The thyroid is a gland located in the area of the neck. It plays an important role in many body functions. The hormones that it produces and secretes (T3 and T4) affect the body's metabolism. If the gland secretes too much or too little of these hormones problems ensue. An underactive thyroid, or hypothyroidism, is more common in dogs. In cats, an overactive thyroid is more common, and this condition is called hyperthyroidism.
Causes of hyperthyroidism
Hyperthyroidism tends to affect middle aged to older cats. It is estimated that up to 10% of senior and geriatric cats may develop hyperthyroidism. The majority of these cats develop a tumor in the thyroid gland itself which causes an increased secretion of thyroid hormones. The vast majority of these tumors are benign adenomas, but as many as 2% of cats with hyperthyroidism may develop a malignant tumor (thyroid carcinoma).
The thyroid tumors may be singular or solitary, however cats often develop additional masses in the area of the thyroid gland itself and in the chest. The tumors will continue to enlarge over time.
Signs of hyperthyroidism
Diagnosing hyperthyroidism
A cat may be suspected of being hyperthyroid based on its history and clinical signs. In many cases, an enlargement of the thyroid or a nodule may be visible or palpable during the physical examination by a veterinarian.
Over 90% of cats with hyperthyroidism will have increases in their measured thyroid hormone (T4 and/or freeT4) levels. TSH, a hormone produced by the pituitary gland, may also be measured. In cases of hyperthyroidism, the levels of TSH are often low. In up to 10% of affected cats, these blood tests may be normal.
All cats should have a complete blood work-up to assess their general health including a CBC, chemistry panel, and urinalysis.
The thyroid gland is not a structure easily seen on a radiograph (x-ray). Scintigraphy, or a thyroid scan, is one way in which the gland can be imaged and assessed for the presence of tumor(s) or enlargement. This test does not require anesthesia. The cat is administered a very small dose of a radioisotope (technetium-99m pertechnetate) under the skin. This injected compound concentrates in the thyroid and salivary gland tissues. By assessing the uptake of the radioisotope in these tissues, the diagnosis of hyperthyroidism can be confirmed as well as help to determine the stage of disease. The thyroid scan can show whether there is a single tumor, multiple tumors, tumor in the chest, and whether the tumor might be malignant.
If a cat has suspected heart disease based on examination findings (heart murmur, irregular rhythm, difficulty breathing), chest radiographs (x-ray) and/or echocardiography should be performed. Blood pressure monitoring is also recommended.
Treatment of hyperthyroidism
The mainstay of medical treatment for hyperthyroidism is with an oral antithyroid drug such as methimazole. This drug blocks the production of thyroid hormone. Although this drug is effective in blocking excessive thyroid hormone production, it does not destroy thyroid tumor(s). In fact, because the thyroid tumor(s) will continue to grow over time, higher doses of methimazole may be required to control thyroid hormone levels over time. Most cats require twice daily dosing to effectively control their disease.
Side effects of methimazole use in cats can include anemia and an allergic reaction leading to skin lesions.
Diets very low in iodine may be beneficial in decreasing thyroid hormone production by thyroid tumors. Iodine is an important factor in thyroid hormone production. The long-term effects of feeding iodine-restricted diets to cats is unknown.
Thyroidectomy, or surgical removal of the thyroid gland, is a consideration for solitary disease confined to the thyroid gland. However, in cases where there are multiple tumors or affected tissue outside of the thyroid gland (chest cavity, etc.) surgery may not achieve a definitive cure of disease.
The goal of I-131 therapy is to destroy abnormal thyroid (tumor(s)) while preserving normal thyroid tissue. A single dose of radoiodine (I-131) can be curative. 85% of cats have normal thyroid hormone levels within 2 to 4 weeks of treatment, and 95% cats are normal within 3 months. Only 5% of cats may have persistent hyperthyroidism after an initial radioiodine treatment. These cats may undergo retreatment, and almost all can be cured by a second dose.
Prognosis
The prognosis for cats with hyperthyroidism is variable. The prognosis is better for cats with benign thyroid tumors (compared to those with malignant carcinomas) and those that undergo treatment. Without treatment, the excessive thyroid hormone levels can lead to a severe catabolic state and weight loss, muscle wasting, heart problems and high blood pressure.
Recent studies have shown that over time, the disease does progress. Even cats that are treated, long-term, with methimazole, show signs of disease progression. Specifically, over time, in spite of medical management, tumors will grow, thyroid hormone levels will rise, and the likelihood that tumors can become malignant increases. Because the risks associated with radioiodine therapy are low, and because this non-invasive therapy is so effective, cat owners should speak with their veterinarian about whether earlier referral for definitive treatment is a reasonable consideration.
