Hyperthyroidism, or, overactive thyroid gland, is a very common disorder of older cats. It is caused by excessive of thyroid hormone from the thyroid glands, which are situated in the neck. Thyroid hormones play an important role in controlling the body’s metabolic rate and thus the general activity level. Therefore, cats with hyperthyroidism tend to burn up energy too rapidly and typically suffer weight loss despite having an increased appetite and food intake.
In the vast majority of cases, the increased thyroid hormone production is due to a benign tumor of one or both thyroid glands, called thyroid adenoma. Both of the thyroid glands are involved, although one gland may be more severely affected than the other. A malignant tumor known as a thyroid adenocarcinoma can also be an underlying cause of some cases of hyperthyroidism.
Fortunately this is rare, and is only the cause in around one to two per cent of all hyperthyroid cats. When a thyroid adenocarcinoma is present treatment is much more difficult as metastasis (spread to other tissues) is common.
Clinical Signs of Hyperthyroidism In Cats
Hyperthyroidism is typically seen in middle- to old-aged cats, and is rarely seen in cats less than seven years of age. Male and female cats are affected with an equal frequency and there is no known established breed predilections at this time. Cats affected with hyperthyroidism usually develop a menagerie of clinical signs, which are usually quite subtle at first, but then become more severe as the disease progresses. Also, since this disease occurs mostly in older cats, some affected 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 despite an increased appetite (polyphagia), increased thirst (polydypsia), increased irritability, and restlessness or even hyperactivity. Many affected cats have a rapid heart rate (tachycardia) and develop an unkempt coat. Mild to moderate diarrhea and/or vomiting is also quite common. Some affected cats will be noticeably intolerant of heat and seek out cooler places to sit, and some (especially advanced cases) may pant when they are stressed. Most hyperthyroid cats will show some degree of polyphagia (excessive appetite) and restlessness, but in some advanced cases there will be generalized weakness, lethargy and loss of appetite and the signs will be less hallmark.
Secondary Disease Manifestations
Thyroid hormones influence all the organs in the body. It is therefore 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, as well as a stronger contraction of the heart muscle. Over time, a cat affected with hyperthyroidism will experience enlargement of the muscle of the largest chamber in the heart (the left ventricle) – so called ‘left ventricular hypertrophy’. 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, and in some cases, resolve completely.
Hypertension (high blood pressure) is another potential complication of hyperthyroidism may 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 to reduce the risk of damaging other organs, at least until the hyperthyroidism can be controlled. As with heart disease, following successful resolution of hyperthyroidism, the high blood pressure will sometimes resolve and permanent therapy may not ultimately be necessary.
Kidney disease does not occur as a direct effect of hyperthyroidism, but the two diseases often occur concurrently because both common in older cats. Care is needed where both these conditions are present, as the hyperthyroidism tends to increase the blood supply to the kidneys, which may improve their function. Thus blood tests taken to assess kidney function in a hyperthyroid cat may show normal or only mild changes, but potentially more severe renal failure may be masked by the presence of the hyperthyroidism. For this reason, regardless of what treatment is chosen for long-term management of the hyperthyroidism, it is usually advisable to start on medical treatment initially and to monitor the response with repeat blood and urine tests to look at thyroid function and kidney function together. Occasionally, successful treatment of the hyperthyroidism results in a dramatic decline in kidney function. If this is occurs, it may be necessary to reduce the dose of therapy so that the hyperthyroidism is not fully controlled but kidney function is not too severely compromised.
Large Swelling Of The Neck
If you or your veterinarian suspects hyperthyroidism, diagnosis is confirmed through physical examination and blood tests. On examination, one or two enlarged thyroid glands can often be palpated (felt) as a small, firm, nodular mass in the neck (these are often about the size of a pea or a baked-bean in hyperthyroid cats). However, in many hyperthyroid cats there is no palpable thyroid enlargement, and this can be because the overactive tissue is present in an unusual (ectopic) site..
A blood test looking at thyroxine (T4) concentration is usually all that is required for a confirmed diagnosis of hyperthyroidism, as this is usually significantly elevated in clinical cases. Other laboratory tests may also be abnormal. For example liver enzymes are commonly increased secondary to hyperthyroidism, and
assessment of routine blood and urine tests is usually advised to help rule out any other concurrent disease (such as renal failure). Where possible, blood pressure should also be checked with hyperthyroid cats, and if secondary heart disease is suspected then an electrocardiogram (ECG – electrical tracing of heart activity), and a chest X-ray or ultrasound may be also be warranted. In a minority of cases, hyperthyroidism may be strongly suspected on the basis of the clinical signs, but blood testing may reveal a normal thyroid hormone (T4) concentration. There are a number of potential reasons for this and often on a T4 recheck, it is found to be be in fact elevated. If not, additional tests may need to be performed to confirm or rule out hyperthyroidism.
Technetium scanning is a technique that is available at a few specialist referral centers, and can be a useful diagnostic in the investigation of some hyperthyroid cats. This technique can be used both to diagnose hyperthyroidism and also to locate exactly where the abnormal thyroid adenomal tissue is anatomically located. This can be particularly helpful if surgery is being considered but there is not a clearly identifiable enlarged thyroid on physical examination. Using this technique a very small dose of a radioactive chemical (technetium) is injected into the cat’s vein. The technetium is selectively taken up by abnormal thyroid tissue, and this can be detected using a special camera called a gamma camera. This is a simple, safe and easy procedure that may be helpful in some situations.
There are three main options for the treatment of hyperthyroidism, each with advantages and disadvantages:
Medical management (drug therapy)
Anti-thyroid drugs act by reducing the production and release of thyroid hormone from the thyroid gland. They accomplish this by constantly causing destruction of thyroid tissue. They do not provide a cure for the condition, however, but they do allow either short-term or long-term control of hyperthyroidism. Methimazole (trade name Tapezole ®) is commonly given at a starting dose of 5 mg twice daily. With treatment, thyroid hormone concentrations usually regulate within the normal range within two to three weeks, at which time the dose can often be reduced. Treatment is adjusted according to response, but to maintain control of the hyperthyroidism with drug therapy, they need to be given daily for the rest of the cat’s life.
For most cats methimazole is a safe and effective treatment for hyperthyroidism. Side effects are uncommon, but if they do occur they are usually mild and reversible. Poor appetite, vomiting and lethargy are the most common side effects and often resolve after the first few weeks of treatment and/or by temporarily reducing the dose of treatment and administering the tablets with food. More serious problems, including low white blood cell counts, reduced platelet counts (important component of the body’s ability to clot), liver disorders, or skin irritation are rare, but if they do occur then an alternative treatment must be considered.
Anti-thyroid drug treatment has the advantage of being readily available and economical, but, as previously stated, it is not curative. Life long treatment, usually involving twice daily oral dosage, will be required and for some owners, and some cats, this may be difficult to achieve. Routine blood tests should be checked periodically during treatment to monitor the effectiveness of therapy, to monitor kidney function, as well as to look for potential side effects.
Surgical removal of the affected thyroid tissue (thyroidectomy) can produce a permanent cure and is and, before the advent of radioactive iodine therapy (see below), was a common treatment for many hyperthyroid cats. In general, this is a very successful procedure and is likely to produce a long-term cure or permanent cure in many hyperthyroid cats. However, even after successful surgery, occasionally signs of hyperthyroidism develop again at a later time if previously unaffected thyroid tissue becomes diseased.
To reduce anesthetic and surgical complications, where possible it is always recommended that hyperthyroid patients are initially stabilised with anti-thyroid drugs for three to four weeks prior to surgery. Any associated heart disease must also be treated when needed. Success and avoidance of post-operative complications depends on the skill and experience of the surgeon. The major risk associated with surgery 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 constant blood calcium levels. Damage to these glands could result in a life-threatening fall in blood calcium concentrations, or hypocalcemia. This is occurs most commonly when both thyroid glands are removed at the same time. To minimise the risk of this complication in those cats that require removal of both thyroid glands it may be appropriate to perform the procedure in two stages. The two stages consist of removing the most affected gland first, then allowing six to eight weeks for recovery of parathyroid hormone production before removing the second thyroid gland.
It is usually recommended that cats remain hospitalized for a few days after surgery for careful monitoring of blood calcium levels. Clinical signs of low blood calcium include muscle twitches and weakness, which can progress rapidly to seizures. Treatment is to stabilize the patient with supplemental calcium by injection and then orally. Additional treatment with vitamin D3 is also needed to allow the calcium given by mouth to be utilized by the body effectively. Once stable, continued treatment can be administered at home, but in most cats the damage to the parathyroid glands is only temporary and treatment may only be needed for a few days or weeks.
Radioactive iodine therapy
Radioactive iodine (I 131 ) is the gold standard for providing a safe and effective cure for hyperthyroidism. Like surgical thyroidectomy, it has the advantage of being curative in most cases with no ongoing treatment required while having the added benefit of more readily preserving the parathyroid glands. Radioactive iodine is administered as an injection given under the skin. The iodine is then taken up by the active (abnormal) thyroid tissue, but not by any other body tissues, resulting in a selective local accumulation of radioactive material in the abnormal tissues only. The radiation destroys the affected abnormal thyroid tissue, but does not damage the surrounding tissues or the parathyroid glands.
The advantages of radioactive iodine are that it is curative, has no serious side-effects, does not require an anaesthetic and is effective in treating all affected thyroid tissue at one time, regardless of its location. However, it does involve the handling and injection of a radioactive substance. This carries no significant risk for the patient, but precautionary protective measures are required for people who come into close contact with the cat. Also, the feces and urine remain radioactive for a certain period of time. For this reason, the treatment can only be carried out in certain specially licensed facilities where a treated cat has to remain hospitalized until the radiation level has fallen to within acceptable limits. This usually translate into the cat being hospitalized for between three and six weeks following treatment. Most treated cats have normal thyroid hormone concentrations restored within three weeks of the treatment.
A single injection of radioactive iodine is curative in approximately 95 per cent of all hyperthyroid cases, and in the few cats where hyperthyroidism persists the treatment can be safely repeated. Occasionally a permanent reduction in thyroid hormone levels (hypothyroidism) occurs following radioactive iodine treatment. and if this is accompanied by clinical signs (lethargy, obesity, poor haircoat) then thyroid hormone supplementation may be required.
The Hills corporation, a pet food manufacturer that specialized in disease specific nutrition, has recently released a food called Y/D, a diet engineered for management of feline hyperthyroidism. Among many characteristics unique to this diet, it is devoid of iodine, a key nutrient utilized by the thyroid to make thyroid hormone. Thus, by denying the feline hyperthyroidism patient iodine, the thyroid gland is unable to overproduce thyroid hormone. This diet is a relatively new treatment modality, but initial case success looks very promising. One main drawback, however, is that in households with multiple cats, normal cats without hyperthyroidism cannot eat this diet, as it will lead to hypothyroidism, or unreactive thyroid disease. Therefore, if multiple cats in one household cannot be separated from one another’s food, then this diet is not a good idea.
Treatment of thyroid adenocarcinoma
Rare cases of thyroid adenocarcinoma (malignant tumour) are more difficult to treat and the prognosis is guarded to poor. Some can be successfully treated using very high doses of radioactive iodine, however, external beam radiation therapy (conventional radiotherapy) is likely the best option. Before even considering treating thyroid adenocarcinoma, a full diagnostic work-up must first be performed to rule out metastasis.
Roger L. Welton, DVM
Founder and Chief Editor, Web-DVM.net
President, Maybeck Animal Hospital
Article updated 10/12/2017