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Key takeaways

Hypothyroidism in cats refers to abnormally low levels of thyroid hormones.

  • The condition is not common and nearly all cases of feline hypothyroidism develop during treatment for hyperthyroidism (overactive thyroid gland)
  • Rarely kittens are born with hypothyroidism (cretinism) and cats can develop hypothyroidism through abnormal immune system activation or brain injury
  • Symptoms include lethargy, weight gain, cold aversion, appetite reduction, hair loss, and dry, thickened skin
  • Diagnosis involves blood tests to measure thyroid hormone levels and medication administration to stimulate thyroid hormone production
  • Treatment involves lifelong administration of oral artificial hormone medications to replace the thyroid’s production
  • Most cats have a good prognosis with treatment
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A closer look: Hypothyroidism in Cats

Hypothyroidism refers to abnormally low thyroid hormone secretion. Thyroid hormones stimulate the body’s metabolism. In their absence, cats become lethargic, gain weight easily, avoid the cold, and have reduced appetite.

Hypothyroidism is very rare in cats. Most cats with hypothyroidism develop it through treatment for hyperthyroidism. Although hypothyroidism is a lifelong condition, it has a good prognosis with hormone replacement treatment.

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Risk factors

Hypothyroidism is rare in cats and is usually associated with treatment for hyperthyroidism (which describes overactive secretion of thyroid hormone). Cats in treatment for hyperthyroidism are at the highest risk of developing hypothyroidism.

Hypothyroidism affects hair growth, so skin conditions are common in affected cats.

Cretinism is a specific hypothyroid condition where cats are born with hypothyroidism. Without a stimulus to grow, these cats develop dwarfism. The characteristic appearance of a dwarf cat includes very short limbs, round heads, and short bodies. Their growth rate is extremely slow, and they have a poor prognosis.

Possible causes

The most common way cats develop hypothyroidism is during treatment for hyperthyroidism, a condition where the thyroid overproduces hormones. The goal of treatment for hyperthyroidism is the reduction of thyroid hormone production, and sometimes hormone levels drop too low in response to the therapy.

Other causes of hypothyroidism are extremely rare in cats, but include:

  • The immune system reacting to thyroid tissue
  • Damage to the brain preventing activation of the thyroid, e.g. from injuries or tumors
  • Kittens produced from queens experiencing iodine deficiency during pregnancy
  • Inherited defects preventing thyroid hormone production
  • Failure to produce normal thyroid tissue during development

Main symptoms

Thyroid hormone is responsible for increasing metabolism in multiple body systems, so the symptoms of hypothyroidism vary.

Testing and diagnosis

Diagnosis of hypothyroidism involves blood tests to identify how much thyroid hormone is present. These tests include hormone level testing and hormone stimulation testing. If thyroid hormone levels remain low after stimulation, the test confirms a diagnosis of hypothyroidism.

Steps to Recovery

Hypothyroidism is a lifelong condition, as the thyroid cannot repair itself. Most cats have a good prognosis with well-monitored hormone replacement treatment.

Hypothyroidism is treated with oral medication that replaces the lacking thyroid hormones. Follow-up blood tests and monitoring are necessary to ensure that the level of supplemented thyroid hormone is appropriate. Most cats require lifelong thyroid hormone replacement after a diagnosis.

Hyperthyroid cats treated with oral medication (as opposed to surgery or radioactive iodine therapy) who develop hypothyroidism often do not require oral hormone supplementation. In these cases, the symptoms of hypothyroidism are expected to resolve when the medication for hyperthyroidism is discontinued.


Hypothyroidism can not be prevented. It is usually incidental. It is not contagious.

Is Hypothyroidism in Cats common?

Hypothyroidism is rare in cats.

Typical Treatment

  • Hormone supplementation
  • Adjustment to existing medication schedule


Mark E. Peterson, Janice E. Kritchevsky - Writing for Merck Veterinary Manual
Dr. Rhea Morgan - Writing for PetPlace
Boyd R. Jones - Writing for World Small Animal Veterinary Association
No Author - Writing for Carolina Veterinary Specialists

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