Hypothyroidism is a condition in dogs which results from an underproduction of thyroid hormones from the thyroid gland. This lack of thyroid hormone in the bloodstream results in several changes in the body as thyroid hormones are responsible for maintaining a normal calorie-burning level, normal tissue repair levels and a healthy immune system.
Hypothyroidism is more common in medium to large-sized dogs. Some breeds are affected more commonly, including: Golden retrievers, Dobermans, Poodles, Boxers, Great Danes, Airedale terriers and Old English sheepdogs. It is more common in middle-aged dogs from four years and older.
The most common signs are increased tiredness, weight gain, patchy hair loss or excessive shedding, recurrent skin infections and dullness (often noted as a decrease in interactions with people and other dogs, particularly a reduction in play behaviours). Sometimes you might notice your dog appears colder than it used to, trying to seek out warmth more regularly. Occasionally, hypothyroidism can contribute to the development of aggression problems. These clinical signs often develop slowly over a number of months.
There are a number of causes of hypothyroidism but the most common is thought to be an immune mediated condition, where the body’s own immune system creates antibodies that attack the normal thyroid hormones. A small proportion of cases can be caused by a cancerous destruction of the thyroid gland.
The vet will examine your dog to look for signs that suggest hypothyroidism. It is a common finding to see bilateral symmetric baldness (alopecia) on the trunk of the body and the tail, rarely on the head. This hair loss is usually not itchy and the bare skin can feel thickened and be darker than other normal areas of skin. Skin infections are common in dogs with hypothyroidism due to the weakened immune system, and this infection can lead to red areas and spots, which are often itchy. Your dog may show signs of generalised weakness and a stiff, stilted gait, sometimes alongside the development of a puffy face. Occasionally, they may also show signs of in-coordination and imbalance. Your dog should be weighed to assess for any abnormal weight gain.
To confirm the diagnosis of hypothyroidism, your vet will need to perform some blood tests. This will often start with a test to measure the level of thyroid hormones. It may also be appropriate to perform a generalised blood test at the same time, to ensure there are no additional internal diseases that might have triggered the low levels of thyroid hormone and also to check there are no additional problems that might affect the successful treatment of hypothyroidism. There are times when the routine blood screens do not confirm a case which is highly suspicious of hypothyroidism. In these cases, we will recommend that we perform further tests, including a blood test after an injection, which should stimulate the thyroid gland to assess if it is able to react normally and produce thyroid hormones.
Dogs with hypothyroidism normally respond well to treatment with synthetic thyroid hormones given to them daily in the form of a tablet. The appropriate dosage varies between individual dogs, so it may be necessary for us to repeat the blood tests to assess if the correct dose has been found. We recommend that once this correct dose is found, your dog should have regular blood tests to monitor that the thyroid levels remain within normal limits; the damage to the thyroid gland can be ongoing and overtime, we may have to increase the amount of synthetic hormone your dog receives in order to keep them healthy. Treatment will be lifelong, but most symptoms resolve over a few weeks or months. Dogs with well-managed hypothyroidism have an excellent prognosis and life expectancy is normal.