Underactive Thyroid (Hypothyroidism) in Dogs
Hypothyroidism is the most common hormonal disorder in dogs. It tends to occur in young to middle-aged large breed dogs such as the Doberman Pinscher, Golden Retriever and Red Setter. Signs are variable and often vague, and diagnosis of hypothyroidism in dogs can be difficult as thyroid levels are affected by many factors.
However, it is a very treatable disorder, and as with hypothyroidism in people, treatment usually results in a great improvement in the patient's quality of life. If your dog has an underactive thyroid, he will need lifelong treatment.
It is often tempting to blame a dog's weight gain or obesity to hypothyroidism, but it is important to avoid overdiagnosing the condition and administering excess levels of thyroid hormone to animals that do not genuinely require them.
Normal thyroid function
The thyroid gland is often described as being a butterfly-shaped organ, lying over your dog's airway in the neck. Iodine from the diet is absorbed into the bloodstream, then concentrated in the thyroid, where it is used to manufacture the hormones T3 and T4. These are then stored in the gland until needed.
The secretion of these thyroid hormones is regulated by the hypothalamus and pituitary glands. When circulating T3 and T4 levels drop the hypothalamus releases TRH (thyrotropin-releasing hormone), which in turn stimulates the release of TSH (thyroid stimulating hormone) from the pituitary gland. TSH acts on the thyroid gland to release T3 and T4 into the circulation.
Disease of any part of this hypothalamo-pituitary-thyroid axis can cause the symptoms of hypothyroidism.
90% of cases of hypothyroidism are caused by an autoimmune process, similar to that causing Addison's disease. The dog's immune system turns against his/her thyroid, destroying the follicular cells which produce thyroid hormone, a process known as autoimmune thyroiditis.
Around 10% of cases are caused by a failure of the thyroid to develop in the first place in young animals (referred to as cretinism). This congenital hypothyroidism is more common in Giant Schnauzers and Fox Terriers.
Signs of an Underactive Thyroid
As stated above, symptoms of this condition are variable; however, the most common are listed below:
- Lack of energy and reduced fitness
- Weight gain without an increase in food intake
- Reduced tolerance to cold temperatures
- Slow heart rate
- Symmetrical hair loss (especially on flanks- see photo)
- Skin changes- thickening and pigmentation of skin, cold and clammy to the touch, recurrent skin infections, blackheads and greasiness
- Female infertility
- Corneal lipidosis (pale deposit in the eyes)
- Less commonly nerve disorders- dragging feet, facial/laryngeal paralysis
It has been suggested that dogs with hypothyroidism may show signs of aggression prior to developing other symptoms. It is thought that this may be due to pain associated with the autoimmune attack on the gland. Any behavioural changes of this sort will be transient rather than permanent.
Diagnosis of Hypothyroidism
As well as being a very common endocrine (hormone) disease, hypothyroidism is also over-diagnosed in dogs. It is important that your pet shows symptoms of the disease as well as testing positive in the laboratory. Many dogs with normal thyroid function could be mistakenly put on thyroid supplements if the results of laboratory tests were taken in isolation. There are a number of thyroid function testing options available, the problem with all being that they give a large number of false positive results. This is a condition where the history you provide and your veterinarian's skill combine to make correct treatment decisions.
One of the reasons for the high number of false positives is the fact that thyroid function may be suppressed by many factors. For example, many commonly used drugs may reduce thyroid levels for up to two months. Likewise, stress and illness anywhere else in the body will depress your dog's thyroid function until these other conditions are corrected.
Diagnosis will require routine blood and urine tests as well as a thyroid function test. The specific thyroid function test usually chosen is the T4:TSH ratio, which essentially measures whether your dog's thyroid is producing as much hormone as his pituitary gland in the brain is telling him to. If the results of this test do not give a clear-cut answer then further tests may be necessary.
Treatment of Hypothyroidism
Treatment is straightforward and usually quite cheap using thyroid hormone supplements (eg L-thyroxine). Your dog's energy levels should improve dramatically within a couple of weeks of starting treatment, otherwise it is necessary to question whether the correct diagnosis has been made in the first place. Skin and coat changes as well as obesity can take much longer to resolve, up to several months. Your veterinary surgeon will recommend follow-up blood tests to ensure that the thyroid hormone dose your dog is being given is the correct one. The outlook for your dog is excellent with treatment.