Hypothyroidism in Cats

What Is Hypothyroidism in Cats?

Hypothyroidism in cats is a rare condition caused by a deficiency of thyroid hormone.

Thyroid hormones are essential in regulating metabolism and supporting normal development throughout the body. 

Most cases of hypothyroidism in cats are acquired later in life, typically occurring when treatment for hyperthyroidism (an overactive thyroid) reduces thyroid hormone levels too much, resulting in an underactive thyroid.

Types of Hypothyroidism in Cats

Hypothyroidism can be divided into two categories: congenital (present at birth) and acquired.

Congenital Hypothyroidism

Congenital hypothyroidism is a rare form present in kittens at birth. Thyroid hormone is critical for proper nervous system and skeletal development. Kittens that lack sufficient thyroid hormone may show signs such as mental dullness and smaller-than-normal body proportions.

Acquired Hypothyroidism

Acquired hypothyroidism occurs in adult cats and is also uncommon. It can be divided into three categories:

• Primary hypothyroidism: Believed to result from immune-mediated destruction of thyroid tissue (such as lymphocytic thyroiditis) or from thyroid gland atrophy, where the gland shrinks and loses function.
• Secondary hypothyroidism: Extremely rare; reported in one case involving head trauma that reduced production of thyroid-stimulating hormone (TSH) by the pituitary gland.
• Iatrogenic hypothyroidism: Most commonly seen in cats treated for hyperthyroidism. Surgery or radioactive iodine therapy may permanently reduce thyroid hormone levels below normal. This form is irreversible but manageable.

Iatrogenic hypothyroidism may also temporarily occur during methimazole treatment (oral or transdermal) while veterinarians adjust dosing. This form is typically reversible once the appropriate dose is established.

Symptoms of Hypothyroidism in Cats

The symptoms of hypothyroidism in cats vary depending on whether the condition is present at birth or acquired later in life.

Congenital Hypothyroidism

Kittens with congenital hypothyroidism may show:

• Mental dullness

• Smaller size compared to littermates

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

• Lethargy

• Wobbly or “drunken” gait (ataxia)

• Dry skin

• Thin or poor-quality 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 depend on whether the condition is congenital or acquired.

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 gland development.

Congenital feline hypothyroidism has been seen in domestic shorthair cats as well as Abyssinian cats.

Acquired Hypothyroidism

Acquired hypothyroidism in adult cats is most commonly iatrogenic, meaning it develops as a result of medical treatment.

This occurs when therapy for hyperthyroidism—such as surgery, radioactive iodine treatment, or medication—reduces thyroid hormone levels too much.

Balancing thyroid treatment can be challenging, as individual cats may respond differently to therapy.

How Vets Diagnose Hypothyroidism in Cats

Diagnosing hypothyroidism in cats requires specialized blood testing to measure thyroid hormone levels.

Your veterinarian will perform a feline thyroid panel, which includes multiple hormone measurements. In cats with hypothyroidism, the total T4 level is typically 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.

Further testing may include measuring:

• Free T4, which is not bound to blood proteins and is available to tissues

• T3 (triiodothyronine)

• TSH (thyroid-stimulating hormone)

In kittens with congenital hypothyroidism, X-rays may reveal skeletal abnormalities, as thyroid hormone is essential for normal bone and nervous system development.

Your veterinarian may also see high cholesterol levels 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 focuses on restoring and maintaining normal thyroid hormone levels in the bloodstream.

This is typically managed by giving an oral thyroid medication supplement, in either liquid or tablet form. The drug sodium levothyroxine (T4) is usually given twice daily. Some cats may also require a second medication, called synthetic sodium liothyronine (L-T3).

Cats with congenital and most forms of acquired hypothyroidism (primary, secondary, and iatrogenic from surgery or radioactive iodine) will have to be on medication forever.

However, cats that develop iatrogenic hypothyroidism due to excessive methimazole dosing (oral or transdermal) may not need permanent therapy. In these cases, adjusting the methimazole dose can restore appropriate thyroid hormone levels over time.

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.