Thyroid tumors in cats
Especially older cats are often affected by diseases of the thyroid gland
In the USA, thyroid tumors are very common in cats and are increasingly diagnosed in Europe as well. Thyroid neoplasms in cats are usually benign (thyroid adenoma or hyperplasia) and occur mainly in old cats (average age 13 years), without any breed or sex predisposition. In contrast to dogs, the majority of thyroid tumors occurring in cats are hormonally active and lead to a more or less pronounced hyperthyroidism. Most adenomas are relatively small. Usually both thyroid glands are neoplastically altered. The rare carcinomas show a tendency to infiltrate and are therefore less displaceable than adenomas. The incidence of metastasis in carcinomas is between 40% and 70%.
The majority of patients is presented with symptoms of hyperthyroidism. Characteristics include weight loss with good nutritional intake, chronic vomiting and/or diarrhea, hyperactivity, panting, PU/PD, and symptoms of hypertrophic heart disease (tachycardia, heart murmur, arrhythmias). In rare cases anorexia, muscle weakness or tremors, increased claw growth, or alopecia occur. Older cats presented with weight loss in spite of good food intake and chronic vomiting or diarrhea should be examined with regard to a possible thyroid tumor.
Cats with hyperthyreose often show leukocytosis and low-grade erythrocytosis with macrocytosis. Increased liver enzymes (AP, AST, ALT) are found in a large proportion of patients, possibly due to a hepatotoxic effect of hyperthyroidism. Some patients show raised renal values (BUN, creatinine). Documentation of a heightened thyroid hormone level (T4) will confirm the diagnosis.
Surgical resection is one form of therapy for thyroid tumors in cats. Hypocalcemia is the most important postoperative complication. By way of treatment with thioamides (mainly carbimazole 5 mg/cat 2-3 x daily) or subsequent thiamazole, a euthyroid state may be achieved in most cases of thyroid adenomas. (For stabilization of feline hyperthyroidism prior to surgical thyroidectomy and for long-term therapy of feline hyperthyroidism, the recommended initial dose is 5 mg thiamazole per day). The total daily dose should be divided into two administrations (morning and evening). For improved stabilization of the hyperthyroid patient, the same feeding and treatment regimen should be maintained daily. Hematology, biochemistry, and serum total T4 should be assessed prior to initiation of treatment and at 3, 6, 10, and 20 weeks and every 3 to 6 months thereafter.
At each of the recommended monitoring intervals, the dose should be adjusted based on total T4 levels and clinical presentation. Dose adjustments should be made in increments of 2.5 mg thiamazole (0.5 ml animal drug). Achieving the lowest possible dose should be the goal. At very high baseline levels (T4 > 8 ng/dl), long-term control is sometimes not achievable. Side effects of carbimazole therapy may include anorexia, vomiting, and bone marrow suppression. Surgical treatment or radioiodine therapy is indicated, if clinical or hematologic side effects occur.
Other options for treatment
Radioactive iodine (131I) may also be used very successfully to treat thyroid adenomas. Due to radioactive radiation, however, patients must be kept in an isolation ward for 2 to 4 weeks. For radiation protection reasons, radioiodine treatment is only available in certain university institutions in Europe. The prognosis for benign tumors of the thyroid gland is good, provided there are no perioperative complications. In some patients there is a recurrence of hyperthyroidism which makes a new therapy (medicinal, surgical, radioiodine) necessary. While little is known about the prognosis of malignant tumors, in the case of metastases it is considered unfavorable. Thyroid scintigraphy is very well suited for visualizing recurrences or metastases in malignant tumors.