----------------------------------------------------------------------------------------------------------------------------------
Feline Hyperthyroidism
You can find more about cats here
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.
