glands, and in most cases (more than 70%) both are involved in the disease, and become enlarged (a change called ‘nodular hyperplasia’, which resembles a very benign tumour). The underlying cause of this change is currently unknown, but is very similar to one of the two main causes of hyperthyroidism in humans called ‘toxic nodular goitre’.Rarely (in less than 1-2% of cases) a malignant (cancerous) tumour (thyroid adenocarcinoma) can be the underlying cause of the disease.
Signs of hyperthryroidism
Hyperthyroidism is usually seen in middle-aged and older cats, rarely being seen in cats less than 7 years of age. Male and female cats are affected equally.In affected cats, a wide variety of signs usually develop, but these are usually subtle at first, and become more severe over time as the underlying disease gets worse. Also, as it is mostly older cats that are affected, some cats will have other diseases that can complicate and even mask some of the clinical signs.
The ‘classic’ signs of hyperthyroidism are:
- Weight loss
- Usually a good or increased appetite (polyphagia)
- Increased thirst (polydipsia)
- Increased activity, restlessness or irritability
- An increased heart rate (tachycardia)
- A poor and unkempt hair coat
Mild to moderate diarrhoea and/or vomiting develop in some cats, and some will be noticeably intolerant of heat and seek out cooler places to sit. Some (especially advanced cases) may also pant when they are stressed, which is very unusual in cats.Although most hyperthyroid cats show a good or increased appetite and restlessness, in some cases there may be generalised weakness, lethargy and loss of appetite.
Clinical Signs of Hyperthyroidism in Cat
Your cat doesn’t need to have all of these clinical signs or symptoms in order to have an overactive thyroid problem. But if your cat has one or more of these signs, especially if he or she is older than 10 years of age, you should definitely have your cat tested for hyperthyroidism.
1. Weight loss
Weight loss despite a normal to increased appetite is the classic and most common sign of hyperthyroidism in cats. Hyperthyroidism is so common that it should always be considered as a possibility in any middle-aged to older cats that has lost weight, even in none of the other clinical features of the disease are present.The weight loss associated is generally progressive and is usually first noticed by the owner as a loss of muscle mass around the cat’s back (spine).
With time, severe muscle wasting, emaciation, and death from starvation can occur if the cat’s hyperthyroidism is left untreated.
2. Increased appetite
The weight loss characteristic of cats with hyperthyroidism is typically associated with an increase in the cat’s appetite. This increase in appetite can be dramatic, with some cats doubling the amount of food eaten and frequently begging for food. Hyperthyroid cats eat more in an attempt to compensate for their higher-than-normal metabolic rate by increasing the number of calories ingested. Unfortunately, most cats cannot fully compensate and continue to loss weight even if they have a good to increased appetite.Although most hyperthyroid cats have an increased appetite, some cats with mild hyperthyroidism will maintain a normal appetite, with no obvious change in amount of food consumed.A few hyperthyroid cats will even develop a reduced appetite that is improved after treatment of their hyperthyroidism. Most of these cats with a poor appetite have a rare form of hyperthyroidism called “apathetic hyperthyroidism,” which is described in more detail below .
3. Hyperactive, increased energy, or nervous behavior
Hyperactivity, exhibited particularly as nervousness or restlessness, is relatively common in cats with hyperthyroidism. In extreme hyperthyroidism, muscle tremor or twitching may be apparent, and affected cats are often described as having an anxious or frantic facial expression.In some cats, especially in the earlier stages of their hyperthyroidism, their owner can misconstrue such increased activity and energy as a positive sign of health. One must remember that cats normally become quite sedentary as they age so this change of behavior leading to increased energy and activity is almost always a manifestation of an underlying disease processes.
Some of these behavior signs — especially restlessness or aggression — are often more noticeable to the examining veterinarian than to the cat owners themselves. Many hyperthyroid cats have an impaired tolerance for stress and do not wish to be held or restrained. Some will even develop a “panic attack” as a result of an impaired tolerance for examination or restraint during blood sampling. These cats may also develop panting, overt respiratory distress, weakness, and even collapse during these episodes.
4. Increased thirst and urination
About half of cats with hyperthyroidism will show signs of increased thirst (polydipsia) and urination (polyuria). Veterinarians often refer to polyuria and polydipsia simply as “PU/PD.”There are a number of possible reasons for the increase in thirst and urination seen in hyperthyroid cats. The most common explanation for the PU/PD is mild, concurrent kidney disease. About a third of all cats older than 12 years of age will develop kidney disease, so the finding of concomitant kidney disease in a hyperthyroid cat should not be surprising.
Two other causes for polyuria and polydipsia in cats with hyperthyroidism include the following:
1. Increased kidney blood flow occurs secondary to hyperthyroidism, which can lead to what’s called “renal medullary washout.” This leads to the inability of the cat to concentration their urine normally.
2. Compulsive water drinking secondary to a behavior problem may also occur in some cats with hyperthyroidism.
In both groups of these cats, the increased thirst and urination return to normal after treatment of the cat’s hyperthyroidism.
5. Vomiting or regurgitation
Gastrointestinal signs including intermittent vomiting or regurgitation are fairly common in cats with hyperthyroidism. Vomiting may result from a direct action of thyroid hormones on an area of the brain called the chemoreceptor trigger zone. In other cats, the vomiting can result from gastric stasis (e.g., delay emptying of the stomach).Vomiting appears to be more common in cats from multi-cat households and usually occurs shortly after feeding. Therefore, vomiting and regurgitation in most hyperthyroid cats may simply be related to rapid overeating. In other words, these cats eats too much food too fast, overfill their stomach, and then vomit.
One solution to this problem during the early stages of treatment is too feed the cat smaller amounts of food more frequently (every 3 to 4 hours, if possible).
6. Anxiety, night yowling, confusion, aimless pacing
The behavioral sign most obvious to owners is night yowling (a long loud mournful cry). This yowling most often occurs in the middle of the night for no apparent reason, and generally causes the owner to awaken. Aimless pacing and easily interrupted sleep patterns may also occur in some hyperthyroid cats.All of these behavior signs appear to reflect a state of confusion, anxiety, or restless associated with the increased central nervous stimulation caused by the hyperthyroidism.
7. Diarrhea, soft stools, voluminous stools
Soft stools and diarrhea can occur in about a third of cats with hyperthyroidism. Other cats develop large voluminous stools with frequent defecation.It is likely that accelerated gastrointestinal transit contributes to the increased frequency of defecation, soft stools and diarrhea. Some of these cats also develop malabsorption, in which they do not normally absorb the food from the intestinal tract.
8. Fast respiratory rate, panting, difficulty breathing
Respiratory abnormalities, including a rapid respiratory rate, panting, or difficulty in breathing at rest, are also common. Respiratory signs tend to occur most frequently during periods of stress. However, some affected cats will be noticeably intolerant of heat and seek out cooler places to sit, and some (especially advanced cases) may pant or breath more rapidly in warm or hot home environments.In the absence of heart failure, weakness of the respiratory muscles due to chronic hyperthyroidism is the most probable reasons for these signs. However, central nervous or psychogenic effects also play a major role in development of these respiratory signs, especially in states of stress.
When severely stressed, some cats with hyperthyroidism will develop severe respiratory distress, rapid breathing, and panting; in addition, these cats may eventually become extreme weak or even collapse from exhaustion and shortness of breath. Hyperthyroid cats should therefore be handled carefully and in the veterinary office.
9. Skin, hair coat, and nail changes
Skin and hair coat changes often develop in hyperthyroid cats. The hair coat, especially in long-haired breeds, is often unkempt, dull, and may even be matted.Some hyperthyroid cats can groom obsessively resulting in alopecia (baldness) or a miliary dermatitis (crusty rash, often with intense itching). This is sometimes associated with an underlying skin allergy, but the skin problem is magnified by the cat’s apparent obsessive and compulsive behavior associated with increased licking and grooming.Excessive nail growth may also occur, especially in cats with chronic and advanced hyperthyroidism. These nails appear thickened and may be more fragile that normal.
10. Apathetic hyperthyroidism
A small percentage (less than 5%) of hyperthyroid cats will show atypical signs where hyperexcitability or restlessness is replaced by depression, apathy, or weakness. Although weight loss is present in these cats, it is accompanied by poor appetite, instead of increased appetite. These disparate signs heighten the importance of clinicians having a high index of suspicion for such a common disease.Many cats with apathetic hyperthyroidism will have concurrent severe non-thyroidal illness such as kidney failure, heart disease, or cancer. Therefore, it is wise to do a complete workup in these cats searching for secondary or concomitant disorders. Occasionally, however, no other medical problem can be identified and the cats respond completely to treatment of hyperthyroidism alone, with resolution of the depression and poor appetite.
Complications of hyperthyroidism
Uncontrolled hyperthyroidism has important consequences on the heart, causing increased heart rate but also changes in the muscular wall of the heart that will eventually cause heart failure if untreated.
Hypertension (high blood pressure) is another potential complication of hyperthyroidism, although seen less commonly, and this can cause damage to several organs including the eyes, kidneys, heart and brain. If hypertension is diagnosed along with hyperthyroidism, drugs will be needed to control the blood pressure.
Kidney disease (chronic kidney disease) does not generally occur as a direct effect of hyperthyroidism, but the two diseases often occur together because they are both common in older cats. Care is needed where both these conditions are present, as managing hyperthyroidism can sometimes have adverse effects on kidney function.
Diagnosis
Although the thyroid glands usually enlarge with hyperthyroidism, this is not usually visible. Detection of enlarged glands will usually require careful palpation (examination by touch) by your vet. However, in some cats there is no obvious enlargement, often because the overactive tissue is in an unusual site (such as the chest cavity).
To confirm a diagnosis, a blood test is needed to measure the level of thyroid hormones in the blood. Usually, measuring the thyroxine (T4) concentration is all that is required to confirm a diagnosis, but sometimes additional tests may be needed (such as measurement of ‘free T4’ in the blood).
Other laboratory tests may also be abnormal – for example liver enzymes are commonly increased with hyperthyroidism, and both blood and urine tests are usually advised to help rule out concurrent problems (such as kidney disease). Blood pressure should also be checked where possible with hyperthyroid cats, and an electrocardiogram (ECG – electrical tracing of heart activity), and a chest X-ray or ultrasound may be helpful where heart disease is suspected.
Technetium scan
A technetium scan is a technique that is available at some specialist centres, and can be useful in some hyperthyroid cats. This technique can help confirm a diagnosis, but importantly will locate exactly where the abnormal tissue is and so can be very helpful if surgery is being considered but where there is no clearly identifiable enlarged thyroid on examination.
A technetium scan involves injecting a very small dose of a radioactive chemical (technetium) into the cat’s vein. The technetium is selectively taken up by abnormal thyroid tissue, and this can be detected using a special camera (a ‘gamma camera’). This is a simple, safe and easy procedure.
Treatment
Successfully treated cats, irrespective of what treatment is used, will usually have complete reversal of all the signs of hyperthyroidism. Further evaluation will usually be needed to check kidney function and ensure long-term goals of thyroid hormone concentrations are being achieved, but most cats do remarkably well when treatment is administered. There are four main options for treatment, each with advantages and disadvantages:
Medical management (anti-thyroid drug therapy)
The most commonly used and effective anti-thyroid drugs belong to a group known as thioamides. This includes both methimazole and carbimazole that are widely used in the management of both feline and human hyperthyroidism. These drugs are available in tablet form and they reduce both the production and release of thyroid hormones from the thyroid gland. They do not provide a cure for, but they do allow either short-term or long-term control of hyperthyroidism. However, to maintain control of the disease medication has to be given at least on a daily basis (and often twice daily).
The treatment dose is adjusted to effect and these drugs are usually both safe and effective. Side effects are uncommon. Poor appetite, vomiting and lethargy are most commonly seen, and these effects often resolve after the first few weeks of treatment. However, if gastric irritation persists alternative treatment may be necessary. More serious problems (bone marrow suppression, liver disorders, or skin irritation) are rarely seen.
Even if other surgery or radioactive iodine is considered (see below), drug therapy is often used initially to monitor the effect of successful treatment on kidney function.
Surgical thyroidectomy
Surgical removal of the affected tissues (thyroidectomy) can produce a permanent cure and is a common treatment for many hyperthyroid cats. In general this is very successful and can produce a long-term or permanent cure in most cats. However, occasionally signs of hyperthyroidism develop again at a later time if previously unaffected thyroid tissue becomes diseased.
To reduce anaesthetic and surgical complications, where possible cats should initially stabilised with anti-thyroid drug therapy before surgery is performed. Any significant heart disease should also be managed. The major risk associated with the surgery itself is inadvertent damage to the parathyroid glands – these are small glands that lie close to, or within, the thyroid glands themselves, and have a crucial role in maintaining stable blood calcium levels. Because there is a small risk that temporary interference with calcium regulation may occur, it is usually recommended that cats remain hospitalised for a few days after surgery, and blood calcium concentrations are monitored during this time.
After surgery, occasional blood tests are recommended to ensure normal thyroid hormone levels are being maintained.
Radioactive iodine therapy
Radioactive iodine (I-131) is a very safe and effective treatment for hyperthyroidism. It has the advantage of being curative in most cases with no ongoing treatment. The radioactive iodine is administered as a single injection, usually simply given under the skin. The iodine is taken up by the active (abnormal) thyroid tissue, but not by any other tissues, resulting in a selective local accumulation of radioactive material in the abnormal tissues. The radiation destroys the affected abnormal thyroid tissue, but does not damage surrounding tissues or the parathyroid glands.
There are no significant side-effects with this treatment, but because cats are temporarily radioactive they have to be kept hospitalised for a short period after treatment as a precaution. Unfortunately, because this treatment requires handling of radioactive drugs it is also only available at certain centres.
A single injection of radioactive iodine is curative in around 95 per cent of all hyperthyroid cats, but following treatment occasional blood tests are recommended to ensure normal thyroid hormone levels are being maintained.
Dietary treatment
A new option that has more recently become available for managing hyperthyroid cats is to feed them on a special diet that has strictly controlled levels of iodine. As iodine is used by the thyroid gland to make thyroid hormones, if there is only sufficient iodine in the diet to make normal levels of these hormones, this can help to control the disease.
This is an interesting form of therapy that appears to be quite successful, at least in a number of cats. It does require feeding a special therapeutic diet that is only available from your vet, and cats need to consume this diet exclusively to ensure effective control. Nevertheless, many cats can have their disease controlled in this way and it is an alternative to other forms of therapy for a number of cats.
Treatment of thyroid adenocarcinoma
The rare cases of thyroid adenocarcinoma (malignant tumour of the thyroid) are more difficult to treat, but can sometimes be successfully managed using much higher doses of radioactive iodine.
rowth in the thyroid gland can cause hyperthyroidism, producing more active cells. However, these cells can develop out of control and instead produce thyroxine continually instead of the amount that is needed. High levels of thyroxine in cats can force the body into overdrive. The body is kept constantly at an unnaturally high level of energy, which places enormous strain on other systems, including the heart.
Left untreated, cat hyperthyroidism can be fatal and early diagnosis is key to ensuring other organs are not affected. Fortunately, there are several treatment options available that can help to manage an overactive thyroid in cats effectively.
Side-effects of treatment for hyperthyroidism
As with any medication or treatment for pets, there can also be side effects. While side effects are temporary and can be treated fairly easily, it’s still important to be aware of them so you can provide the best care for your pet.
Side effects of thyroid medications for cats can include:
- Vomiting
- Loss of appetite
- Itchiness around the face
- A lack of white blood cells, which can reduce immunity to other diseases
Side-effects of surgery, though rare, include problems with calcium levels. While this is curable, it may involve intensive treatment and hospitalisation. Calcium supplementation may have to be continued at home. Problems with infection or wound breakdown may also occur, although this is also very rare.
Compiled & Shared by- Team, LITD (Livestock Institute of Training & Development)
Image-Courtesy-Google
Reference-On Request.
