Hypothyroidism in Cats: What Pet Parents Need To Know

Hypothyroidism in cats is a rare condition where the body doesn’t produce enough thyroid hormone—most often occurring after treatment for hyperthyroidism. It can cause symptoms like weight gain, low energy, and a poor coat, but is typically manageable with lifelong medication. 

If your cat seems unusually sluggish, is gaining weight without eating more, or has changes in their coat, it’s worth talking to your veterinarian. While uncommon, hypothyroidism affects multiple body systems and requires proper diagnosis and treatment. 

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 who 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. 

  • Latrogenic hypothyroidism: Most 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 under-the-skin/transdermal) while veterinarians adjust dosing.  

This form is typically reversible once the appropriate dose is established. 

Symptoms of Hypothyroidism in Cats

Symptoms of hypothyroidism in cats depend on the type.  

Signs of congenital hypothyroidism include: 

• Mental dullness 

• Smaller kitten size compared to littermates  

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

• Lethargy 

• Wobbly gait (ataxia) 

• Dry skin 

• Thin or poor-quality fur 

Symptoms of acquired hypothyroidism may include: 

• 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: These cases are commonly iatrogenic, meaning they develop because of medical treatment. This occurs when therapy for hyperthyroidism—such as surgery, radioactive iodine treatment, or medication—reduces thyroid hormone levels too much. 

How Veterinarians Diagnose Hypothyroidism in Cats

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

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 (CBC) 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 liquid or tablet form. The medication used for treatment is levothyroxine.  

Cats with congenital and most forms of acquired hypothyroidism (primary, secondary, and iatrogenic from surgery or radioactive iodine) must be on medication for the rest of their lives. 

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

Cats typically see improvement in many of their symptoms, such as lethargy and weight gain, after just two weeks of treatment. Skin and coat issues may take a couple of months to get better. 

Finding the correct dose of thyroid hormone to give your cat can take time.  

Generally, your cat will be started on a dose and then his bloodwork will be rechecked six weeks later. If the levels of thyroid hormone are within the normal range, your cat can continue his current dose.  

If levels are too low or too high, his dose will need to be adjusted, and he will need another recheck six weeks later.  

Cats being treated for hypothyroidism generally need bloodwork every six to 12 months to ensure their condition is stable.  

Prevention of Hypothyroidism in Cats

Congenital hypothyroidism is not preventable, since it is present at birth, and the exact underlying cause is unknown.  

However, ensuring a pregnant mother is fed a high-quality commercial diet, such as Purina Pro Plan Kitten Food, can reduce the risk.  

For cats who are being treated with hyperthyroidism, ensure you give her medication exactly as prescribed by your veterinarian to avoid iatrogenic hypothyroidism. Always bring your cat in for rechecks as recommended by the veterinarian to ensure the dose remains appropriate.