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September 13, 2018 – Thyroid function - what you need to know

The thyroid gland is one of the most important, yet frequently misunderstood, organs in the body. Hypothyroidism, a condition characterized by low levels of circulating thyroid hormones, is not only one of the most common diseases of dogs but also one of the most commonly misdiagnosed diseases. Understanding more about how the thyroid gland functions, as well as knowing more about thyroid gland abnormalities can help us make better treatment decisions. As dog owners, it’s important to get the facts about the thyroid in health and disease.

Normal thyroid function

The thyroid gland is a crucial player in regulating metabolism as well as playing a significant role in fetal development. The thyroid produces hormones that help regulate these processes and is part of a feedback loop that includes several structures in the brain as well as cells within the thyroid itself. Low levels of circulating hormones stimulate production – once levels increase, the process stops until levels decrease once more.

The bald, the sluggish, the overweight – hypothyroidism in dogs

Many veterinary experts report that hypothyroidism is one of the most common endocrine (glandular) problems diagnosed in dogs. A defect in any part of the thyroid hormone production pathway can result in low thyroid hormone levels leading to signs of hypothyroidism. The most commonly reported signs of hypothyroidism in dogs include:

  • Skin problems – usually hair loss but can include dull haircoat, increased skin pigmentation and recurrent skin and ear infections
  • Obesity
  • Lethargy and weakness
  • Slow heart rate
  • Reproductive problems

Many of the signs of hypothyroidism are similar to a lot of other diseases, and special testing is needed to confirm a diagnosis of hypothyroidism.

Confirming the diagnosis – harder than you think

Although it seems obvious that simply measuring the level of thyroid hormone in the blood would be a straightforward way to confirm a suspicion of hypothyroidism it turns out that many other factors outside the thyroid can influence thyroid hormone levels. These include:

  • Drug administration including glucocorticoids, phenobarbital, and sulfonamide antibiotics
  • Concurrent illnesses
  • Age
  • Breed

There are several types of thyroid tests routinely run on dogs to try to pinpoint a diagnosis. Many are familiar to dog owners because they’re often included on routine blood panels. These tests include:

  • Total T4 (often reported simply as T4) – T4 is the most common type of thyroid hormone secreted by the thyroid gland. Once secreted into the bloodstream, 99 percent of this hormone binds to plasma proteins. Protein-bound T4 acts a reservoir to help maintain steady thyroid hormone levels.
  • Free T4 – this small fraction of total T4 is not bound to proteins in the blood and is biologically active.
  • Thyroid-stimulating hormone (reported as TSH) – this hormone is produced by the pituitary gland in the brain in response to low blood levels of thyroid hormones. In cases of chronically low thyroid hormone levels, TSH is high (such as hypothyroidism).

In combination, these tests are commonly used to rule in or out a diagnosis of hypothyroidism. There are other tests available in cases where it simply isn’t clear if a patient is truly hypothyroid or not, but these are much more specialized and often require submission to specific testing laboratories. Your veterinarian can guide you on what tests are appropriate – sometimes re-testing is best; in some cases, a “wait and see” approach is indicated; and in other patients, more extensive testing is warranted.

If the diagnosis of hypothyroidism is tricky, the treatment is straightforward. Oral supplementation of thyroid hormone is very effective although it can take several months for many abnormalities (such as hair loss) to completely resolve. Periodic testing and adjustment of thyroid dose is necessary to find the optimal supplementation regime for each patient. The most common reasons for treatment failure are an inappropriate diagnosis of hypothyroidism and failure to treat concurrent illnesses.

Hypothyroidism is one of the secondary endpoints of the Golden Retriever Lifetime Study. So far, we have 66 dogs (about 2 percent of the study cohort) diagnosed with hypothyroidism but we expect more diagnoses as our cohort ages. Once identified, we’ll continue to follow these dogs for concurrent illnesses as well as continue to bank biologic samples that might be of interest to researchers wanting to learn more about this common disease.