Canine Hypothyroidism

Hypothyroidism is the condition where one has inadequate active thyroid hormone. It is a common human ailment and is also the most common hormone imbalance of dogs. It seems like hypothyroidism would be a straight forward problem: the body does not produce enough thyroid hormone, disease results, a test shows thyroid hormone level is low, additional hormone is taken as a pill, problem solved. Unfortunately, the situation is not quite so straight-forward in real life. Here we review the issues relevant to this condition and the pitfalls that keep it from being a simple problem.

What is the Thyroid Gland and What does it do?

The dog’s thyroid glands are located on either side of the windpipe (trachea).

The human thyroid glands actually connect across the windpipe forming what is classically decribed as an H.

The thyroid gland forms an H in a human throat as shown in the graphic above, although in dogs, the thyroid exists as two separate halves. The thyroid produces two forms of thyroid hormone: T3, which is the active form of the hormone, and T4, which is the inactive form created to circulate in the bloodstream. When T4 is absorbed into tissue cells, it is converted into T3. Most of the circulating T4 is carried by blood proteins and is not available for tissue absorption; the portion that is not carried by proteins (the so-called “free T4”) is the portion that is able to enter tissues for activation. The thyroid gland itself can also activate T4 into T3. In dogs about half of the T3 comes from the thyroid gland and the remaining half comes from the body’s other tissues. In humans, 80% of T3 comes from activation in the body’s other tissues.

Production of T4 is regulated by the pituitary gland at the base of the brain. (This gland is called the master gland as it regulates hormone production in the adrenal system, the thyroid system, the reproductive system, and more). The pituitary produces a substance called TSH, which stands for thyroid stimulating hormone. When T4 levels are dropping, the pituitary gland stimulates the thyroid gland to make and release more T4.

Active thyroid hormone serves as a sort of a volume dial for metabolism. Since virtually every cell in the body can be affected by reduced levels of thyroid hormone, it is not surprising that reduced levels of thyroid hormone lead to symptoms in multiple body systems.

What is Hypothyroidism?

In short, hypothyroidism is the natural deficiency of thyroid hormone. This deficiency is produced by immune-mediated destruction of the thyroid gland, by natural atrophy of the gland, by dietary iodine deficiency, or as a congenital problem. In dogs, the first two causes listed account for almost all cases, although currently the feeling is that atrophy of the gland actually represents the end result of earlier immune-mediated destruction of the gland.

Hypothyroidism generally develops in middle aged or elderly dogs. Breeds with definite predisposition to develop hypothyroidism include: the Doberman pinscher, the golden retriever, the Irish setter, the Great Dane, the dachshund, and the boxer.

Manifestations of Hypothyroidism

Hypothyroidism is a classical disease with a classical collection of clinical signs. One particularly well-published survey of 162 confirmed hypothyroid dogs showed the following common findings:

• 88% had some kind of skin abnormality
• 40% had hair loss (often this starts on the tail leading to a rat tail appearance or a bald area around the collar is created.)
• 22% had skin infection (often dogs are scaly and smelly due to an excessively oily coat.)
• 14% had brittle or dry coats (often the outer hairs break off leaving a short, softer under coat, classically described as a “puppy-like coat.”)
• 49% were obese
• 48% were described as lethargic or listless at home
• 36% were anemic (had a reduced number of red blood cells due to slowed red blood cell production in the bone marrow.) This is a good place to note that elevated blood cholesterol levels are also considered a common finding of hypothyroidism.

One classical finding in hypothyroid dogs is a thickening of some tissues, especially of the face and head. The skin in particular thickens, leading to more skin folds and what is classically referred to as a tragic face. This thickening is called myxedema and can occur in some other tissues as well.

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