Hypothyroidism in Cats

Clare Deming
Fat cat resting on sofa

The thyroid glands are a pair of tiny organs located near the larynx in the neck of your cat. In all mammals, the thyroid plays a vital job in regulating the basal metabolic rate by producing a hormone called thyroxine. Although hypothyroidism is a common condition for humans, spontaneous hypothyroidism in cats is rare. Hypothyroidism in cats is usually caused by overtreatment of hyperthyroidism, a much more common condition in cats. Your best bet in identifying the cause of any unusual symptoms in your cat is to head to your veterinarian for an expert opinion.

Types of Feline Hypothyroidism

The most common type of feline hypothyroidism is iatrogenic hypothyroidism which is when a cat's thyroid function is too low because of treatment for an overactive thyroid. Other types that can occur are spontaneous hypothyroidism and congenital hypothyroidism.

Iatrogenic Hypothyroidism

When cats are diagnosed with hyperthyroidism (an overactive thyroid), there are four ways to treat the disease. These treatment options include:

  • Lifelong treatment with methimazole, a medication that suppresses thyroid function
  • Surgery to remove the thyroid glands
  • I-131 treatment - an injection of a radioactive pharmaceutical that destroys the abnormal thyroid gland
  • A diet that is extremely low in iodine which is required for thyroid function

While surgery is no longer done as often as it once was, I-131 treatment is a very successful technique and cats usually respond very well to it. After treatment for hyperthyroidism, your vet will need to monitor your cat's thyroid levels to make sure that treatment has worked. In the first few months after treatment, it is common for the thyroid levels to be below the normal range. This does not necessarily indicate hypothyroidism, but can result from any leftover thyroid cells recovering from the condition.

If your cat's thyroid function remains low for some time after I-131 treatment or surgery, your vet may decide to treat her for iatrogenic hypothyroidism. According to DVM360, symptoms of iatrogenic hypothyroidism and other findings that may prompt treatment include:

  • Lethargy
  • Weight gain
  • Elevated kidney values

Cats with iatrogenic hypothyroidism and kidney disease may have longer survival rates when treated with replacement thyroid hormones.

Spontaneous Hypothyroidism

When thyroid function decreases without other causes, this is called primary or spontaneous hypothyroidism. It is very rare in cats, with only a handful of cases being reported, although more may exist. In one case, symptoms included:

  • Profound lethargy
  • Low body temperature
  • Poor hair growth
  • Severe crusting of the skin
  • A puffy face

In another cat, symptoms were similar:

  • Lethargy
  • Weight gain
  • Decreased appetite
  • Poor hair coat
  • Ear infections

Congenital Hypothyroidism

This is also a rare form of feline hypothyroidism but is reported occasionally. In congenital hypothyroidism, a kitten is born with a defect in the thyroid gland that limits its ability to produce thyroxine. Symptoms of congenital hypothyroidism include:

  • Stunted growth
  • A round head and short legs
  • Lethargy
  • Mental dullness
  • Constipation
  • Low body temperature
  • Low heart rate
  • Persistent baby teeth
  • Cold intolerance
  • Persistent fluffy kitten fur

What Else Could It Be?

If your cat has symptoms of feline hypothyroidism, there are more likely other causes because spontaneous feline hypothyroidism is very rare. Other causes of weight gain, lethargy or weakness, decreased appetite, and changes to the skin and coat in cats could be:

How Is Feline Hypothyroidism Diagnosed?

Blood tests to measure a variety of thyroid hormones are used to diagnose feline hypothyroidism. However, if your cat does not have symptoms consistent with hypothyroidism, the thyroid level can still be decreased. This is called euthyroid sick syndrome and can be seen with any other illness. Thyroid function remains normal, despite low blood levels and this does not need to be treated.

If your vet suspects congenital or spontaneous hypothyroidism, more complicated hormone testing or referral to a specialist may be needed.

Treatment for Feline Hypothyroidism

Hypothyroidism itself is not life-threatening in cats, but can contribute to progressive kidney disease in geriatric cats. Treatment for feline hypothyroidism consists of either decreasing the medication being used to treat hyperthyroidism or supplementing thyroid hormones in cats that have received surgery or I-131 therapy for hyperthyroidism. The most common medication used is called L-levothyroxine, but can go by other names. Your cat will need to be on this oral thyroid hormone replacement for the rest of its life.

The good news is that the hormone replacement therapy for iatrogenic hypothyroidism in cats will usually show signs of efficacy within a few weeks. For cats with spontaneous or congenital hypothyroidism, the prognosis is unclear because these conditions are rare.

Hypothyroidism in Cats