The thyroid gland is one of the most important glands in the body. It is located in the neck near the trachea (windpipe) and has two lobes, one on each side of the trachea. This gland is controlled by the pituitary gland, which is located at the base of
the brain.

The thyroid gland regulates the body's metabolic rate. If the thyroid is overactive (hyperthyroidism), the body's metabolism is elevated. If it is underactive (hypothyroidism), the metabolism slows down.

Causes

Hypothyroidism is usually caused by one of two diseases: lymphocytic thyroiditis or idiopathic thyroid gland atrophy. The former disease is the most common cause of hypothyroidism and is thought to be an immune-mediated disease. This means that the immune system decides that the thyroid is abnormal or foreign and attacks it. It is unclear why this occurs. In idiopathic thyroid gland atrophy the normal thyroid tissue is replaced by fat tissue. This condition is also poorly understood.

These two causes of hypothyroidism account for more than 95% of the cases in dogs. The other five percent are due to rare diseases, including cancer of the thyroid gland.

Clinical Signs

When the metabolic rate slows down, virtually every organ in the body is affected. Most dogs with hypothyroidism have one or more of the following signs: 

 Weight gain without an increase in appetite

 Lethargy and lack of desire to exercise

 Cold intolerance (get cold easily)

 Dry, dull hair with excessive shedding and flaking

 Very thin to nearly bald hair

 Increased dark pigmentation in the skin

 Increased susceptibility and occurrence of skin and ear infections

 Failure to re-grow hair after clipping or shaving

 High blood cholesterol

Some dogs also have other abnormalities such as:

 Thickening of the facial skin so they have a "tragic facial expression"

 Abnormal function of nerves causing non-painful lameness, dragging of feet, lack of coordination, and a head tilt

 Loss of libido and infertility in intact males

 Lack of heat periods, infertility, and abortion in females

 Fat deposits in the corneas of the eyes

 Keratoconjunctivitis sicca (KCS) or dry eye due to lack of proper tear production

Diagnosis

The most common screening test is a Total Thyroxin (TT4) level. This is a measurement of the main thyroid hormone in a blood sample. If it is low to below normal and clinical signs are present, this is suggestive of hypothyroidism. Definitive diagnosis is then made by performing a Free T4 by Equilibrium Dialysis (Free T4 by ED). If this test is low, then your dog has hypothyroidism. Some pets will have a low TT4 and normal Free T4 by ED. These dogs do not have hypothyroidism. Additional tests may be necessary based on your pet's condition.

Treatment

Hypothyroidism is treatable but not curable. It is treated with oral administration of thyroid replacement hormone. This drug must be given for the rest of the dog's life.

Dosage

There is a standard dose that is used initially based on the dog's weight. After one month of treatment, further testing is done to verify that the thyroid hormone levels are normal. The dose will need to be further adjusted and maintained by performing TT4 level tests every six months on your pet. Close communication with your vet is necessary in order to ensure that your dog is neither over nor under-dosed.

Overdosing

Signs of hyperthyroidism can result. These include hyperactivity, lack of sleep, weight loss, and an increase in water consumption. If any of these occur, please contact your vet immediately.

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