Hypothyroidism in Dogs

What is hypothyroidism?

Hypothyroidism arises when the thyroid gland fails to produce enough thyroid hormone.

The thyroid gland is located in the neck, although extra thyroid tissue can sometimes be found within the chest. The thyroid gland secretes thyroid hormone which has a wide range of important effects on the body. Thyroid hormone helps the body to keep its functions working at the correct pace. The main cause of hypothyroidism is destruction of the thyroid gland, although the underlying cause of this is normally unknown.

Are there any particular breeds that are predisposed to hypothyroidism?

Any breed can develop hypothyroidism; however, certain breeds such as the Golden Retriever and Doberman pinscher have been reported to be at higher risk. The average age at diagnosis is seven years but dogs as young as six months have been reported to have the disease. Hypothyroidism is rare in small breed dogs.

What are the typical presenting signs in dogs with hypothyroidism?

hypothyroidism in dogs

An example of hair loss in a hypothyroid dog

Clinical signs of hypothyroidism are usually non-specific as thyroid hormones influence the function of many organs in the body. Weight gain occurs in approximately 50% of hypothyroid dogs; tiredness, cold intolerance and unwillingness to exercise can also be seen. These signs are often very gradual in onset.

Skin changes are seen in 60-80% of hypothyroid dogs and this is often the most common reason for presentation to a vet. Hair loss frequently begins in areas of friction including the tail and neck, and can progress to result in symmetrical hair loss over both sides of the body.

The hair coat is often dry and the skin is sometimes scaly. Secondary infections can lead to the animal becoming itchy. Very occasionally dogs with hypothyroidism can present comatose.

How is the diagnosis of hypothyroidism made?

Your vet will need to take blood samples and run some blood tests if they suspect that your dog has hypothyroidism

Your vet will need to take blood samples and run some blood tests if they suspect that your dog has hypothyroidism

Routine blood work can reveal changes that may be suggestive of hypothyroidism; however, routine blood testing does not provide a definitive diagnosis. Measurement of thyroid hormone (T4) and the hormone that causes thyroid hormone production (TSH) is necessary to make a definite diagnosis of hypothyroidism. It is important that both T4 and TSH are measured for a diagnosis of hypothyroidism to be made. The diagnosis should not be made on the basis of thyroid hormone (T4) measurement alone. Unfortunately thyroid blood test results can be affected by other diseases and other drugs that the animal may be receiving and therefore the diagnosis is not always completely straightforward. Certain breeds of dog such as Greyhounds and Whippets have different normal blood thyroid levels when compared to other dogs.

How is hypothyroidism treated?

Dogs with hypothyroidism require lifelong medication to replace their thyroid hormones

Dogs with hypothyroidism require lifelong medication to replace their thyroid hormones

Hypothyroidism is managed with thyroid hormone replacement. Lifelong therapy is necessary and animals are treated either once or twice daily. Both an oral liquid and tablet are available for the treatment of hypothyroidism. The dose of medication is adjusted based upon the response to treatment and blood testing. Blood tests are normally taken 4-6 hours after tablet administration and are also occasionally taken just before the medication is due.

Following treatment, a dog’s energy level will often improve within 1-2 weeks. However, skin changes may take months to resolve and the coat may initially appear worse as hairs are shed.

Comatose dogs require hospitalisation and intensive management.

An absent or inadequate response to treatment may be the result of an inadequate drug dose, poor absorption of the drug from the bowel or occasionally an incorrect diagnosis.

Can cats get hypothyroidism?

Hypothyroidism is rare in cats. Most cats with low thyroid hormone levels have another disease that is causing reduced thyroid hormone levels. Reported signs are similar to those described in dogs and the diagnosis is based on thyroid hormone testing. Treatment and monitoring is the same as in dogs.