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

The thyroid gland is one of the most important glands in the body. It is located in the neck near the windpipe and is composed of two lobes, one on each side of the windpipe. This gland is controlled by the body's master gland, the pituitary gland, located at the base of the brain.

The thyroid gland regulates the rate of metabolism (body chemistry). If it works more than normal, metabolism speeds up. If it is less functional than normal, metabolism slows down. The latter is the basis for the clinical signs of hypothyroidism. Excess thyroid function is extremely rare, or does not occur, in dogs.

Why does it happen?

Hypothyroidism is almost always caused by one of two diseases:

the gland gets "invaded" by a certain type of white blood cells (lymphocytic thyroiditis), or

it shrinks and stops working, but the cause of this is unknown (idiopathic thyroid gland atrophy).

The first 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 not known why the immune system does this. The second disease is also poorly understood. Normal thyroid tissue is replaced by fat tissue in what is considered a "degenerative disease".

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

How does this disease manifest itself?

When the rate of metabolism slows down, virtually every organ in the body is affected in some manner. Most affected dogs have one or more of several "typical" physical and/or chemical abnormalities. These include:

a. Weight gain without an increase in appetite, or inability to lose weight despite dieting
b. Lethargy and lack of desire to exercise
c. Cold intolerance (gets cold easily)
d. Dry haircoat with excessive shedding
e. Very thin haircoat to near baldness
f. Increased "blackness" (pigmentation) in the skin
g. Failure to re-grow hair after clipping or shaving
h. High blood cholesterol, and fatty blood
i. High blood muscle enzyme levels
j. Anaemia (lack of red blood cells)
k. Seizures (rare)

Some dogs also have other abnormalities that are not the typical findings. These include:

a. Thickening of the skin around the face so they have a "tragic look on their face"
b. Abnormal function of nerves causing non-painful lameness, dragging of feet, lack of co-ordination, and a head tilt
c. Loss of libido and infertility in intact males
d. Lack of heat periods, infertility, and abortion in females
e. Fat deposits in the surface (corneas) of the eyes
f. So-called "dry eye" (Keratoconjunctivitis sicca) due to lack of proper tear production.

How is it diagnosed?

The most common test is for the T4 level (T4 is a hormone produced by the thyroid gland). This is a measurement of the main thyroid hormone in a blood sample. However, testing for the T4 level can be misleading because some dogs that are not hypothyroid may have levels of T4 lower than normal. This happens when another disease is present or when certain drugs are given. The test may need to be performed a number of times to diagnose the disease.

Can it be treated?

Hypothyroidism is treatable but can't be cured for good. It is treated with oral administration of a thyroid replacement hormone. This drug (hormone) must be given for the rest of the dog's life. The level of medication will initially need to be monitored by blood test to determine the correct dose.

What happens if the medication is overdosed?

Although overdosing is difficult, excess medication may produce signs identical to that of excessive thyroid gland function. These include hyperactivity, lack of sleep, weight loss, and an increase drinking. If any of these occur, please let us know immediately.