Hypothyroidism in Cats

What Is Hypothyroidism in Cats?

Hypothyroidism in cats is a lack of thyroid hormone. Hypothyroidism in cats is rare and can be divided into two categories: congenital (present at birth) and acquired.

The most common scenario is when a cat that has hyperthyroidism (overactive thyroid) has treatment that overcorrects and then causes them to have (acquired) hypothyroidism.

Congenital Hypothyroidism

Congenital hypothyroidism is a rare form that is present in kittens at birth. The thyroid hormone is important for the development of the nervous and skeletal systems. Kittens that lack this hormone will have related symptoms, such as mental dullness and smaller than normal proportions.

Acquired Hypothyroidism

Acquired hypothyroidism in cats is a lack of thyroid hormone in adult cats, which is also very rare. Acquired hypothyroidism can further be divided into three categories: primary hypothyroidism, secondary hypothyroidism, and iatrogenic hypothyroidism. Each type has a different cause.

Primary hypothyroidism is thought to result from either an immune-mediated disorder that destroys thyroid tissue, such as lymphocytic thyroiditis, or from thyroid gland atrophy, a shrunken and shriveled gland that does not function properly.

Secondary hypothyroidism has only been reported in one cat that suffered head trauma, which caused a decrease in the production of thyroid-stimulating hormone (TSH) by the pituitary gland.

Iatrogenic hypothyroidism can often occur in cats that are being treated for hyperthyroidism, which is the overproduction of thyroid hormone.

Surgery or radioactive iodine therapy to regulate overactive hormone production can sometimes leave cats with the opposite problem: their levels of thyroid hormone are now too low. This will cause irreversible hypothyroidism in your cat, but it can be managed.

Your cat can also have iatrogenic hypothyroidism if you are treating them with methimazole (in a pill or transdermal gel) as the veterinarian tries to find the right dose for your cat. However, this is reversible over several months after finding the right dose.

Symptoms of Hypothyroidism in Cats

The symptoms you’ll see depend on the type of hypothyroidism your cat has.

Congenital Hypothyroidism

Kittens with congenital hypothyroidism can display the following symptoms:

• Mental dullness

• Small size compared to their littermates

• Abnormal body proportions, such as a large and wide head, enlarged tongue, and short limbs

• Lethargy

• Wobbly, drunken gait (also known as ataxia)

• Dry skin

• Thin fur

Acquired Hypothyroidism

Cats with acquired hypothyroidism may display the following signs:

• Lethargy

• Excessive weight gain

• Lack of appetite

• Dry, dull coat

• Hair loss

• Scaly skin

Causes of Hypothyroidism in Cats

The causes of hypothyroidism in cats are also dependent on the type.

Congenital Hypothyroidism

Causes of congenital hypothyroidism can be divided into goitrous and nongoitrous forms. Goitrous refers to the enlargement of the thyroid gland.

Goitrous congenital hypothyroidism is thought to be caused by an inherited defect or dietary issues with iodine in the pregnant mother cat or newborn kitten.

Nongoitrous congenital hypothyroidism is usually caused by incomplete thyroid organ development. Congenital feline hypothyroidism has been seen in domestic shorthair cats as well as Abyssinian cats.

Acquired Hypothyroidism

Adult-onset acquired hypothyroidism is most commonly caused by iatrogenic hypothyroidism. This condition occurs when a cat’s treatment for hyperthyroidism (an overactive thyroid) results in an underactive thyroid.

Finding the balance an individual cat needs and anticipating their body’s response to therapy can be challenging. 

How Vets Diagnose Hypothyroidism in Cats

Diagnosis of hypothyroidism in cats requires blood samples to be drawn and put through a specific test called a thyroid panel. A feline thyroid panel consists of multiple measurements.

In a cat with hypothyroidism, their total T4 measurement is usually low. However, certain medications as well as a syndrome called euthyroid sick could also be responsible for lowering your cat’s total T4, so additional testing may be required.

This additional testing may consist of a blood sample for measuring free T4 (not bonded to your cat’s blood protein, which means the T4 is able to penetrate body tissues), T3, and TSH (thyroid-stimulating hormone).

In kittens with congenital hypothyroidism, X-rays may reveal bony changes. The thyroid hormone is essential for normal development of a kitten’s skeletal and nervous systems, so a lack of the hormone leads to changes seen in these systems.

Your veterinarian may also see high cholesterol and anemia on a complete blood count and chemistry panel.  While these issues are not unique to a diagnosis of hypothyroidism, seeing them in your cat may lead your vet to order more specific tests, including thyroid measurements. 

Treatment for Hypothyroidism in Cats

Treatment for hypothyroidism in cats is aimed at restoring normal blood levels of thyroid hormone. This is typically managed by giving an oral thyroid medication supplement, in either liquid or tablet form.

The drug sodium levothyroxine is usually given twice daily. Some cats may also require a second medication, called synthetic sodium liothyronine (L-T3).

Cats with congenital and acquired hypothyroidism (primary, secondary, and iatrogenic from surgery or radioactive iodine) will have to be on medication forever. But cats with acquired iatrogenic hypothyroidism from too high of a dose of oral or transdermal methimazole can usually be treated by adjusting the dose.

Recovery and Management of Hypothyroidism in Cats

The goal of oral medication for hypothyroidism in cats is to improve the clinical signs of the disease. Follow-up bloodwork is important to assess the absorption and appropriate dosing of medication.

An overdose of medication can have significant consequences, so your cat’s blood levels of T4 are an important indicator of appropriate regulation.

In cats with acquired iatrogenic hypothyroidism after treatment for hyperthyroidism, additional follow-up testing—including complete blood count, serum biochemistry, and urinalysis—is recommended.

With some of these cats (that were given oral or transdermal medication for hyperthyroidism), hypothyroidism can be reversed when your vet finds the ideal dosage of long-term hyperthyroid medication for your cat.

Cats that were treated for hyperthyroidism with permanent treatments like surgery or radioactive iodine that led to hypothyroidism will need the long-term oral thyroid hormone.