Hyperthyroidism in Cats
What Is Feline Hyperthyroidism?
Feline hyperthyroidism is a common endocrine disorder in older cats, caused by the overproduction of thyroid hormone. Left untreated, hyperthyroidism can result in heart failure and other complications, and can even lead to death.
The thyroid is a gland in the neck that produces hormones that regulate metabolism and many other processes in the body. Cats have two thyroid lobes, one located on each side of the trachea (windpipe) in the neck. More than 80% of hyperthyroid cats have a condition known as 'hyperplasia', resulting in excessive function and enlargement of both thyroid lobes. However, both lobes cannot always be felt, even if both are functioning excessively. About 15% of hyperthyroid cats have a benign tumor (adenoma), and usually only one of the two lobes is involved. Approximately 2-3% of hyperthyroid cats have a carcinoma (malignant tumor) that can be variable in presentation, involving one lobe or creating multiple nodules. Some cats also have abnormal functional thyroid tissue located in the chest.
What are the clinical signs of feline hyperthyroidism?
The overproduction of thyroid hormone leads to a multitude of clinical signs, most commonly weight loss, increased appetite, increased activity and/or vocalization, vomiting, diarrhea, increased water consumption, and increased urination. Some cats show all of these signs, but many show just one or two. Other signs such as decreased activity, reduced food consumption, or problems related to cardiovascular disease occur less commonly.
How is feline hyperthyroidism diagnosed?
During the initial exam, medical records will be reviewed, and a complete history and physical exam will be performed. Laboratory tests include a minimum of kidney values (BUN and creatinine) and a urinalysis in conjunction with thyroid testing. Additional tests can be required if concurrent illnesses are detected on physical exam or blood work, and a technetium scan might be recommended.
A technetium scan (scintigraphy) is a safe, minimally invasive test that provides information on the diagnosis and sites of involvement with hyperactive thyroid tissue and can alter treatment recommendations. Technetium, a short acting radioactive substance, is administered through a catheter into a vein, and concentrates in the thyroid tissue. Images of the thyroid gland are acquired using a gamma camera to provide information on nodule size, the presence of single or multiple nodules, and the location of the abnormal thyroid tissue. Although not perfect, this is the most reliable test for determining if a cat has one or two thyroid lobes involved or if abnormal tissue is located in the chest. Cats need to be hospitalized for 24-hours following this scan to prevent owner exposure to radiation.
How is feline hyperthyroidism treated?
Treatment options for hyperthyroidism include surgery (if one lobe is involved), diet to reduce iodine exposure (using a prescription diet), medical treatment with methimazole (Tapazole®), or I-131 radioactive therapy. These treatment options have different pros and cons and should be discussed with a veterinarian. Not all treatment options are suitable for all feline patients.
Methimazole (Tapazole®) - Methimazole can be used as a temporary treatment for hyperthyroidism and is sometimes used long-term. Some cats with hyperthyroidism also have kidney disease (a common condition of older cats), but their thyroid condition increases blood flow to the kidney so that it masks (but does not improve) the underlying kidney problem. Therefore, before embarking on a permanent treatment (surgery or radioactive iodine) that might cure the hyperthyroidism but unmask kidney disease, it is important to evaluate what happens to kidney function when the hyperthyroidism is treated with a reversible treatment such as methimazole. If deterioration in kidney function does not occur, permanent treatment with I-131 or surgery is typically a viable option. If deterioration in kidney function does occur, it might be preferable to avoid permanent treatment.
Methimazole can be associated with side effects such as vomiting, facial scratching, or abnormalities in blood tests, and strict follow-up with a veterinarian is required after this medication is prescribed. It is available in both an oral and a trans-dermal form.
Diet - A prescription diet (Hills Y/D) has been shown to be effective in management of feline hyperthyroidism. It can be used in multi-cat households with proper veterinary assistance. It must be the exclusive diet of the hyperthyroid cat to be effective.
Surgery - Surgery is typically recommended if one lobe of the thyroid is involved or if there is concern for a thyroid carcinoma, a malignancy of the thyroid gland. This is rare, but it can occur. Typically, methimazole would be used to create normal thyroid hormone levels and ensure that kidney function remains stable before surgery is advised. If only one thyroid lobe is involved, it is important to realize that the other can become hyperfunctional in months to years. Surgery involves an incision down the front of the neck for removal of the abnormal gland, and a surgeon will discuss possible complications and hospitalization times with you.
I-131 (radioiodine) treatment - I-131 is radioactive iodine that is administered as a single injection under the skin. Iodine is the main ‘building block” for creating thyroid hormone and it concentrates in the thyroid gland. Other tissues do not require iodine and its concentrations are low elsewhere in the body. I-131 concentrates in the thyroid gland where it destroys overactive thyroid tissue. Neighboring thyroid tissue that has been suppressed by the overactive cells is mostly spared, as are other tissues in the body. Side effects of I-131 treatment are uncommon but similar to other treatments for hyperthyroidism. Adverse effects of treating hyperthyroidism include unmasking underlying kidney disease and development of hypothyroidism (decreased production of thyroid hormone).
What is the prognosis for feline hyperthyroidism?
With proper management and veterinary care, the prognosis for most cats is excellent, although recurrence of the disease is possible in some cats.
How can feline hyperthyroidism be prevented?
The exact cause of feline hyperthyroidism is unknown, so there are currently no preventative measures. Early diagnosis often leads to a better prognosis.
*This article may not be reproduced without the written consent of the UC Davis School of Veterinary Medicine.