Hyperthyroidism is an endocrine disease that is more common in cats from 8 years of age. Know its risk factors, typical symptoms, and the best treatment options for your feline.

Hyperthyroidism in Cats, Symptoms, and Treatment

Hyperthyroidism can also affect cats, in fact, more than we think. For example, a recent study estimated that 10% of Spanish cats suffer from this problem that affects their metabolism, which is weight loss despite increased appetite.

The hyperthyroid cats suffer thyroid dysfunction, an endocrine gland located in the neck, adjacent to the larynx, consisting of two parts or lobes connected and responsible for the secretion of two hormones, thyroxine (T4) and triiodothyronine (T3). 

These substances control the body’s metabolism, essential for regulating growth, temperature, heart rate, and energy level. In turn, the production of T4 and T3 is influenced by the secretion of thyrotropin (TSH) in the pituitary gland and TRH in the hypothalamus, two areas of the brain relevant to the endocrine system.

The thyroid gland of felines with this alteration autonomously and uncontrollably secretes large amounts of T4 and T3. In 99% of cases, this is due to thyroid hyperplasia or adenoma, which are benign thyroid growths. Only 1% has its origin in a malignant tumor: thyroid carcinoma.

Despite the benign nature of adenomas or hyperplasia, hyperthyroidism can seriously affect the cat’s metabolism, with fatal consequences for its body if proper treatment is not applied.

Symptoms of feline hyperthyroidism

The symptoms of feline hyperthyroidism are soft at the beginning and become more severe as the disease progresses. The main signs that may indicate that your cat suffers from this endocrine problem are:

  • Weightloss.
  • The increased appetite (polyphagia).
  • Polydipsia-polyuria syndrome (increased thirst and urine production).
  • Behavioral changes: hyperactivity, irritability, and anxiety or stress.
  • For the careless.
  • Heat intolerance; the animal looks for more incredible places and sometimes gasps.
  • Tachycardia: increased heartbeat rate.
  • Diarrhea or vomiting due to increased intestinal motility.
  • Muscle weakness, drooping neck, and neurological signs due to lack of potassium.

In very advanced cases, especially if other diseases concur, the symptoms are the opposite: loss of appetite, lethargy, and anorexia.

How hyperthyroidism affects the cat’s organs?

Thyroid hormones are essential for the proper functioning of many organs, so that excess T4 and T3 can negatively affect the animal and cause:

  • Heart problems: the increase in the rate and force of the heartbeat leads to hypertrophy of the heart muscle, which eventually leads to heart failure.
  • High blood pressure: Increased blood pressure can damage vital organs such as the brain, eyes, kidneys, or heart.
  • Chronic kidney disease (CKD): it is due to increased protein catabolism, hypertension, heart failure, or pre-existing CKD, something common in older cats. Initially, hyperthyroidism increases renal blood flow, improving its function to mask the symptoms of CKD.

Risk factors for feline hyperthyroidism

To date, the exact cause of feline hyperthyroidism is not known, but there are scientific studies that link certain environmental factors with the increase in cases, such as:

Low iodine concentration in the diet: iodine is essential for thyroid function. Suppose it is not available; the gland atrophies and does not produce T4 and T3. This alarms the hypothalamus and pituitary, which increase the secretion of TSH and TRH so that the thyroid secretes more hormones (positive feedback). Ultimately, it causes the thyroid gland to become overactive, causing hyperthyroidism.

Canned wet diets: some containers of these food preparations are made with materials and additives such as resorcinol and bisphenol A, which reduce the efficiency in the production of T4 and T3, also causing ‘positive feedback.’

Age: cats from 8 years of age. In those over 13 years of age, the prevalence increased by 14%.

Breed: Siamese and Himalayan cats are believed to be less predisposed to the disease.

Diagnosis of feline hyperthyroidism

If we observe symptoms compatible with hyperthyroidism in our cat, we should go to the vet as soon as possible to confirm the diagnosis through exploration and tests, such as:

General examination: includes palpation of the thyroid gland in the neck (it will usually be enlarged), cardiac auscultation (tachycardia and murmurs may be detected), and blood pressure measurement (frequently elevated).

Blood and urine tests: in hyperthyroidism, the levels of thyroid hormones (T4 and T3), liver transaminases (ALT, AST, and alkaline phosphatase), and renal function indicators (creatinine, urea, SDMA, and phosphorus) are increased. Potassium blood levels are usually low. High levels of protein can be detected in the urine.

Radiography, ultrasound, and electrocardiography: valid for diagnosing cardiac complications.

Scintigraphy: This test uses contrast and a special counting camera to detect functional thyroid tissue in unusual locations. It is only done in atypical cases of hyperthyroidism.

Treatment of feline hyperthyroidism

Hyperthyroidism in cats can be treated with drugs, surgery, or radioactive iodine. Antithyroid medications, such as methimazole or carbimazole, work by decreasing the production and release of thyroid hormones. They are administered to the feline orally or as a transdermal gel daily. They cannot cure the disease, but they can control it in the medium-long term.

They have few side effects and are pretty well tolerated by the cat, but it is necessary to perform blood tests periodically to adjust the dose. The hyperthyroidism results on other organs are usually corrected once treatment is started, but sometimes it is required to add antihypertensive drugs or improve kidney function.

Hyperthyroidism in Cats, Symptoms, and Treatment

Surgical treatment and radioactive iodine therapy

The surgical removal of abnormal thyroid tissue can be curative, although, in some healthy cats, not resected thyroid disease may develop long term. It requires prior antithyroid treatment for 3-4 weeks and stabilization of pre-existing heart or kidney diseases. However, it is not without complications, such as damage to the parathyroid glands or the laryngeal nerve.

Radioactive iodine therapy is an effective and safe alternative that does not require anesthesia. Instead, it consists of an injection of this element (I131), which is selectively fixed in the diseased thyroid tissue, destroying it without affecting the surrounding tissues. The drawbacks are that it is only performed in specialized centers, the cost is high, and hospitalization of the feline is necessary for a few days until the radioactivity falls to levels acceptable to humans.

Prevention and early detection of hyperthyroidism in cats

Feline hyperthyroidism is challenging to prevent since the causes that originate it are not known precisely, although they seem to be related to diet. Therefore, offering a high-quality diet with a balanced amount of iodine and avoiding the abuse of canned food can benefit our cat.

In the same way, it is highly recommended to carry out an annual blood test, especially from the age of 8, to detect the disease early and treat it as soon as possible.