Have Your Senior Cat Suddenly Lost Weight? Beware of Hyperthyroidism!

When we hear the following “compliments” from middle-aged and senior cat parents, we tend to be on edge because they might be caused by “hyperthyroidism,” including:

• My cat is always very hungry!
• I feel like my cat has become rejuvenated, acting like a kitten again!
• I can’t believe my cat is already 16 years old!
• My cat has become thin because they’ve been exercising!
• My cat has finally succeeded in losing weight!
• My cat’s kidney index is getting better and better!

Cats with hyperthyroidism often give the illusion of becoming younger when they first become ill.

Why Does My Cat Have Hyperthyroidism?

Causes of Feline Hyperthyroidism

The thyroid gland is an endocrine gland located on both sides of the trachea in the neck of a cat, and the “thyroxine” it secretes mainly regulates the metabolic rate of the whole body. When thyroxine is secreted in increased amounts for some reason, it leads to “metabolically robust” symptoms throughout the body, which is hyperthyroidism.

The most common cause of hyperthyroidism in humans is the activation of autoantibodies to the thyroid-stimulating hormone (TSH) receptor, known as Graves’ disease, but so far, no autoimmune antibodies have been found in cats with thyroid disease.

The causes of increased secretion of thyroxine in cats mostly come from benign nodular hyperplasia of the thyroid, similar to Plummer’s disease in humans, which causes glandular hyperplasia and excessive release of thyroxine. About 2% of cats with hyperthyroidism are found to have malignant tumors upon diagnosis.

The causes of nodular hyperplasia are not very clear, and only a few factors are “possibly” responsible, including:

• Genetics: May affect susceptibility to the disease, such as Siamese and Burmese cats, which have a lower risk of developing hyperthyroidism.
• Increased lifespan: After 1970, due to changes in cat-raising habits, the proportion of indoor cats increased, and the use of commercial feed and longer lifespans have affected the prevalence of the disease.
• Epidemiological surveys have compiled a list of suspects, but no experimental studies have confirmed this so far:
  • Phenolic compounds or halogenated hydrocarbons
  • Deodorizing cat litter
  • Canned ingredients: Bisphenol A, Phthalates (commonly known as plasticizers)
  • Common ingredients in cat food: such as soy isoflavones
  • Environmental pollutants: flame retardants
  • Iodine content in food

Is Hyperthyroidism Common in Cats?

With the advancement of technology and the dedication of cat owners, health checks are becoming more and more valued, and the lifespan of cats is getting longer, and the incidence of hyperthyroidism is also increasing year by year. Hyperthyroidism is now the most common endocrine disease in middle-aged and senior cats, with a prevalence of about 2-4% among all cats.

• The prevalence in cats over 9 years old is at least 6%
• Some literature even points out that for cats over 10 years old, one in ten will develop hyperthyroidism!

How Do I Know If My Cat Has Hyperthyroidism?

Cats with hyperthyroidism used to be taken to the hospital with typical symptoms and then diagnosed with hyperthyroidism after blood tests showed high levels of total thyroxine (TT4).

Typical symptoms include:

1. Weight loss
2. Increased appetite
3. Excessive urination (defined as more than 50 mL per day per kilogram, for example, a 4 kg cat urinates more than 200 mL of urine per day)
4. Excessive drinking (defined as more than 50 mL per day per kilogram, for example, a 4 kg cat drinks more than 200 mL of water per day)
5. Vocalizing
6. Restlessness, increased activity
7. Increased respiratory rate, increased heart rate
8. Vomiting, diarrhea
9. Untidy fur (not grooming)
10. Lack of energy, lack of appetite, lethargy

However, due to the increased awareness of this disease nowadays, hyperthyroidism is also increasingly diagnosed in cats before the appearance of the above typical symptoms.

Routine testing of thyroxine has been recommended in the 2021 American Animal Hospital Association / American Association of Feline Practitioners’ Life Stage Guidelines to be included in the routine semi-annual to annual physical examination items for cats over 6 years old. If there are high indexes, suspected symptoms, etc., it may be suggested to retest after 2 weeks to 6 months.

Thyroid Testing

After a complete medical history and physical examination, hyperthyroidism in cats can be preliminarily assessed by blood tests for total thyroxine (TT4) in the outpatient department (but usually cats suspected of hyperthyroidism will also undergo a complete examination of blood +/- imaging +/- urine +/- blood pressure). If necessary, tests for free thyroxine (free T4) and thyroid-stimulating hormone (TSH) will be added.

However, it is important to remember that hyperthyroidism is a chronic endocrine disease involving multiple organs and hormones that need to be coordinated, and it is not suitable to make a definitive diagnosis of ‘having’ or ‘not having’ hyperthyroidism based on a single value. The nuances of this need to be left to the clinical veterinarian to design a plan for examination and monitoring, and then step by step towards a ‘diagnosis’ or ‘exclusion’ of hyperthyroidism.

What If My Cat Really Has Hyperthyroidism? Will It Die?

As mentioned earlier, although hyperthyroidism is a chronic endocrine disease, if it is not diagnosed early and treated continuously after diagnosis, it can still be life-threatening. There is currently no research on the survival time of untreated cats, but if left untreated, complications such as malnutrition, hypertension, heart disease, thyroid storm, and death may occur.

The survival time after treatment was statistically about 2 years in older studies, but recent studies show that if the cat does not have chronic renal failure, the survival time of cats with hyperthyroidism can reach 5.3 years. Most of them can still have a longer and comfortable life, and it has also been found that the key to survival time is the presence and severity of other diseases that the cat has simultaneously, rather than the progression of the hyperthyroidism disease.

Although hyperthyroidism can make cats look younger, the chronic damage to other organs is very large, so it still needs to be treated!

How to Treat Hyperthyroidism? Are There Side Effects?

4 Common Treatment Options for Feline Hyperthyroidism

1. Radioactive iodine therapy
2. Medical treatment: using drugs that inhibit thyroid synthesis
3. Surgery: thyroidectomy
4. Diet: using iodine-restricted food

The Most Common Medical Treatment

Drugs that inhibit thyroid synthesis include methimazole and carbimazole. Both drugs are metabolized into the same active ingredient in the body, so the side effects are usually the same, and the choice will be based on the veterinarian’s usage habits, the owner’s cooperation, the animal’s acceptance, the effect after taking the medication, and the cost.

Common side effects usually occur within the first three months of use, including:

• Gastrointestinal symptoms: such as vomiting, diarrhea, loss of appetite, accounting for about 10%
• Lack of energy, change in vocalization
• Partial blood test abnormalities: about 15% of cats may have abnormalities within the first 8 weeks of medication, usually recovering after stopping the medication
• Hematological abnormalities (including severe leukopenia, thrombocytopenia, anemia): usually occurring within the first 1-2 months of treatment, with an incidence of 4%, requiring medication withdrawal
• Facial itching and scratching: usually occurring within the first 3 weeks of medication, with an incidence of about 2-15%, requiring medication withdrawal
• Rare but potentially life-threatening side effects requiring immediate medication withdrawal: liver damage and hematological abnormalities (including severe leukopenia, anemia, thrombocytopenia)

If your hyperthyroid cat experiences the above side effects after taking medication, it is recommended to schedule a return visit as soon as possible, so that the doctor can provide symptomatic treatment for the side effects and assess whether it is serious enough to adjust or discontinue thyroid treatment medication. However, in clinical practice, it is often the case that the ‘side effects’ that cat owners think occur after the hyperthyroid cat starts medication treatment are actually from the ‘suppression of hyperthyroid symptoms by the medication’, making the cat’s energy and appetite not as vigorous as when it was hyperthyroid, but in fact, the medication is restoring the cat’s thyroid function from an overactive state to normal. At this time, the cat’s energy and appetite are the ‘normal state’. Parents of hyperthyroid cats need to have a certain understanding of this disease, so as not to be scared by the reaction after treatment, and then panic to stop the medication, which delays the treatment.

*Cost: According to statistics, no matter which of the four treatments the cat receives, the lifetime cost is almost the same, only the difference is that radioactive iodine and surgery have a higher one-time cost, while drug treatment and iodine-restricted diet are long-term treatment, monitoring, and the cost of purchasing special prescription food.

The side effects of hyperthyroidism medication seem very serious, is it better not to take the medicine?

The treatment of hyperthyroidism, no matter which method, may be accompanied by certain side effects, but remember, if left untreated, the disease will continue to deteriorate and even cause death.

How to choose the treatment method must be based on the cat’s age, personality, existing complications, the cat owner’s ability to bear the treatment cost, the difficulty of obtaining treatment, the clinical veterinarian’s advice, etc., and it is recommended to discuss carefully with your own veterinarian after understanding the advantages and disadvantages of the treatment, and choose the best treatment method for the cat and the cat owner.

Summary

Hyperthyroidism is a common chronic disease in senior cats, which can cause problems in various systems throughout the body. However, the symptoms can make parents think that ‘the cat seems to have become younger’ and delay the time to seek medical treatment. Compared with other endocrine diseases, the diagnosis of hyperthyroidism is relatively easy, and there are many treatment options, so the health check items for middle-aged and senior cats are recommended to include screening for total thyroid concentration (TT4) for early diagnosis and early treatment.