Tuesday, June 7, 2011

Top 12 Physical Exam Findings in Cats with Hyperthyroidism

Hyperthyroidism is the clinical syndrome caused by overproduction of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3). In cats, hyperthyroidism is most commonly due to a benign tumor affecting one or both lobes of thyroid gland.

Thyroid hormones normally regulate 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, including weight loss and muscle wasting. High thyroid hormone concentrations also interact with and stimulate the central nervous system, which commonly leads to increased activity, restlessness, aggression, and panic attack.

Clinical features 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.

When you take your cat to your veterinarian, there are a number of findings that should alert us to that fact that your cat might be hyperthyroid and that specific diagnostic testing should be undertaken.

If Your Cat Is Hyperthyroid, What Will Your Veterinarian Find on the Physical Examination?

Here I’ve listed the top 12 most common physical exam findings that a veterinarian may find on routine physical examination of the feline hyperthyroid patient. Your cat doesn’t need to have all of these physical exam abnormalities in order to have an overactive thyroid problem. But if your cat has one or more of these findings, especially if he or she is older than 10 years of age, additional testing to rule out hyperthyroidism may be warranted.

1. Enlarged thyroid gland (Goiter)
Because of its small size and flat shape (only 2-3 mm thick), even an experienced veterinarian cannot palpate (feel) the normal feline thyroid gland. In contrast, palpable nodular enlargement of the thyroid gland (goiter) is present in the vast majority of cats with hyperthyroidism.

The thyroid gland is made up of 2 separate halves or lobes. About 70% of hyperthyroid cats have enlargement of both thyroid lobes (bilateral tumors), whereas 30% have involvement of only one side (unilateral tumor).

A palpable goiter is an expected finding because since all hyperthyroid cats have a tumor as the underlying cause of the disease. Almost all of these tumors are benign (i.e., adenomas) but malignant tumors (i.e., carcinomas) develop in about 5% of the cats with hyperthyroidism.

Veterinarians use a variety of techniques to palpate an enlarged thyroid gland in cats. With the classic palpation technique, the cat is placed in sitting position and the front legs held still. The neck of the cat is extended, and the veterinarian’s thumb and index finger are placed on each side of the trachea and swept downwards from the larynx to the sternum. Palpation of a freely movable, subcutaneous nodule or a “blip” that slips under the fingertips determines the presence of a goiter.

Thyroid lobes are loosely attached to the surrounding tissues and tend to migrate ventrally (downward toward the chest) as they grow and become larger. In fact, these large thyroid tumors occasionally are pulled down from their normal location adjacent to the larynx (voice box) into the chest cavity.

In some cats with very large thyroid tumors, the goiter is visible and palpation is not even required to identify it.

2. Muscle wasting
Weight loss despite a normal to increased appetite is the classic and most common sign of hyperthyroidism in cats. These cats lose weight because their metabolic rate is increased: they are burning up their food calories faster than they can consume their daily meal.

The weight loss characteristic of hyperthyroidism is generally progressive and can usually be first noticed as a loss of muscle mass around the cat’s back (spine). Despite this muscle atrophy, most hyperthyroid cats retain their “belly” during the initial stages of their thyroid disease.

With time, however, severe muscle wasting, emaciation, cachexia, and death from starvation can occur if the cat’s hyperthyroidism is left untreated. Concomitant with the progressive muscle wasting, cats with advanced or chronic hyperthyroidism may become weak and are easily fatigued.

3. Hyperactive, Nervous behavior
Hyperactivity or restlessness is relatively common in cats with hyperthyroidism. These signs may be more obvious when attempts are made to restrain the cat and are therefore often more noticeable to the veterinarians during the physical exam than to owners themselves. Many of these cats do not wish to stay on the exam table or will tolerate being handled for only short periods.

See video of severe anxiety in a cat with severe untreated hyperthyroidism here:

4. Too thin (Low body condition score)
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.

Your veterinarian should weight your cat at the time of every examination. With this information, it is easy to document the progressive weight loss that occurs in cats with untreated hyperthyroidism.

Veterinarian can also determine your cat’s body condition score as a semi-quantitative method of evaluating body fat and lean body tissue. To score your cat’s body condition, the veterinarian checks for palpable fat covering the ribs, the degree of abdominal tuck, and the presence and degree of the abdominal fat pad. Because they lose too much weight, cats with advanced hyperthyroidism obviously will develop a low body condition score.

5. Fast heart rate
It is very common for hyperthyroid cats to have an abnormally fast heart rate, especially when examined at the veterinarian’s office. At home, the normal cat's relaxed heart rate is 140 to 180 beats per minute (bpm). Even in a normal cat, however, the heart rate will often be slightly fast in the clinic or hospital situation due to stress or fear.

About half of cats with hyperthyroidism will have very rapid heart rates (over 250 bpm) found at time of their physical examination. Some of these cats will even have heat rates over 200 bpm when they are relaxed at home.

In addition to the rapid heart rate, the intensity of each heartbeat is often more pronounced in cats with hyperthyroidism. In some of these cats, each heartbeat can easily be felt by placing ones hand on the cat’s chest cavity. When the veterinarian listens to a hyperthyroid cat’s heart, one hears the intense “pounding” of the overactive heart.

6. Low-grade fever
The high metabolic rate of hyperthyroid cats sometimes causes hyperthyroid cats to have a mildly elevated rectal temperature (103 F, 39.4 C). Such low grade “fever” can easily occur during a visit to your veterinarian simply due to the stress of the visit.

Almost none of these hyperthyroid cats have a true fever. Rather, the slightly high body temperature reflects the heat intolerance resulting from the increased metabolic rate of hyperthyroidism.

7. Heart murmurs
Cardiac murmurs are detected in about half of hyperthyroid cats on the physical examination. Hyperthyroidism is the single most important factor for the development of murmurs in older cats.

8. Panting or difficulty breathing
Respiratory abnormalities are common findings on physical examination and may include a rapid respiratory rate, panting, or difficulty in breathing at rest. Because of their increased metabolic rate, hyperthyroid cats generate more body heat than normal. Because of that, they are more sensitive to higher temperatures than they were when not hyperthyroid.

Cats do sweat, but not how you might think. They only sweat through their paws, but this is too small a surface area to do much cooling. Because of their decreased ability to sweat when very hot, they must breath faster (or pant) in order to dissipate excess body heat.

These respiratory signs tend to occur most frequently during periods of stress, such as a car ride or visit to the veterinarian’s office.

When severely stressed, cats with hyperthyroidism may develop severe respiratory distress, rapid breathing, and extreme panting and open-mouth breathing. During their exam or blood sampling, these cats may become extreme weak or even collapse from exhaustion and shortness of breath. Hyperthyroid cats should therefore be handled carefully and gently, especially in the veterinary office during these stressful periods.

9. Poor hair coat
Skin and hair coat changes are often detected on examination of hyperthyroid cats. The hair coat, especially in long-hair breeds, is often unkempt, dull, and may even be matted.

Long and thickened nails may also be noted, especially in cats with chronic and advanced hyperthyroidism.

10. Aggression, Panic attack
Some hyperthyroid cats will suddenly show signs of aggression during their exam. They may attempt to bite or scratch the veterinarian, technician, or even the owner during these episodes.

Even more frightening, other hyperthyroid will develop a “panic attack” as a result of the impaired tolerance for examination or restraint during blood sampling. These cats may develop panting, overt respiratory distress, weakness, and even collapse during these episodes.

11. Irregular heartbeat
An irregular heartbeat or abnormal heart rhythm (also called an arrhythmia) can occasionally be heard during the cardiac examination of cats with hyperthyroidism. Gallop rhythms can also occur.

An electrocardiogram (EGG) is commonly performed in these cats to better define the arrhythmia and determine if specific treatment is needed.

12. High blood pressure
Mild to moderate hypertension is not an uncommon finding in untreated hyperthyroid cats. However, these cats are typically only mildly hypertensive and we do not generally see severe complications associated with high blood pressure (e.g., blindness, retinal hemorrhage or detachment).

When hypertension is mild, it may simply reflect the reduced tolerance of hyperthyroid cats to stressful situations such as a veterinary examination and may explain why typical hypertensive complications are rarely seen. This is commonly referred to as “white-coat” hypertension.

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