Hypothyroidism in Cats

Summary

Hypothyroidism refers to abnormally low levels of thyroid hormones. The condition is not common in cats, and nearly all cases of feline hypothyroid develop the condition during treatment for hyperthyroidism, or an overactive thyroid gland. Very rarely, kittens are born with hypothyroidism, which is called cretinism. In rare cases a cat can develop hypothyroidism through abnormal immune system activation or injury to the brain. 

Thyroid hormones stimulate the body’s metabolism. In their absence, cats are lethargic, gain weight easily, avoid the cold, and have reduced appetites. Hypothyroidism also affects the skin, causing hair loss and dry, thickened skin. 

Diagnosis of hypothyroidism involves blood tests to measure thyroid hormone levels. If levels are low, hypothyroidism is suspected. To confirm the diagnosis, a medication that stimulates thyroid hormone production is given. If hormone levels remain low after this stimulus, the diagnosis is confirmed.

Oral administration of synthetic thyroid hormone treats hypothyroidism. Hypothyroid cats require hormone replacement medications for life, as the thyroid gland cannot repair itself. Most cats have a good prognosis with treatment.

Risk Factors

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.

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. Symptoms include:

• Reduced appetite • Unexplained weight gain • Lethargy • Seeking warm places to rest

Detailed Characterization

Hypothyroidism affects hair growth, so skin conditions are common in affected cats. Secondary symptoms include:

• Dry skin • Thickened skin • Hair loss • Thin hair coat • Unkempt appearance

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.

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.

Prevention

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

Is Hypothyroidism Common in Cats?

Hypothyroidism is rare in cats.

Typical Treatment

The typical treatment of hypothyroidism is thyroid hormone supplementation. In cases where it is related to treatment for hyperthyroidism, adjustment of the treatment protocol for the original condition is the treatment of the incidental hypothyroidism.