Thyroid Function Tests in Dogs and Cats

The thyroid gland is found just below the larynx and consists of two lobes lying either side of the trachea (windpipe). This paired gland is responsible for the production of thyroid hormones, the major one of which is thyroxine (T4). A small amount of another thyroid hormone, tri-iodothyronine (T3) is also produced by the thyroid gland. The function of the thyroid gland is controlled by the pituitary gland in the brain through a hormone called TSH – thyroid stimulating hormone.

What do thyroid hormones do?

Thyroid hormones control the body's metabolic rate (rate of working) and have far reaching effects on the body.

Are thyroid dysfunctions common in cats and dogs?

Thyroid disorders are quite common in cats and dogs.

Cats

In the cat the most common type of thyroid gland disorder is hyperthyroidism. This is an excessive production of thyroid hormone (T4). It is usually due to overactive thyroid tissue in one or both lobes of the gland. Less than 2% of hyperthyroid cats have malignant tumours of the gland and so once the condition is diagnosed, surgical removal of the benign tumour often effects a spectacular cure.

Hypothyroidism, decreased production of thyroid hormone, is rare in cats but this is the most common thyroid problem in dogs.

Dogs

In dogs the most common thyroid problem involves insufficient circulating thyroid hormone, hypothyroidism. The majority of cases are due to two main causes

• Lymphocytic thyroiditis which is an inflammatory condition of the thyroid gland or
• Idiopathic thyroid gland atrophy which is a gradual shrinking and wasting of the thyroid gland due to an unknown cause.

These two conditions are responsible for approximately 95% of cases of canine hypothyroidism. The remaining 5% are very rare and causes include failure of the pituitary gland to stimulate the thyroid gland (secondary hypothyroidism), thyroid gland tumours and congenital birth defects in thyroid gland hormone production.

How are these conditions diagnosed in cats and dogs?

Because of the widespread effects of thyroid hormones on the body, any imbalance is often reflected in changes in blood and serum biochemistry tests (please see separate handouts). Changes in the initial screening tests often help to identify the presence of either hypo or hyperthyroidism and may sometimes uncover other conditions associated with the problem.

"Because of the widespread effects of thyroid hormones on the body, any imbalance is often reflected in changes in blood and serum biochemistry tests."

Hyperthyroidism will often be indicated by an increase in kidney related markers. i.e. blood urea nitrogen (BUN) and creatinine as well as mild increases in the liver enzymes.

Hypothyroidism usually results in an increase in cholesterol (or hypercholesterolemia). This is seen in over 75% of hypothyroid dogs. Urine analysis tests can also be used to support or refute biochemistry findings.

What specific tests are used to diagnose thyroid problems?

The thyroid gland produces T4 (thyroxine) and T3 (tri-iodothyronine). T4 is the main hormone produced by the thyroid, usually in response to stimulation by the pituitary by TSH. Often the total T4 is affected by the presence of other underlying illnesses and so a test measuring a more specific form of T4 called free T4 is performed. T3 has a somewhat lesser role. Although produced in the thyroid, T3 is also produced by breakdown of T4 hormone in the tissues. All that is necessary is to carry out a T4 test on a small blood sample. In hyperthyroid cats T4 concentration is usually very elevated whereas dogs with hypothyroidism will show a reduced level of T4 compared with the normal. If the results are borderline in cats or dogs, further tests may have to be carried out including assessment of T3 and TSH.

Are there any other tests that will be needed for a positive diagnosis?

The vast majority of thyroid dysfunctions can be diagnosed with a combination of clinical history, initial laboratory screening tests and the determination of circulating T4 levels. Occasionally other blood tests or imaging studies involving the thyroid gland may have to be undertaken.