Poor Kitty? Living With an Overactive Thyroid

Feline hyperthyroidism (FHT) is one of the most common diseases in older cats. Diagnosis and treatment are not easy, but therapy and healing are possible.

About 20% of cats over the age of ten are diagnosed with an overactive thyroid. Nevertheless, we have to assume that there is a not inconsiderable number of undetected diseased cats. In cats with hyperthyroidism, also known as feline hyperthyroidism (FHT), the diseased thyroid tissue produces more hormones and releases them into the bloodstream as T4 (thyroxine) and T3 (triiodothyronine).

The disease has only been known to affect cats since 1979. Much research and observation have been made since then. Countless studies have processed case numbers, laboratory data, and therapy successes, so that today, only 40 years later, we can already show a great deal of evidence-based knowledge about this new disease.

Is it the most common internal disease or the most common tumor in older cats? Hyperthyroidism is caused in the majority of cases by benign tumor cells, known as a functional adenoma (adenoma = benign tumor of the glandular tissue), the cells of which are usually organized into nodules 2–20 mm in size. Very rarely, in about 2% of cases, we also find adenocarcinomas in the thyroid gland, the malignant form of hyperthyroidism. The likelihood of carcinoma increases with the duration of drug treatment; after four years it is 20%.

In 70-75% of cases, changes can be found in both thyroids. 20% of the diseased cats have tumor cells not only in the thyroid but also ectopically, i. H. elsewhere, mostly mediastinal in the thorax.

Diagnosis and management

Early feline hyperthyroidism is often detected during routine blood tests because the early symptoms of the disease are very non-specific. If the disease is more advanced, the cat shows classic symptoms such as weight loss despite increased food consumption, increased thirst, or gastrointestinal disorders.

Classic symptoms of FHT depending on disease stage:

  • weight loss
  • Polyphagia (increased feed intake)
  • Polyuria (PU, increased urine output)
  • Polydipsia (PD, increased fluid intake)
  • unkempt fur
  • vocalization
  • restlessness
  • aggressive behavior
  • Tachycardia (increased heart rate)/tachypnea (increased breathing rate)
  • vomiting/diarrhea
  • Apathy, loss of appetite, lethargy

Cat owners often mistake the changes associated with an overactive thyroid as normal signs of aging and therefore only take their kitten to the vet when the disease is in an advanced stage. Patients have often already lost 10-20% of their body weight and muscle mass.

Diagnosis is made with a blood test. T4 (thyroxine) is measured routinely. The determination of serum T4 has a sensitivity of 90% and a specificity of 100%, which means that it can be used very well to confirm the diagnosis. The reference range depends on the laboratory device and is always included in the reports. An increase in the concentration of this hormone in the blood in connection with the corresponding clinical symptoms leads to diagnostic certainty. Other blood changes may include increased ALT (alanine aminotransferase) and increased alkaline phosphatase.

In unilateral disease, the enlarged thyroid can sometimes be detected by palpation and comparison to the other side. However, many cats are neither abnormal on palpation nor have T4 values ​​above the reference range. However, if clinical signs suggest hyperthyroidism, these cats should be retested in 2-4 weeks. In addition, other diseases with similar symptoms should be excluded.

Other well-known thyroid laboratory tests such as the determination of free T4 in equilibrium dialysis, TSH tests, T3 suppression tests, and TSH/TRH stimulation tests are either not possible for the cat does not add any value to the diagnosis.

Cats with clinical symptoms and T4 values ​​in the upper half of the reference range should be classified and treated as hyperthyroid. The same applies to cats that do not (yet) show any classic symptoms but showed T4 values ​​above the reference range in two measurements. Diseases with symptoms similar to FHT include:

  • diabetes mellitus,
  • gastrointestinal malabsorption/maldigestion,
  • gastrointestinal neoplasia, e.g. B. alimentary lymphoma.

Clarify possible concomitant diseases

Hyperthyroid cats tend to be middle-aged to advanced in age and are therefore also predisposed to other geriatric diseases. These patients should receive treatment for both FHT and other disorders and be monitored very regularly. The following diseases are commonly associated with FHT:

  • heart disease,

  • high blood pressure,

  • retinal diseases,

  • chronic kidney disease (CKD),

  • gastrointestinal disorders, cobalamin deficiency, malabsorption,

  • insulin resistance,

  • pancreatitis.

To get an overall picture of an affected cat’s condition, laboratory tests, blood pressure measurements, eye exams, x-rays/ultrasound scans, and – depending on the symptoms – other follow-up tests should be carried out.

Tests for suspected FHT depending on further findings

  • Blood test T4
  • Blood test hematology
  • Blood test clinical chemistry (esp. kidney values, liver values, glucose, fructosamine)
  • Urinalysis (specific gravity, urine protein creatinine ratio/UPC)
  • for gastrointestinal symptoms also Spec.PL (pancreas-specific lipase) and cobalamin
  • Palpation of the thyroid glands and abdomen
  • blood pressure measurement
  • Auscultation heart, chest X-ray
  • echocardiography
  • abdominal ultrasound
  • Eye/retinal exam
  • Possibly scintigraphy

make therapy decisions

After an overall picture of the patient has been created, the therapy decision follows. The first goal is stabilization, because the cats are often extremely emaciated, unappetizing, and presented with gastrointestinal disorders. A serious complication of hyperthyroidism is acute or chronic recurrent pancreatitis. Affected cats require IV treatment and symptomatic therapy until they can feed themselves again. Insertion of a feeding tube can support the therapy.

The next step is to restore the euthyroid state as quickly as possible, i. H. a condition in which the level of T4 in the blood is in the lower half of the reference range. The first check-up after the start of drug therapy takes place two to three weeks later. The kidney values ​​should always be checked during this check-up. Hyperthyroidism can mask CKD (chronic kidney disease) by lowering kidney values ​​through increased renal perfusion and increased water intake. In addition, due to the loss of muscle mass in affected animals, the creatinine is falsely low and the existing CKD cannot be detected. In these cats, after successful initiation of therapy and normal thyroid hormone levels, CKD appears as a side effect of the drug. Cat owners should be made aware during the first therapy session that this can happen because there is a possibility that their cat already has undetectable kidney disease.

Contrary to other advice, cats with recognized CKD and azotemia (too much urea in the blood) on thyroid therapy should always be treated in the same way as cats with healthy kidneys. The goal must be to treat the cat’s T4 below the middle of the reference range. Attempting to keep kidney levels artificially low by leaving the cat “a bit hyperthyroid” from under-treatment of FHT gives us a false sense of security. In contrast, elevated T4 leads to activation of the renin-angiotensin-aldosterone system (RAAS) leading to increased cardiac output, volume overload, sodium retention, renal hypertension, and glomerular sclerotherapy, ultimately leading to the progression of CKD and worsening of the condition. However, checks must be carried out very regularly to avoid iatrogenic (doctor-induced) hypothyroidism at all costs.

About one in five cats with an overactive thyroid also has an elevated BI. This increase in blood pressure may be caused by FHT and treating it may return blood pressure to normal. Checking blood pressure during therapy control of hyperthyroidism is essential to identify and treat non-FHT-associated hypertension. The same is true for cardiac symptoms, which may be FHT-related and may improve markedly with euthyroid cessation. Nevertheless, an echocardiographic examination should be undertaken in these cases.

Therapy options

FHT is a life-threatening condition and must be treated to establish a euthyroid condition in the cat. Medication, dietsurgery, and radioiodine therapy are available for treatment.


The active ingredient methimazole is approved for cats as a tablet and as a tasty solution to be given twice a day. Carbimazole, also approved for cats, is metabolized to methimazole in the body and has the same effect. Both block thyroid peroxidase and thus reduce the biosynthesis of thyroid hormones.

Treatment with these agents can be lifelong or temporary to stabilize the cat pending surgery or radioiodine therapy. In about 18% of all patients, however, methimazole or carbimazole causes side effects. This can be:

  • anorexia
  • vomit
  • pruritus and excoriations on the face
  • lethargy
  • hepathopathies, jaundice
  • increased tendency to bleed

These side effects can occur immediately or only after administration for one to two months. Vomiting and loss of appetite are mostly dose-dependent and disappear after dose reduction. In the event of any other side effects, the drug should be discontinued immediately and other treatment options should be considered.

When adjusting to thyroid medication, the cat owner must be instructed in detail. The active ingredients may have a teratogenic (malformation-causing) effect in humans, which is why it is advisable to wear gloves when handling them and tablets must not be split. Administration with so-called “pill pockets” or “trojans” in which you can hide the pills is a good idea. The methimazole solution is very palatable and most cats willingly take it.

An alternative that has not yet been approved for cats in Germany is a methimazole gel that allows the active substance to be absorbed transdermally. Here, too, gloves must be worn during the application. For cats that require a high dose, the amount of gel to apply is very large. But this drug application is very well tolerated by many cats.

Checks of the T4 blood level and, if necessary, other parameters are advisable after three, six, ten, and 20 weeks. Even stable patients should have a blood test every 12 weeks because FHT is a tumor disease and can get worse with tumor growth, whereupon the dose must be adjusted.

Another problem with drug therapy is owner compliance. Unfortunately, the symptoms do not immediately deteriorate after stopping the tablets, but only a gradual disease process. We often only see the cats again when the condition is dramatic to life-threatening.


The diet is a good therapeutic option for cats that live alone and indoors. The effect is based on a diet in which the iodine content is reduced to a necessary minimum. Since the thyroid glands cannot synthesize thyroid hormones without iodine as a basic building block, production is significantly reduced. However, it must be ensured that the cat has no other food sources from which it can consume iodine.


Surgical removal of the thyroid gland is the easiest but not the best option for treating FHT. It can be useful if only one side is affected and if there is no ectopic thyroid tissue in inaccessible places, e.g. B. in the thorax. Even previously very high T4 values ​​are already in the normal range on the day after the operation. Unfortunately, thyroid adenomas tend to spread to both sides, leading to timely recurrences when the tumor in the remaining gland begins to grow. The removal of both thyroid glands is not the method of choice because, firstly, there is a risk that too few parathyroid glands (epithelial bodies or parathyroid glands) remain in the body, which leads to a life-threatening lack of parathyroid hormone.

radioiodine therapy

The gold standard in the treatment of FHT is radioiodine therapy. It is the only option that leads to healing. In most cases, a single treatment is enough and almost 95% of treated cats are healthy for life. Radioactive iodine accumulates in the thyroid cells. It concentrates almost exclusively on the much more active tumor cells and destroys them. No anesthesia is necessary for the treatment. The disadvantage of this therapy is the necessary length of hospitalization, which, however, varies greatly from place to place (at least four days, up to four weeks, also depending on the legislature, e.g. ten days in the Norderstedt veterinary clinic). During this time, the kitten is not allowed to be visited. Another disadvantage is that this form of therapy is not available everywhere. There are different statements as far as the costs are concerned: radioiodine therapy is as expensive as drug therapy including the necessary blood tests per year or over the remaining lifespan. According to studies, the life expectancy after radioiodine therapy is twice as long as that of cats treated with methimazole.


It is important to educate the owner and develop an individual treatment plan. Animal welfare is paramount. The goal is to get T4 levels in the lower half of the reference range and keep them there. Other diseases such as CKD, cardiomyopathies, high blood pressure, etc. should also be treated and included in regular monitoring. This monitoring is important because geriatric diseases, especially tumor disease FHT, are subject to progression, and treatment protocols must be constantly adapted to maintain the patient’s quality of life.

Frequently Asked Question

How does a cat with an overactive thyroid behave?

Typical symptoms of an overactive thyroid in cats are Restlessness. Hyperactivity. Cravings (polyphagia).

How long can a cat with an overactive thyroid live?

The gold standard in the treatment of FHT is radioiodine therapy. It is the only option that leads to healing. In most cases, a single treatment is enough and almost 95% of treated cats are healthy for life.

How do you know if a cat is suffering?

Pulling back, tenderness to touch, aggressiveness, a crouched posture, or limping indicate the animal is suffering. In addition to behavior, you can also look for other symptoms that will give a more accurate indication of why your cat is suffering.

What to feed cats with an overactive thyroid?

Cats with an overactive thyroid should only be fed Hills Feline y/d, as the higher iodine content of other feeds negates the effect of the treatment.

What drug for hyperthyroidism in cats?

Therapy for hyperthyroidism always begins with the administration of tablets containing the active ingredients thiamazole and carbimazole. These are optimally administered twice a day and prevent the production of thyroid hormones, the higher the dose, the lower the production.

What helps hyperthyroidism in cats?

Hyperthyroidism in cats can be treated with pills. The two drugs “Thiamazol” and “Carbimazole” reduce the production of thyroid hormones. This normalizes the excessive hormone levels in the blood. The dose should be given twice a day.

Can a cat cry?

Like humans, cats can cry and feel emotions. However, there is no connection between the tear and the feeling, because cats express their emotions differently.

How does a cat sound when it’s crying?

Acoustic crying: pitiful mewing, meowing, or yelling. Decreased pupils. Rapid twitching and flicking of the tail.

Mary Allen

Written by Mary Allen

Hello, I'm Mary! I've cared for many pet species including dogs, cats, guinea pigs, fish, and bearded dragons. I also have ten pets of my own currently. I've written many topics in this space including how-tos, informational articles, care guides, breed guides, and more.

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