Hypothyroidism in cats: symptoms, causes, treatment and FAQ

What is hypothyroidism in cats?

Hypothyroidism in cats is a rare disease caused by a deficiency of thyroid hormones.

Thyroid hormones are essential for the regulation of metabolism and normal development of the entire body.

Most cases of hypothyroidism in cats occur later in life, usually 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 essential for the proper development of the nervous system and skeleton. Kittens who lack sufficient thyroid hormones may exhibit signs such as mental dullness and smaller than normal body proportions.

Acquired hypothyroidism

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

• Primary hypothyroidism: It is thought to result from immune destruction of thyroid tissue (such as lymphocytic thyroiditis) or thyroid gland atrophy, where the gland shrinks and loses function.
• Secondary hypothyroidism: Extremely rare; reported in a case involving head trauma that reduced the 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 can permanently reduce thyroid hormone levels below normal. This form is irreversible but manageable.

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

Symptoms of Hypothyroidism in Cats

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 exhibit:

• Mental boredom

• Smaller size compared to littermates

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

• Lethargy

• Wobbly or “drunk” gait (ataxia)

• Dry skin

• Thin or poor quality fur

Acquired hypothyroidism

Cats with acquired hypothyroidism may exhibit the following signs:

• Lethargy

• Excessive weight gain

• Lack of appetite

• Dry and 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

The causes of congenital hypothyroidism can be divided into goitrous and non-goitrous forms. Goitrous refers to the enlargement of the thyroid gland.

• Congenital goitrous hypothyroidism It is thought to be caused by a hereditary defect or iodine-related dietary problems in the pregnant cat or newborn kitten.
• Congenital non-goitrous hypothyroidism is usually caused by incomplete development of the thyroid gland.

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

Acquired hypothyroidism

Acquired hypothyroidism in adult cats is most often iatrogenic, meaning it develops following medical treatment.

This happens when treatment for hyperthyroidism, such as surgery, radioactive iodine treatment, or medications, reduces thyroid hormone levels too much.

Balancing thyroid treatment can be difficult, as each cat may respond differently to treatment.

How Vets Diagnose Hypothyroidism in Cats

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

Your veterinarian will perform a feline thyroid panel, which includes several hormonal measurements. In cats with hypothyroidism, total T4 levels are usually low.

However, certain medications as well as a syndrome called euthyroid disease could also be responsible for a decrease in your cat's total T4, so additional testing may be necessary.

Additional tests may include measuring:

• Free T4which is not bound to blood proteins and is available in tissues

• T3 (triiodothyronine)

• TSH (thyroid stimulating hormone)

In kittens with congenital hypothyroidism, x-rays may reveal skeletal abnormalities because 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. Although these problems are not unique to a diagnosis of hypothyroidism, observing them in your cat may lead your veterinarian to order more specific tests, including thyroid measurements.

Treatment of hypothyroidism in cats

Treatment for hypothyroidism in cats focuses on restoring and maintaining normal levels of thyroid hormones in the blood.

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

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

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

Recovery and Management of Hypothyroidism in Cats

The goal of oral medications for hypothyroidism in cats is to improve the clinical signs of the disease.

Follow-up blood tests are important to assess absorption and proper dosing of the medication.

A medication overdose can have significant consequences, which is why your cat's T4 blood levels are an important indicator of proper 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.

In some of these cats (who have received oral or transdermal medications for hyperthyroidism), the hypothyroidism can be reversed when your veterinarian finds the ideal dose of long-term hyperthyroid medication for your cat.

Cats that have been treated for hyperthyroidism with permanent treatments such as surgery or radioactive iodine that led to hypothyroidism will need oral thyroid hormone long term.