One of the common mantras taught to veterinary students is that “age is not a disease.”

Certainly as pets age they are more predisposed to diseases such as osteoarthritis and cancer.

So many times we hear a pet's parents saying with resignation that their pet is just slowing down due to age.

But there must be an underlying cause and sometimes once discovered and treated, the pet's final years can be improved.

One such disease of older dogs is hypothyroidism.

The thyroid glands sit in the neck along side the trachea. They produce thyroid hormone, which helps maintain normal metabolic function of all cells in the body.

Thyroid hormone levels are tightly regulated by communication between the thyroid glands and pituitary gland, as too low or too high a level will cause problems, hypothyroidism and hyperthyroidism, respectively.

Hypothyroidism in dogs is usually related to inflammation and gradual destruction of the glands preventing hormone production.

Puppies can be born without or with poorly functioning glands, often resulting in stunted growth.

In rare cases, the pituitary gland may stop signaling the thyroid glands to produce the hormone.

Hypothyroidism may result in symptoms from almost any organ, as most cells depend on thyroid hormone to function normally.

Behavior changes may be noted first. Decreased activity, mental dullness and lost interest in physical activity may be noted.

The slowed metabolism also may result in weight gain, with or without a change in appetite.

Since the skin is the most visible organ of all, changes in it are most easily observed. A coarse, dull hair coat, dry scaly skin, hair loss on the tail (rat tail), increased skin pigmentation, increased oiliness of the coat (seborrhea), skin infections and diffuse hair thinning are often seen.

Sometimes thickening of the skin may occur, called myxedema. This may cause the face to droop, creating a sad expression.

Nerves and muscles also are affected by a decreased level of thyroid hormone.

This may present as a generalized weakness or even just what appears to be lameness in one limb. If left untreated for a long period, the brain may be affected.

One of the more serious consequences can be seen if the esophageal muscle is affected. Since dogs eat in a horizontal position (not vertical like us) they are entirely dependent on this muscle to carry food and water to their stomach. If this muscle is not functioning properly (megaesophagus) then they may regurgitate. When regurgitating, the airway is not closed (as with vomiting) so these dogs are at a great risk of developing aspiration pneumonia.

The muscles and nerves that affect the larynx also may be affected.

This may be noted as a change in the sound of the bark. More seriously, it may result in difficulty breathing, which can be life-threatening.

Dogs used for breeding may be infertile, skip their heat cycles and have difficultly carrying puppies to term.

As you can see, hypothyroidism can show multiple symptoms. Some just limit the quality of life while others can lead to death.

Fortunately, hypothyroidism is usually easy to diagnose and treat.

Once diagnosed, an oral pill twice a day is all that is required to treat. Periodic blood testing is needed to ensure the correct dose is being given.

Hypothyroidism is a relatively common hormonal disorder in dogs. That old, tired dog whose symptoms were being chalked up to just “old age” can suddenly have a new lease on life.

Dr. Henri Bianucci and Dr. Perry Jameson are with Veterinary Specialty Care LLC. Send questions to petdocs@postandcourier.com.