Hyperthyroidism

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Feline Hyperthyroidism

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Introduction

Hyperthyroidism (overactive thyroid glands) is a very
common endocrine disorder of older cats (> 7-12 years).
Feline Hyperthyroidism can show up at four years of age but
generally appears around ten years of age. Male and female
cats, neutered and unneutered are all susceptible to the
disease. Hyperthyroid cats lose weight but yet eat well,
and seem otherwise healthy. It is caused by an increase in
production of thyroid hormones from the thyroid glands,
which are situated in the neck. Clinical signs associated
with hyperthyroidism can be quite dramatic and cats can
become seriously ill with this condition. However, in most
cases hyperthyroidism is treatable and most cats will make
a complete recovery.

Thyroid glands produce the hormone, thyroxine (T4), which
governs rate of metabolism. The more hormone that is
produced, the higher the metabolic rate and the more
calories the cat burns as energy. Excess thyroxine affects
every organ in the cat’s body. Muscles, heart, stomach and
intestines, urinary and nervous system are all affected.
The disease will also change your cat’s behavior.
Complications resulting from hyperthyroidism include heart
enlargement, increased blood pressure, kidney damage and
intestinal problems. The most common cause of
hyperthyroidism is the presence of hormone-producing,
hyper-functional, nodules within the gland. Much less
frequently, a true tumor of the thyroid gland is the cause
of the disease.



This cat
is showing weight loss and poor coat – typical signs of
hyperthyroidism



Clinical
signs

Some or all of the following symptoms may be
present in a hyperthyroid cat:

• Weight loss

• Hyperactivity,
difficult to examine,

Irritability or nervousness

• Polyphagia
(increases appetite)
• Tachycardia
(more rapid beating)
• Polyuria
(excessive thirst) and polydipsia (excessive urination)
• Cardiac murmur

• Vomiting

• Diarrhea

• Increases fecal volume

• Decreased appetite

• Lethargy

• Polypnea (panting) and dyspnea

• Muscle weakness

• Muscle tremors

• Congestive heart failure

Not
all symptoms will be seen in every cat with
hyperthyroidism, but any one or two of them should be a
strong indication that a veterinary examination is
indicated.

Most cats with hyperthyroidism lose weight, become restless
or even hyperactivity and neglect the care of their
haircoat. They can be difficult to handle during a physical
examination. Some cannot tolerate the stress of being taken
in the car and then restrained for veterinary examination,
and they can develop serious respiratory distress, cardiac
arhythmias, and even cardiac arrest.

Enlargement of one or both thyroid lobes can be detected by
palpation in over 90 per vent of hyperthyroid cats. Both
lobes are affected in about 70 per cent of cases.

Increases appetite (sometimes extreme) is common and if
extra food is provided, it may temporarily prevent loss of
weight, but eventually weight loss occurs is almost all
cases and is sometimes severe. In about 20 per cent of
cases there are alternating periods of increased volume of
faeces, diarrhea and vomiting are olso reported.

Polyuria and polydipsia (increases thirst) occur in about
50 per cent of hyperthyroid cats. Renal azotemia is common
in middle-aged and older cats and is commonly found in cats
with hyperthyroidism, but does not appear to be caused by
the pyperthyroidism. It may become worse after treatment of
the hyperthyroidism.

Thyroid hormones have effects on virtually all the organs
in the body, and therefore it is not surprising that this
disease can sometimes cause secondary problems that may
lead to the necessity for additional investigations and
treatment.

The effect of thyroid hormones on the heart is to stimulate
a faster heart rate (more rapid beating of the heart) and a
stronger contraction of the heart muscle. Cardiac
abnormalities found on physical examination include
tachycardia, systolic murmurs, arrhythmias and signs of
congestive heart failure (e.g., dyspnea, muffled heard
sounds, ascites) are fairly common in cats with
hyperthyroidism. Hyperthyroidism can induce a secondary
form of cardiomyopathy in the cat, either
hypertrophic (the muscle of the largest chamber in the
heart, the left ventricle, enlarges and thickens – so
called ‘left ventricular hypertrophy‘) or,
less commonly dilatated. Chest x-rays and cardiac
ultrasound may reveal secondary hypertrophic
cardiomyopathy.

If left untreated and unmanaged, these changes will
eventually compromise the normal function of the heart and
can even result in heart failure. This means that in some
cats with hyperthyroidism, additional treatment may be
required to control secondary heart disease. However, once
the underlying hyperthyroidism has been controlled, the
cardiac changes will often improve, or may even resolve
completely.The hypertrophic form of cardiomyopathy is
usually reversible but the dilated form is usually not.

A secondary complication of this disease is hypertension
(high blood pressure). This develops as a consequence of
the increased pumping pressure of the hearts. In some cats,
blood pressure can become so high that retinal hemorrhage
or detachment will occur and result in blindness.

On about 10 per cent of cats with hyperthyroidism, the
dominant clinical signs are depression and weakness rather
than hyperexcitability or restlessness. Weight loss is
still common but may be accompanied by anorexia rather than
polyphagia (increased appetite). These cats also frequently
have cardiac abnormalities.

Diagnosis

In most cases, a single measurement of plasma T4
concentration is sufficient to confirm the diagnosis. In
some hyperthyroid cats, plasma T4 concentration fluctuates
above and below the upper limit of reference values and it
is necessary to make repeated measurements to confirm the
diagnosis.

Thyroid scans can be used not only to confirm the diagnosis
but more importantly to demonstrrate conclusively whether
one or both lobes of the thyroid lobes is involved. This is
important in determining the method of treatment.

Treatment

Feline hyperthyroidism can be treated by

• surgical thyroidectomy,

• by administering radioactive iodine,

• or by chronic administration of an antithyroid
drug.


Surgical thyroidectomy (removal of the affected thyroid
lobe) is preferred when there is unilateral thyroid
enlargment. Since bilateral involvement occurs in 70 per
cent of cases, the anterior (externel) parathyroid should
be preserved with an intact blood supply if at all
possible, in the event that the opposite lobe must be
removed at a later date.

An antithyroid drug is available in tablet form and can be
used if necessary to bring severe hyperthyroidism under
control prior to surgery. For most cats this is a safe and
effective treatment for hyperthyroidism. Treatment for 2-4
weeks is usually sufficient. It can also be used as the
sole treatment in cats in which neither surgery nor
radioiodine therapy are possible, for whatever reason.

Several antithyroid drugs have been used for this purpose
in cats.

Side
effects


Mild side effects are relatively common (about 15 per cent
of the cats) and include anorexia, vomiting and lethargy.
In most cases these are transient and treatment does not
heve to be discontinued.

Severe signs, necessitating discontinuance of the drug,
include persistent anorexia and vomiting, self-induced
excoriations of the face and the neck, and hepatic toxicity
with anorexia, vomiting, lethargy, jaundice and elevation
of the liver enzymes. Most signs disappear within a few
days of stopping the drug, but jaundice and abnormal liver
enzymes may persist for several weeks.

Sometimes there are mild hematologic side-effects.

Most of the serious side effects develop within the first
few weeks of treatment, but they cn occur at any time.
After the first 3 months, follow-up measurements of T4
alone can be made at 6-months intervals, to adjust the dose
of the antithyroid drug as needed.

Other labaratory measurements can be made if indicated by
clinical findings.