Nutritional nuggets for hyperthyroid cats

In the past, the three most common treatment options for hyperthyroid cats were medical management, surgery, or radioactive iodine (I-131) therapy. Now there is another option—dietary management.

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The disease

The thyroid gland is a small, paired gland located in the neck adjacent to the trachea. The gland produces hormones that are critical to the proper functioning of many organs, including the heart, liver, brain, and kidneys. One of these hormones is thyroxine (T4), a hormone involved with the body's metabolic rate. T4 is converted in the liver, producing triiodothyronine (T3), an active form of thyroid hormone that affects the metabolic rate of every cell in the body.

When these hormones are secreted in excess from the thyroid glands, which become enlarged, an animal develops a condition known as hyperthyroidism. In most cases in cats, the enlarged thyroid glands are caused by a benign tumor called an adenoma. Rarely, the cause is a type of malignant tumor called a thyroid adenocarcinoma.

Complications

Since thyroid hormones affect so many organs in the body, secondary disease processes can often be identified in addition to hyperthyroidism. Excess thyroid hormones result in a faster heart rate and stronger heart muscle contractions. If left untreated, this can compromise the function of the heart and eventually lead to heart failure.

Hypertension is another possible complication of hyperthyroidism. Untreated, additional damage may occur to other organs, including the kidneys, brain, and eyes. Hypertension increases renal blood flow and the glomerular filtration rate, which could mask concurrent chronic renal failure. Initially, the hypertension may need to be treated with drugs. But as the hyperthyroidism is managed, the associated hypertension will often resolve, requiring no further treatment.

Hyperthyroid cats may also develop changes in glucose and insulin metabolism. Sometimes an untreated hyperthyroid cat can develop diabetes mellitus. This may worsen over time, even after successful treatment of hyperthyroidism.

Prevalence and predilections

Since the disease was first described in the late 1970s, hyperthyroidism has become the most commonly diagnosed endocrine disorder in cats. Typically, the cause is adenomatous hyperplasia. It's unclear whether the incidence of the disease is on the rise or whether greater veterinary awareness, improved diagnostics, and an aging cat population account for the increased numbers.

Hyperthyroidism is most often diagnosed in cats older than 8 years of age, although it can be seen in younger animals. No sex or breed predilection has been identified, nor have any definitive risk factors. Some proposed, yet unproven, risk factors include feeding canned food from pop-top cans, ectoparasiticide exposure, and the high iodine content of some cat foods. Another theory is that immunologic, infectious, nutritional, environmental, or genetic factors may interact, causing pathologic changes. Despite many theories, the pathogenesis of hyperthyroidism remains unknown.

Clinical signs and physical examination findings

Clinical signs of hyperthyroidism caused by excessive secretion of thyroid hormone by the thyroid mass include:

• Weight loss, despite a ravenous appetite

• Polyphagia (in some animals, a decrease in appetite may follow a long period of polyphagia)

• Hyperactivity or nervousness

• Excessive vocalization

• Heat intolerance

• Polydipsia and polyuria

• Vomiting

• Diarrhea

• Tachypnea or panting

• Fatigability (open-mouth breathing after even short bursts of activity)

• Decreased grooming.

Clinical signs vary and may be subtle at first, becoming more severe as the disease progresses. Loss of muscle mass may first be noticed along the epaxial muscles running along either side of the spine. In the early stages of the disease, the cat may appear to be overconditioned and may even retain an abdominal fat pad. Ventroflexion of the neck can be seen in some hyperthyroid animals and may be a sign of potassium or thiamine deficiency.

Physical examination findings will also vary but may include:

• Cardiac changes, such as sinus tachycardia, systolic heart murmur, or arrhythmia

• Poor body condition

• Poor hair coat or alopecia

• Dehydration

• Abnormal retinal examination (tortuous retinal blood vessels, retinal tears, retinal detachment)

• A palpable thyroid nodule.

Palpation of a thyroid nodule is one of the most significant findings during the physical examination.

Diagnostics

Laboratory tests generally include a urinalysis, serum chemistry profile, and serum T4 measurement. Most cats with hyperthyroidism will have elevated T4 concentrations. However, a small percentage will have concentrations within the normal range. Additional tests may be needed if hyperthyroidism is still strongly suspected. Diagnostic imaging (thoracic radiography, cardiac ultrasonography) and electrocardiography may also be beneficial in determining the scope of secondary disease conditions. It is also important to rule out concurrent disease processes, such as renal disease, as this is a common diagnosis in older cats.

Traditional treatment options

Three treatment options have traditionally been used to treat hyperthyroidism in cats: medical management, surgery, and radioactive iodine therapy.

Medical management: Managing hyperthyroidism with antithyroid drugs can provide short-term or long-term control, but it is not a cure. Methimazole is most commonly used. It controls the production of thyroid hormone but does not destroy the thyroid tumor. This is a lifetime therapy, and if the drug is discontinued, the signs of hyperthyroidism will reappear.

While methimazole is readily available and inexpensive, it does require twice-daily dosing. So poor compliance is always a concern. Adverse reactions to the medication have been reported, including facial swelling, vomiting, anorexia, and depression, but they are usually transient and resolve without discontinuing the drug. Rarely, self-induced excoriations of the head and neck caused by facial pruritus and icterus caused by hepatopathy can occur. Mild to severe hematologic complications have also been rarely reported. These include agranulocytosis, thrombocytopenia (manifesting in epistaxis and oral bleeding), and, more rarely, immune-mediated hemolytic anemia.

Surgery: Surgical removal of the thyroid glands (thyroidectomy) is an effective cure for hyperthyroidism. To reduce the cardiac and metabolic complications associated with anesthetizing hyperthyroid cats, the cat must undergo medical management and be euthyroid or have a normal functioning thyroid gland before undergoing a thyroidectomy.

Hypoparathyroidism resulting in hypocalcemia is the most common postoperative complication. More than half of cats with the disease will require removal of both lobes to control the disease. Postoperative complications include hypoparathyroidism (which tends to resolve), Horner's syndrome, laryngeal paralysis, and hypothyroidism, all of which are considered rare. Surgery is not performed as often today in areas where radioactive iodine therapy has become more readily available.

Radioactive iodine therapy: This form of therapy consists of a single, relatively painless subcutaneous injection. It has a success rate of about 95 percent. The radioactive iodine is taken up by the thyroid gland where radiation is released, which kills the overactive tumor cells preferentially as well as some normal thyroid cells. Other adjacent tissues sustain no significant damage. The rest of the radioactive iodine is excreted in the urine and feces. To protect others from radiation exposure, the cat must remain hospitalized for about four days, be housed away from other patients, and be monitored until the radioactivity level is low enough to allow the cat to return home.

A new option: Iodine-restricted diets

The latest option for treating hyperthyroidism is dietary management in the form of iodine-restricted diets. Why iodine restriction? Iodine is an essential component of both T4 and T3. Without iodine, the thyroid cannot produce thyroid hormones. This form of treatment may be especially beneficial for patients with underlying or concurrent disease conditions that can make medical management more challenging.

A study conducted by a pet food manufacturer to determine the effects of feeding an iodine-restricted diet to hyperthyroid cats with naturally occurring hyperthyroidism was recently conducted.1 The objective was to determine the maximum concentration of dietary iodine required to maintain normal serum total T4 concentrations in hyperthyroid cats. The results of the yearlong study indicated that the use of a diet with severely restricted iodine concentrations could result in normalization of T4 concentrations in hyperthyroid cats. One hundred percent of the cats tested had normal total T4 concentrations when fed the lowest level of iodine (0.17 ppm). None of the cats in the study suffered any adverse reactions as a result of eating the iodine-restricted food.

Based on this research, a similar diet is now commercially available (Prescription Diet y/d Feline Thyroid Diet—Hill's Pet Nutrition), but it is considered a therapeutic diet and is not recommended for euthyroid cats. Additional veterinary counseling is recommended if this food is fed in a multicat household.

General feeding plan

The goal of nutritional management in cats with hyperthyroidism involves ensuring adequate intake of a nutritionally complete and balanced diet. This is one disease in which decreased appetite is not the cause of weight loss. Hyperthyroidism creates a catabolic state. Cats, being obligate carnivores, have an increased need for dietary protein. Increased protein catabolism, which is evidenced by progressive weight loss and muscle wasting, occur despite increased caloric intake from polyphagia. Animals begin catabolizing their own lean muscle tissue for use as energy.

In the early treatment stages, animals may need to be fed in excess of their maintenance energy requirement (MER). It's important to provide owners with specific recommendations about not only what, but also how much, to feed their cats. There are several equations for predicting energy requirements for individual dogs and cats. These equations are only accurate within ± 25 percent.

When predicting energy requirements for an animal whose needs may further exceed these predictions, it's important to monitor patients closely to ensure that their nutritional and caloric needs are being met. A good place to start is to calculate the MER at the animal's ideal body weight. A simple equation for calculating a lean adult cat's MER is 50 to 60 x weight in kilograms. For example, using this formula, if a patient's ideal body weight is 5 kg, then the daily MER would be 250 to 300 kcal/day.

A high-quality, high-protein complete and balanced diet that's acceptable to the animal should be recommended if the current diet is not appropriate. High-energy foods, such as growth or recovery formulas, may be useful for some patients. Any concurrent disease processes must be taken into consideration before making specific nutritional recommendations. To ensure better monitoring of hyperthyroid cats, instruct owners on how to determine their cat's body condition score, and weigh the cat every 10 to 14 days. At this time, if the animal is losing weight, you may suggest appropriate changes to the feeding plan.

Conclusion

Hyperthyroidism is the most commonly diagnosed endocrinopathy in middle-aged and older cats. Animals with hyperthyroidism are in a hypermetabolic state, and their body's energy expenditure is markedly increased. They are burning calories faster than they can replenish them with their daily meals and may need to be fed in excess of their basal MER to be able to regain and maintain a good body condition score. Patients being treated for hyperthyroidism need to be closely monitored to ensure that their weight and body condition scores do not decline. Clients should know the signs of malnutrition, be able to assign a body condition score, and check their cat's weight regularly at home.

If the cat's hyperthyroidism is left untreated, death from starvation (or other concurrent disease process) may occur. If the animal has a concurrent disease process such as chronic renal insufficiency, which may be unmasked after the hyperthyroidism has been diagnosed, the hyperthyroid cat's health may be further compromised, and its dietary needs more complicated. Perhaps the most important thing to remember and to convey to anxious owners is that there are several safe and effective treatment options for feline hyperthyroidism that are used successfully every day across the country. Management plans can be individualized to optimize treatment for their cats.

Charlotte Higgins, AS, CVT, is a nurse practitioner in nutrition at the MJ Ryan Veterinary Hospital of the University of Pennsylvania in Philadelphia. She's also a charter member of the Academy of Veterinary Nutrition Technicians.

Reference

Melendez, LD, Yamka, RM, Forrester SD, et al. Titration of dietary iodine for reducing serum thyroxine concentrations in newly diagnosed hyperthyroid cats, (abst.). J Vet Intern Med 2011;25(3):683.