Dog - Hypothyroidism

What is the thyroid gland, and what does it do?

The thyroid gland is one of the most important glands of the body. It regulates body chemistry (metabolism). It is controlled by the master gland of the body, the pituitary which is located in the brain. The thyroid gland is located in the neck near the windpipe (trachea). It is composed of two lobes, one on each side of the trachea. When the thyroid gland is under active metabolism slows down and the clinical signs of hypothyroidism occur.

What causes hypothyroidism?

Hypothyroidism is usually caused by one of two diseases:

1. Lymphocytic thyroiditis is the most common cause. This is considered to be an immune-mediated disease, i.e. the immune system decides that the thyroid represents foreign protein and attacks it. The reasons are presently unknown.
2. Idiopathic thyroid gland atrophy. This is another poorly understood problem. Normal thyroid tissue is gradually replaced by fat tissue and hypothyroidism results.

Together these two problems account for approximately 95% of cases. The other 5% are due to other problems including cancer of the thyroid gland.

What are the clinical signs?

As the circulating thyroid hormone reduces, the rate of metabolism slows down.

Virtually every organ in the body is then affected. Most affected dogs have one or more of the following signs.

• Weight gain without an increase in appetite.
• Lethargy and lack of desire to exercise.
• Cold intolerance (dog will appear to feel the cold easily).
• Dry haircoat with excessive shedding.
• Very thin haircoat to near baldness.
• Increased pigmentation in the skin.
• Increased susceptibility to skin and ear infections.
• Failure to re-grow hair after clipping or shaving.
• High blood cholesterol.

Other less common signs include:

• Thickening of the facial skin so-called "tragic facial expression syndrome".
• Abnormal function of nerves causing non-painful lameness, dragging of feet, lack of co-ordination, 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 cornea (clear part) of the eyes.
• Keratoconjunctivitis sicca or so-called ‘dry eye’ due to lack of proper tear production.

How is it diagnosed?

Initial diagnosis depends on a blood test to measure levels of circulating thyroid hormones, initially T4. If the T4 blood level is below normal and correct clinical signs are present, it is likely the dog has a hypothyroid problem. However, further tests usually have to be carried out since some dogs that are not hypothyroid may have subnormal levels of T4. This can happen when another disease is also present or when certain drugs are being given.

If hypothyroidism is suspected but the T4 is normal, other tests may have to be performed. These are more expensive so they are not used as first line tests.

Can hypothyroidism be treated?

Depending on the actual cause, hypothyroidism is usually treatable but not curable. Tablets containing thyroid replacement hormones usually have to be given for the rest of the dog's life.

How is the proper dose determined?

There is a standard dose that is used initially based on the dog's weight. However, after about one month of treatment, further testing is done to verify that the thyroid hormone levels are normal. In some dogs, the dose will need to be further adjusted every 6-12 months. Close liaison with your treating vet is necessary in order to ensure that the dog is neither overdosed nor underdosed.

What happens if the dog receives an overdose of the replacement thyroid therapy?

Signs of hyperthyroidism, a condition which is not uncommon in the cat but is rare in dogs can be caused. Signs include hyperactivity, lack of sleep, weight loss, and an increase in water consumption. If any of these occur, please let us know without delay.