Hypothyroidism in cats

Mandy Cooper

Note:  Many of BCR's cats have experienced and been treated for hyperthyroid, including Jamie, Mack, Pearl, Natasha and Serena.  We recently welcomed another cat suffering from this untreated condition, Gio.  She weighed less than five pounds when she arrived due to not being treated for this disease.  It's important to know all you can to protect and treat your cat!

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 role in regulating the basal metabolic rate by producing a hormone called thyroxine. Although hypothyroidism is a common condition for humans, hypothyroidism in cats is rare.

Hypothyroidism in cats is usually caused by over treatment 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 vet 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 of 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), they 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.
•  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, radioactive 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. I the first few months after treatment, it is common for the thyroid levels to be below 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 radioactive treatment or surgery, your vet may decide to treat them for iatrogenic hypothyroidism. Symptoms of iatrogenic hypothyroidism 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. Symptoms can include:

•  Profound lethargy
•  Low body temperature
•  Poor hair growth
•  Severe crusting of the skin
•  A puffy face
•  Weight gain
•  Decreased appetite
•  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 their 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:

•  Diabetes mellitus
•  Hyperthyroidism
•  Cushing's disease
•  Cancer
•  Liver disease
•  Kidney disease 

What To Watch For

A lack of thyroid hormone affects the metabolic function of many organ processes. As a result, the clinical signs are usually slow to develop. Although there is no one symptom that is diagnostic, several combined signs may make your vet more suspicious. Symptoms may include:

• Lethargy, lack of interest in play, increased amounts of sleeping
• Weight gain and obesity
• Cold intolerance – seeks out warm places to lie down, low body temperature
• Constipation
• Decreased appetite
• Chronic skin disorders, such as dry skin, thinning of the hair coat, excessive hair loss
• Diagnosis of Hypothyroidism in Cats

Hypothyroidism is not always a simple, straightforward disease to diagnose. Various tests are available to diagnose the condition and a combination of tests may be required. Proper diagnosis also includes a thorough history, documentation of clinical signs, a thorough physical examination, and diagnostic tests to assess various organ functions, including thyroid function. A diagnostic work-up may include the following:

•  Complete blood count(CBC)
•  Biochemical profile Urinalysis
•  Thyroxine (T4) level, tri-iodothyronine (T3) level
•  Serum Free T4 by Equilibrium Dialysis (FT4ED)
•  Thyrotropin stimulation test
•  Thoracic (chest) and abdominal radiographs (X-rays) in certain cases
•  Other tests to rule out other hormonal disorders, such as hyperadrenocorticism (overproduction of cortisone hormone) and acromegaly (overproduction of growth hormone)

Treatment of Hypothyroidism in Cats

If the use of methimazole is responsible for the signs of hypothyroidism, the drug is stopped for several days or until the T4 level returns to normal. Methimazole may then be re-introduced at a lower dose.

 Thyroid hormone supplementation is indicated for the treatment of other forms of hypothyroidism, and it is administered for the life of the cat

 Synthetic (man-made) levothyroxine (T4) is the drug of choice for treating hypothyroidism. The dosage and frequency of administration are determined by your vet. Levothyroxine is usually given once a day in cats

 There are both brand name and generic levothyroxine products available commercially. Using a brand name product rather than a generic product is usually preferred. Once a cat is stabilized on a particular thyroid medication, it is also better if the cat remains on that product consistently, rather than bouncing from one product to another.

Follow-up Care

Treatment requires a combination of home and professional vet care. At home it is important to administer all medication exactly as prescribed by your vet. With appropriate therapy, most of the clinical changes associated with hypothyroidism improve within four to six weeks.

Most cats tolerate thyroid supplementation very well, however, over dosage is associated with return of the signs of hyperthyroidism. Watch the cat closely for signs of hyperactivity, increased vocalization (meowing, howling), restlessness, weight loss and diarrhea, and report these signs to your vet promptly.

It is important to follow-up with regularly scheduled visits to your vet so that both your cat's clinical signs and thyroid concentration in the blood can be monitored. Generally, the first follow-up examination is within four weeks after the start of therapy. T4 levels are often measured six to eight hours after the pill is given. Adjustments in the dosage of medication are then recommended depending upon the results of these tests. Additional recheck visits are then scheduled based upon the test results, changes in clinical signs, and any changes in the medication schedule.

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