About Euthyroid Sick Syndrome in Dogs

By Dr. Melissa Nelson

Euthyroid sick syndrome is a poorly understood syndrome in dogs. The syndrome does not necessarily cause symptoms. Instead most dogs with euthyroid sick syndrome will be suffering from a concurrent severe illness with its own set of symptoms. Misdiagnosis of hypothyroidism is a common problem, which can lead to unnecessary and potentially dangerous treatment with thyroid hormones.


Hypothyroidism is a common endocrine disease of middle aged, medium to large breed dogs. The most common symptoms are hair loss and flaky skin. The skin is easily bruised and wounds heal poorly. There is also an increased risk for skin and ear infections. The disease is caused by a faulty thyroid gland that does not release the necessary hormones needed for vital body functions. In contrast, the thyroid gland has no significant damage in euthyroid sick syndrome.

Euthyroid Sick Syndrome

In euthyroid sick syndrome another illness causes the suppression of the blood's concentration of hormones particularly thyroxine and free thyroxine (T4 and fT4). This suppression can be misleading and prompt a diagnosis of hypothyroidism. The reason the hormones become suppressed is not fully understood.


Diagnosis of euthyroid sick syndrome in dogs is a difficult process. Many cases are misdiagnosed or overlooked. A complete medical history and thorough physical exam are warranted to determine if there is a nonthyroidal illness present. Medications can cause suppression of the thyroid gland as well. It is important to take into consideration what medications your dog is taking. Blood tests for thyroxine levels and thyroid stimulating hormone will help in making a diagnosis. Other blood tests that can help distinguish euthyroid sick syndrome versus hypothyroidism include tests for anemia and high cholesterol. These are characteristic of hypothyroidism.


There are many potential causes for euthyroid sick syndrome. Endocrine diseases such as Cushing's disease, hypoadrenocortism and diabetes can suppress thyroid hormones. Severe infections, liver failure, kidney disease and heart failure can lead to euthryoid sick syndrome. Obese or elderly animals can have abnormally low levels of thyroid hormones. Being in heat, pregnancy or fasting also can cause misleading T4 and fT4 level results.


Treatment should revolve around treating the underlying illness or condition which caused the low hormone levels. Medications that may suppress hormone levels should be adjusted or changed if possible. Giving supplemental thyroid hormones to a dog with euthyroid sick syndrome is not recommended. All cases of euthyroid sick syndrome should be carefully monitored by a veterinarian until the condition resolves.