Does your senior cat suddenly seem rejuvenated? Is he more active despite his advanced age? Does he seem to be losing weight though he eats voraciously? These are all signs of an illness called hyperthyroidism.
The Thyroid Gland
The thyroid gland is located in the neck in the area of the trachea and consists of two lobes. The gland is responsible for secreting hormones that control metabolic rate and body temperature. These hormones have a direct impact on the organs of the body.
What Happens in a Patient With Hyperthyroidism?
Hyperthyroidism is an illness caused by an abnormal excess production of hormones by the thyroid gland. It is a chronic, insidious condition that typically affects senior cats (10 years and older), but could also be found in younger patients.
The Cause?
In the majority of cases (about 80%), the condition is caused by a process that affects the two lobes called ‘nodular hyperplasia’ in which the gland becomes enlarged and begins to produce too many thyroid hormones. The reason why some cats develop this illness is still a mystery. A combination of genetic, environmental, and nutritional factors could be the cause.
In about 15% of cases, the illness is caused by the appearance of an adenoma – a benign, non-cancerous tumour within one of the lobes. This adenoma also produces an excess quantity of thyroid hormones.
In only 2-3% of cases the hyperthyroidism is directly associated with a malignant or cancerous tumour that produces too many hormones.
Symptoms
The fact that the illness is progressive explains why an animal may at first present only a few, subtle symptoms. Here are some to look for :
Loss of weight
Hyperactivity, agitation
Changes in behavior, anxiety, nervousness, aggressiveness, excessive vocalisation
Increased appetite and sometimes loss of appetite in advanced stages of the disease
Increased thirst
Increased production of urine
Vomiting
Diarrhea or loose stools
Poor or unkempt hair coat
Nodules in the neck
Increased heart rate, appearance of a heart murmur, arrhythmia, hypertension
Loss of vision, hemorrhages in the eyes, blindness
Eventually, if the disease is left untreated it will cause heart failure, renal failure, blindness, neurological problems, severe dehydration and eventual death.
Diagnosis
The first step in diagnosing hyperthyroidism is to consult your veterinarian. A history of symptoms and the presence of any anomalies, such as the detection of an enlarged thyroid gland will lead to a diagnosis.
A definitive diagnosis is obtained with a blood test which measures the level of thyroid hormones in the blood (thyroxine or T4). Since cats suspected of having a thyroid problem are usually at an advanced age, they are also at risk for other health problems. For this reason, the veterinarian will recommend, as well as the measurement of T4, a complete blood test and urinalysis.
In the majority of cases, the investigations will present a clear diagnosis of hyperthyroidism for which the veterinarian will inform you of its consequences and other related diseases.
Certain rare cases require further follow-up, including control blood tests, ultrasound, or scintigraphy (a test in which radioisotopes are passed through the body to obtain a 3-D image or X-ray).
Treatment
There exist 4 different types of treatment available for hyperthyroidism. Each option has its advantages and disadvantages. Your veterinarian will help to guide you towards the best course of treatment depending on your situation and the condition of your pet.
Dietary treatment with Hill’s Prescription Y/D cat food (available in dry and cans) : The production of thyroid hormones is dependent on the presence of iodine in the body. Without iodine, these hormones cannot be produced. The Hill’s company has developed a cat food formula deficient in iodine, thereby limiting the production of thyroid hormones in cats that eat the food. This option is safe, economical, and has no associated side effects. However, it is recommended only for cats that live indoors and portions must be strictly monitored. No treats, supplements or table food may be offered since the iodine contained in these may cause the thyroid gland to begin to produce too much hormones.
Pharmaceutical treatment with oral medication : Methimazole is an anti-thyroid oral medication marketed in Canada under the name Tapazole. This molecule inhibits the production and release of thyroid hormones by the glands. The medication must be administered on a regular basis, morning and night for the rest of the patient’s life. The dosage may be adjusted with regular follow-up monitoring of the T4 levels in the blood.
The most common side effects, seen in about 10% of cases, are vomiting, loss of appetite and lethargy. These side effects are generally mild and pass after a few weeks of treatment. Other more serious side effects are reversible, but necessitate stopping the medication completely. These include skin irritation on the head and neck (2.3%), hemorrhagic syndrome (2.3%), jaundice related to liver problems (1.5%), hematology problems (between 1.5 and 2.7%) and fever. If these symptoms persist, it may require an alternative medical treatment.
Radioactive Iodine treatment : This treatment cures the disease in the majority of cases. It consists of administering radioactive iodine in a single injection that destroys the abnormal thyroid tissue without destroying healthy tissue. This therapy is available only in centres that handle radioactive materials and requires a hospitalisation of the pet for about 7-10 days, the time it takes for the cat to become non radioactive.
Surgical treatment : Since the appearance of radioactive iodine treatment, a process that is safe and less invasive, the surgical option has become less popular. The patient must be in a condition to undergo a general anesthesia. Complications may arise if the tissues surrounding the thyroid glands are damaged during the intervention.
Medical Follow-Up and Blood Tests
It should be noted that the first two treatment options, nutritional (Hill’s Y/D) or with oral medication (methimazole), do not cure the disease. Both control the production of thyroid hormones which in turn control the symptoms. However, if the treatment is not followed adequately or is stopped, the thyroid gland will continue to produce an excess of hormones. It may take several weeks of treatment before an effect is noted in the symptoms, so it is recommended for the patient to submit to a follow-up blood test about 3-4 weeks after the beginning of treatment.
Also, over time, the sick glands will continue to grow and produce more hormones necessitating an increase in the dosage of Methimazole. It is therefore necessary to routinely test the T4 level in the blood every 3-6 months and adjust the medication as required.
In the case of nutritional therapy, control blood tests help to assure the effectiveness of the diet. A poorly controlled hormone level could indicate that the patient has access to food other than the Y/D, even in small quantities.
In some patients, specific follow-up visits may be necessary to monitor arterial pressure, renal function, or blood tests to determine any adverse effects of the medication. Your veterinarian will make further recommendations based on these clinical tests and examinations.
Prognosis and Quality of Life
Among the most frequently diagnosed illnesses in senior cats – renal insufficiency, diabetes, cancer – hyperthyroidism is considered among the least serious.
Thanks to available treatments, these pets enjoy a very good quality of life. The short-term prognosis is excellent. However, given that the illness usually strikes older patients who may suffer from other associated health problems, the long term prognosis is not as positive. The average life expectancy after diagnosis is two years, not due to the thyroid problem, but more related to the age of the patients at the time of diagnosis.