Hyperthyroidism is a widely recognized disorder of middle-aged and older cats. Most cases are caused by benign thyroid adenomas (tumors). Common clinical findings include weight loss, increased appetite, hyperactivity, restlessness, rapid heart rate, vomiting, increased thirst and urination, heart murmur, diarrhea, increased fecal volume, difficulty breathing, and rapid breathing, and muscle weakness. Some cats exhibit atypical signs such as inappropriate urination, seizures, lethargy, and loss of appetite. A suspicion of feline hyperthyroidism should be based on the history, clinical signs, laboratory tests and physical examination abnormalities, and detection of thyroid gland enlargement by palpation. Diagnosis is confirmed by documenting an elevated serum thyroid concentration.
Hyperthyroidism has progressive systemic consequences and, in the untreated cat, ultimately results in emaciation and severe metabolic and cardiac dysfunction. The aim of treatment is to decrease the excessive production of thyroid hormones as quickly as possible with minimal risk to the cat. Treatment options include antithyroid drugs, surgery, and radioiodine.
Antithyroid medications may be considered as a short-term treatment in a hyperthyroid cat being prepared for surgery or they may be used to control hyperthyroidism indefinitely. When used preoperatively, they are generally administered for 4 to 10 weeks in order to maintain the cat in a stable state for at least 3 weeks before surgery and to minimize the anesthetic and surgical risks. Long-term treatment may be used to control the clinical signs of hyperthyroidism in cats which are not considered candidates for other therapies, because of owner wishes, concurrent disease, or the unavailability of other treatment modalities.
The two antithyroid medications that are widely used for the medical management of feline hyperthyroidism are methimazole (Tapazole) and propylthiouracil (PTU). Both drugs act by inhibiting the production of thyroid hormones in the thyroid gland. Most cats become stable within 2 to 3 weeks of the initiation of therapy with Tapazole or PTU.. Some hyperthyroid cats may have decreased gastrointestinal absorption, resulting in impaired response to therapy with standard doses.
Mild clinical side effects, including vomiting, loss of appetite, and lethargy, are common with medication. In most cases these signs are transient and usually resolve in spite of continued administration of the drugs. In some cats in which the gastrointestinal side effects persist, the signs may be alleviated by discontinuing the drug temporarily and re- instituting therapy at a lower dose. The dose can then be gradually increased over a period of 7 to 10 days until the recommended dose is tolerated. Persistent severe vomiting or appetite loss will, however, necessitate discontinuation of the drug in some cats. Gastrointestinal side effects occur primarily during the first month of therapy and are much more common with PTU than with Tapazole.
Liver disease has been seen infrequently in cats treated with PTU or Tapazole. Jaundice, appetite loss, and vomiting are the predominant clinical signs. Clinical improvement usually occurs within a few days after cessation of therapy. Other side effects that may be seen and occasionally necessitate discontinuation of therapy include skin eruption, facial swelling, and itching. Other more severe clinical signs include anemia and kidney disease.
Tapazole and PTU are effective in lowering serum thyroxine concentrations in hyperthyroid cats, but because PTU is associated with a higher incidence of mild and severe side effects, Tapazole should be considered the drug of choice for preoperative stabilization and long-term treatment of feline hyperthyroidism.
Tapazole should be administered initially at a dosage of 5 mg orally every 8-12 hours. Serum thyroxine concentrations should return to normal concentrations 2 to 3 weeks after initiating therapy. If little or no decrease in serum T4 occurs after this time, the daily dosage should be increased. The total daily dosage of Tapazole should be increased. The total daily dosage of Tapazole should be increased by 5 mg (to 20 mg/day) and the serum thyroxine concentration re- evaluated 2 weeks later. Monitoring of the serum thyroxine concentration and adjustment of drug dosage (in 5 mg/day increments) should be done every 14 days until the serum thyroxine concentration is maintained in the normal range. Although apparent drug resistance may occur in a small percentage of cats with hyperthyroidism, poor owner compliance is the major reason for drug failure.
During the first 3 months of therapy, when side effects are most likely to occur, the cat should be examined at least every 2 to 3 weeks in order to make necessary adjustments in the drug dose and to monitor for adverse effects. At the time of each recheck, serum thyroxine determinations, complete blood counts (CBCs), platelet counts, kidney function tests , and liver tests should be performed. The drug should be discontinued if there is evidence of a severe drug reaction such as decreased platelets, decreased white blood cells, liver disease or kidney disease. In cats developing these reactions, an alternative therapy should be considered.
In cats for which long-term treatment with Tapazole is planned, the goal of treatment should be to maintain serum thyroxine concentrations in the normal range, using as low a drug dose as possible. Serum thyroxine concentrations should be monitored at intervals of 3 to 6 months, as well as kidney, liver, and blood counts. If serum T4 concentrations fall to low or low-normal levels during therapy, an attempt should be made to lower the daily dose by 5 mg. Serum concentrations should be monitored at 2 to 3 weeks after each dose adjustment, until the lowest effective dose is determined. Many cats can be maintained successfully on a dose of 10 mg/day, and 5 mg/day will be adequate in rare cases. Divided doses administered every 8 or 12 hours are the most effective method for maintaining a response to Tapazole; however, in some cats, administration of the total daily dose once daily will maintain stabilization. The administration of Tapazole less often than once daily results in the return of hyperthyroidism.
The long-term administration of Tapazole as the sole treatment for hyperthyroidism warrants periodic monitoring for adverse side effects. Physical examination and a CBC ,platelet count, kidney, and liver test should be performed (every 3 to 6 months) at the time of serum thyroxine measurement.
Antithyroid medications are widely available to the practicing veterinarian. The administration of these medications and the monitoring and control of hyperthyroidism may be achieved in nearly every case without the need for specialized equipment or personnel. No anesthesia or surgery is required, making this treatment fairly safe, even for the critically ill hyperthyroid patient. The incidence of mild undesirable side effects is high, but the incidence of severe side effects is low when Tapazole is administered. A small percentage of cats need to have the drug discontinued due to the development of side effects. Rarely, a cat will be refractory to Tapazole therapy, necessitating consideration of an alternative therapy.
Poor owner compliance is the most common reason for the failure of antithyroid medications to control hyperthyroidism. Antithyroid medications decrease serum thyroxine concentrations and alleviate the signs of hyperthyroidism, but they do not correct the underlying disorder. If the drug is withdrawn or if doses are missed, the signs recur rapidly. It must be stressed that the owner will have to administer medication at least once daily for the rest of the cat’s life. Owner compliance deteriorates as the cats learn to resent and resist the administration of medication.
One must be aware of the costs. This must be discussed with your veterinarian when determining the appropriate course of treatment.
Surgical removal of affected thyroid tissue , thyroidectomy, is indicated as a definitive treatment for feline hyperthyroidism. It is considered by many to be the treatment of choice for the cure of hyperthyroidism.
Hyperthyroid cats have increased anesthetic and surgical risks due to their advanced age and the cardiovascular and metabolic stresses of their disease. Many of these cats are emaciated and most have cardiac abnormalities. Screening for concurrent disease and assessing the severity of the disease namely the induced metabolic and cardiac disease are therefore important before considering or recommending surgical treatment. Chronic renal failure, diabetes mellitus, and gastrointestinal (GI) and cancer are common in older cats and may be accidentally overlooked if the diagnosis of hyperthyroidism is made. A thorough physical examination, CBC, serum biochemistry profile, urinalysis, electrocardiogram (ECG), and chest x-ray and ultrasound are therefore indicated in every hyperthyroid cat before treatment is recommended.
The incidence of surgical and anesthetic complications in hyperthyroid cats can be decreased by proper preoperative management. Ideally, a normal thyroid state should be maintained in each cat for 3 to 9 weeks before surgery. Any cat which has been maintained on antithyroid drugs should have a CBC, platelet count, kidney and liver tests performed immediately before surgery to be certain that significant complications such as anemia and other diseases affecting the kidney etc. have not occurred.
If Tapazole cannot be administered because of adverse reactions, or if it fails to control rapid heart rate, propranolol, a cardiac drug, may be administered to lower the heart rate to less than 200 beats per minute (bpm). Propranolol, a beta- blocker, should be administered initially at a dose of 2.5 mg orally every 8 to 12 hours; the dose is slowly increased until the desired effect is achieved. Most cats will require 2.5 to 5 mg orally every 8 hours for control.. The heart rate will usually decrease after the administration of one to three doses. Treatment for 1 to 2 weeks will help stabilize a hyperthyroid cat prior to surgery.
Hyperthyroid cats should be anesthetized with caution and monitored closely, because they are extremely sensitive to the adverse effects of preanesthetic and anesthetic agents. Electrocardiographic monitoring is essential, because irregular heartbeats develop commonly and are the most frequent cause of mortality during surgery. Induction of inhalation anesthesia may be performed by placing the cat in an anesthesia induction chamber (box) or administering the inhalant via a mask. Isofluorane, although expensive is the inhalant anesthetic of choice because it provides rapid induction and recovery. After anesthetic induction, the cat should be intubated and maintained on an inhalant anesthetic. If an irregular heartbeat develops, the anesthetic concentration should be decreased and the cat ventilated with oxygen. If the irregular heartbeat persists, propranolol may be administered in 0.2-mg intravenous injections, as needed to control the heartbeat. Close observation should continue until the cat has recovered completely from anesthesia.
The feline thyroid gland consists of two lobes that normally lie adjacent to the first five to six tracheal rings. Occasionally, enlarged thyroid lobes will descend down the neck into the thoracic inlet, where they may not be palpable or surgically accessible. In addition, additional thyroid tissue may develop in chest cavity due to an abnormal development as an embryo. Inaccessible hyperfunctional thyroid tissue is responsible for hyperthyroidism in 3 to 5 per cent of affected cats.
Two parathyroid glands (these glands regulate calcium levels in the body) are associated with each thyroid lobe. The external parathyroid gland is located within the front end of the thyroid lobe, whereas the internal parathyroid gland is located within the thyroid gland. The external parathyrid glands are spherical, 2 to 7 mm in diameter, and lighter in color than the surrounding thyroid tissue. Preservation of at least one parathyroid gland is essential for the maintenance of proper calcium levels. If these glands are inadvertently removed, your pet will develop seizures and will require life long administration of calcuim.
Identification of the source of the excessive production of thyroid hormones is necessary for successful surgical correction. Twenty to 30 percent of hyperthyroid cats have disease affecting only one thyroid gland. More often, cats have bilateral disease, with variable enlargement of the two affected lobes. In 15 percent of cats with bilateral lobe involvement, one lobe appears normal at the time of surgery, complicating visual assessment of the extent of disease. Preoperative nuclear thyroid scans, when available, help to determine the extent and location of abnormal thyroid lobes as well as the presence or absence of abnormally located thyroid tissue. This will simplify the surgical decision concerning which tissues to remove. In most cases, no scan is available and vets must make this decision by palpation of the cat’s neck and by intraoperative visualization of the thyroid lobes.
At the time of surgery, both thyroid lobes should be inspected. If both thyroid lobes are clearly enlarged, they should be removed with an attempt to preserve the external parathyroid glands. If one lobe is enlarged and the other is clearly atrophied, removal of the single enlarged lobe is indicated. If one lobe is clearly enlarged but the other lobe appears normal in size, only slightly atrophied, or abnormal in color or texture, bilateral disease should be suspected, and in most cases bilateral thyroidectomy with parathyroid preservation is warranted. Occasionally no thyroid tissue can be located in the proper location, or both thyroid lobes appear atrophied.
Postoperatively cats should be monitored closely. Potentially serious complications of thyroidectomy include hypoparathyroidism and nerve paralysis of the throat. Hypothyroidism, low thyroid production, is common after bilateral thyroidectomy.
Failure to maintain function in at least one parathyroid gland results in hypoparathyroidism, decreased calcium production. This complication is seen most commonly in cats which have undergone bilateral thyroidectomy. Hypoparathyroidism causes hypocalcemia, low calcium levels, the clinical signs of which include weakness, muscle tremors, twitching, and convulsions. Signs usually develop 1 to 3 days postoperatively and may progress rapidly. Calcium concentrations should be monitored at least once daily after bilateral thyroidectomy until the calcium concentrations should be monitored at least once daily after bilateral thyroidectomy until the calcium concentration stabilizes within the normal range.
Hypoparathyroidism can be serious and even fatal complication of thyroidectomy. Inability to monitor these cats adequately should be considered an absolute contraindication for surgical therapy. Some surgeons have recommended that bilateral thyroidectomies be performed as a staged procedure in order to prevent this complication, allowing 3 weeks between the removal of thyroid lobes to allow time for the damaged parathyroid gland to recover. Although this approach requires an additional surgical procedure, results have been favorable.
Serum thyroid hormone concentrations decline to subnormal levels within 24 to 72 hours of bilateral thyroidectomy and remain low if all thyroid tissue is removed. It is unusual, however, for these cats to develop clinical signs suggestive of hypothyroidism. Routine thyroid hormone supplementation may not be necessary. If signs such as obesity, sluggishness, and poor hair coat develop and if the measured serum thyroxine concentration is low, treatment with sodium levothyroxine (0.1 mg/day) is warranted.
Thyroidectomy is curative in most hyperthyroid cats. It is a technically simple procedure that can be performed successfully in most veterinary hospitals. Anesthetic and postoperative complications have been reported to occur commonly, but their incidence and significance decrease as appropriate precautions, monitoring, and treatment are initiated. Relapse has been seen in 5 percent of cats after bilateral thyroidectomy, as a result of tissue inadvertently left behind or because of the presence of ectopic hyperfunctional tissue. Twenty percent of cats treated with unilateral thyroidectomy become hyperthyroid again within 1 year of the initial surgery, illustrating the inadequacy of visual inspection as a means to distinguish unilateral from asymmetric bilateral disease. A small percentage of cats will have hyperfunctional thyroid tissue inaccessible to the surgeon, necessitating consideration of other therapy.
Thyroid cells have a natural avidity for iodine and are unable to distinguish between normal and radioactive iodine. When radioiodine. is administered orally or intravenously, it is taken up by the thyroid gland and trapped within the colloid of functioning thyroid follicles. The most commonly used isotope is 131, which has a half-life of 8 days and emits gamma rays and beta particles. Beta particles travel up to 2 mm (average 400 um) in tissue and are locally destructive, damaging functioning thyroid tissue. Radioiodine therapy is effective in destroying all hyperfunctional tissue, whether it is associated with the gland or in an ectopic location. The chronically elevated serum thyroxine concentrations seen in hyperthyroid cats result in decreased thyroid- stimulating hormone (TSH) and atrophy of normal thyroid tissue. Normal tissue, then, does not take up the radioiodine and is spared.
The goal of radioiodine therapy is to obtain a cure as quickly as possible. Most reports emphasize the calculation of a dosage using an estimation of thyroid gland size and radioiodine kinetics following a tracer dose of 131I. This requires sophisticated and expensive nuclear medicine equipment and trained personnel. Although attempts to precisely determine the dose to be delivered to the thyroid using these calculations have been unsuccessful, the results of therapy using the doses calculated by this method have been promising. Recently, however, similar therapeutic success has been demonstrated using a fixed dose of 4 or 5 mCi of 131I, thereby dramatically decreasing the time, expertise, expense, and equipment required to threat hyperthyroid cats with radioiodine. Functional thyroid carcinomas, seen rarely in the cat.
When considering radioiodine therapy for a hyperthyroid cat, it is important to evaluate the patient carefully. Travel can be stressful and even fatal for the unprepared, unstable hyperthyroid cat. In addition, following treatment, the cat will be radioactive, and laboratory testing and intensive care will be radioactive, and laboratory testing and intensive care will be difficult or impossible for a prolonged period of time (2 to 5 weeks). The cat should, therefore, be relatively stable before radioiodine therapy is recommended. Attempts to stabilize the cat may include propranolol (a cardiac drug) therapy and perhaps even a 2 to 3 month course of antithyroid medication, as was described for the presurgical patient.
The treatment of hyperthyroid cats with radioiodine is a simple and nonstressful procedure. An intravenous catheter is inserted and the iodine is administered as a slow bolus. Although some have advocated oral administration of the iodine dose, it is not recommended, because of the potential for variable absorption, vomiting of the entire or partial dose, and additional human exposure.
After treatment, strict radiation safety precautions must be followed. Each cat is maintained in a metabolic cage in a room with limited access. This room must be inspected and approved by the appropriate government radiation safety officials. Personnel handling the cats must wear laboratory coats, gloves, and dosimeters. All material removed from the cage must be disposed of as radioactive waste. Cats are hospitalized until their surface radiation dose as measured by a geiger counter is below a level deemed safe by radiation safety officials. This is usually a period of 1 week. When the cats are returned to their owners, additional precautions and restricted owner contact may be advised for 2 to 3 weeks.
Adverse effects of radioiodine therapy have not been reported. Most cats seem to improve within days after treatment. There is a rapid decrease in serum thyroxine concentration, with greater than 70 percent of the cats becoming normal by the eighth day after treatment. The owner notices gradual clinical improvement that is most apparent 3 to 8 weeks after therapy.
More than 85 percent of hyperthyroid cats treated with a calculated or empirical dose of radioiodine will be cured by a single dose. However, 10 to 15 percent of cats treated with radioactive iodine will fail to respond completely to an initial dose. These cats improve clinically, but their serum thyroxine concentrations remain elevated. If a second dose of 131I is not administered, these cats become clinically hyperthyroid again 6 to 9 months after therapy. A second dose of 131I is effective in curing most cases. The reason for an incomplete response to a single treatment by some cats is uncertain, but may be related to the initial degree of hyperthyroidism, prior antithyroid drug administration, thyroid gland size, or the presence of thyroid cancer. The present recommendation is to discontinue oral antithyroid medication at least 4 weeks prior to the administration of 131I to maximize the likelihood of successful treatment.
Cats treated with radioiodine should have their serum thyroxine concentration measured 1 to 2 months after therapy, retreatment is recommended. Although serum thyroxine concentrations may be low for a period of time after therapy, treatment with sodium levothyroxine is not indicated unless clinical signs of hypothyroidism occur. Normal thyroid tissue is spared by radioiodine therapy, and it will regenerate best in the absence of exogenous thyroid hormone supplementation.
The administration of radioiodine is a safe, simple, and effective treatment for feline hyperthyroidism. This treatment involves minimal stress to the patient and is rapidly effective. The small percentage of cats not cured by a single dose of 131I will respond to a second treatment. The treatment effectively destroys all hyperfunctioning thyroid tissue, whether it is located in the cervical region or in the thorax.
The major disadvantage of radioiodine therapy is its limited availability. Now many of the universities as well as some private practices are now offering this treatment. One private practice offering RIA treatment is Radiocat (www.radiocat.com). The utilization of a fixed dose of 131I rather than a calculated dose may make this therapy more accessible in the future. In addition to the financial restrictions, however, there are strict regulations imposed on institutions offering this treatment because of the radiation hazard of the radioisotope. In nearly every case, this therapy will be available to the practicing veterinarian only as a referral service.
A disadvantage of radioiodine therapy is that cats, once treated, must remain hospitalized until they no longer pose a threat to human contacts. This can be stressful to a geriatric cat unaccustomed to being separated from its home environment. Although most cats tolerate this long hospitalization quite well, the separation is a common reason for client refusal of this therapeutic option.
The treatment of feline hyperthyroidism is successful and rewarding for clients and veterinarians. May factors influence the choice of treatment for an individual cat. Surgery and radioiodine therapy are potentially curative treatments, whereas the administration of antithyroid medication requires life-long therapy. The age of the cat and the presence or absence of associated cardiovascular disease or concurrent serious medical problems should be considered.
Radioiodine therapy is recommended as an alternative to anesthesia and surgery in the high- risk patient, but its availability is limited. Surgery should be attempted only on a stabilized cat by an experienced surgeon with the ability to provide the necessary intraoperative monitoring and postoperative care. Medical therapy may be considered as the sole therapy if there is serious concurrent disease or if the owner is not prepared to make the major financial commitment required with the other, definitive treatments. The advantages and disadvantages of each treatment should be considered in the context of each case, and an appropriate form of therapy then chosen.
We have found radioiodine treatment to be the most cost effective and least traumatic treatment for your pet. Although, the initial expense is greater, long term it is our treatment of choice.