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Hypothyroidism is a common hormonal condition in dogs and occasionally occurs in cats.

What is canine hypothyroidism?

Hypothyroidism is the consequence of reduced production of thyroid hormones by the thyroid gland. Thyroid hormones have an extensive effect on the body, affecting the function of all organ systems, including controlling the metabolic rate.

This condition is more commonly seen in middle age to older dogs and it is more common in purebred dogs like the English Setter, retrievers and spaniels.

What are the clinical signs of canine hypothyroidism?

The majority of clinical signs are associated with the reduction of the metabolic rate. This will result in lethargy, mental dullness, exercise intolerance and weight gain without an increase in appetite. The skin and coat are also commonly affected, and these changes include dull and dry coat, excessive shedding, delayed regrowth of hair, hair thinning and hair loss usually on either side of the flank and in areas of friction, such as the tail. Less common consequences of the condition are neurological, cardiac and reproductive disturbances (in non-neutered patients).

How is hypothyroidism diagnosed?

The diagnosis of hypothyroidism can sometimes be very challenging. Diagnosis can be achieved with a combination of clinical signs and blood tests. Low thyroid hormone (T4) alone is not optimal as many patients with other diseases can have lower T4 without being hypothyroid. Additional tests such as measuring TSH, and hormone that stimulates the thyroid helps with the establishment of a hypothyroidism diagnosis. On rare occasions, these tests do not allow us to confirm a diagnosis and advanced imaging of the thyroid may be necessary.

How is hypothyroidism treated?

Treatment involves thyroid hormone (T4) supplementation either once or twice daily. Lifelong treatment is needed. As food influences the absorption of T4, it is recommended that the treatment is given on an empty stomach.

What are the adverse effects of the treatment?

Adverse effects of the treatment are rare. Dogs are very resistant to the development of signs associated with excess thyroid supplementation.

What is the treatment monitoring?

An improvement in energy levels is normally noted just days after starting the treatment. Changes in the coat, skin and body weight can take up to several weeks to a month to be seen. Repeat blood tests are important to monitor that an appropriate T4 dosage is being administered. It is not unusual for more than one dose adjustment to be made. Once the correct dose has been achieved, blood tests are usually done a couple of times a year.

What is the prognosis of this disease?

Adequately treated, this disease has an excellent prognosis.