Hypothyroidism is a disease that is caused by lower than normal levels of thyroid hormone in the body. Most cases are diagnosed in middle-aged or older large breed dogs and are due to abnormal immune system activity that results in destruction of the thyroid gland. Very young animals can be afflicted with hypothyroidism if their thyroid glands fail to develop normally.
Adult hypothyroid dogs often are lethargic, gain weight, have poor quality coats and recurrent skin problems, are frequently cold, and may develop neurologic problems or even become comatose in severe cases. Young animals with hypothyroidism also have stunted growth. Thyroid hormone levels can be measured in a sample of blood, but diagnosing the disease is complicated by the fact that thyroid hormone concentrations frequently drop in response to other illnesses. Therefore, diagnosis of hypothyroidism is based on a combination of the presence of typical clinical signs, low levels of circulating thyroid hormone, and the absence of another disease that could explain the results of thyroid testing. Veterinarians may to run routine blood work, a urinalysis, fecal exams, x-rays, and additional forms of thyroid testing before a diagnosis of primary hypothyroidism can be made.
Thankfully, treating hypothyroidism is much easier than is diagnosing the disease. Oral supplementation with synthetic thyroid hormone is typically very effective and relatively inexpensive but does need to continue for the rest of the animal's life. Thyroid hormone levels need to be checked on a regular basis to make sure that dosages remain appropriate. Animals that have low thyroid levels in response to an underlying disease do not benefit from thyroid hormone supplementation.
Written by: Jennifer Coates, DVM
Last reviewed: October 2, 2008