Cat & Dog - Thyroid Function Tests

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 the thyroid hormones, the major two of which is thyroxine T4, and tri-iodothyronine (T3) which is produced in smaller quantities.  The thyroid gland is controlled by the pituitary gland in the brain via 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 problems common in cats and dogs?

Thyroid disorders are not uncommon in cats and dogs.

Cats

In the cat the most common type of thyroid gland disorder is hyperthyroidism.  This is the excessive production of thyroid hormone (T4).  It is usually due to a benign tumour affecting one or both lobes of the gland.  Less than 2% of hyperthyroid cats have malignant tumours of the gland, so that once the condition is diagnosed, surgical removal of the benign tumour often results in a complete cure. Other treatment options include radioactive iodine therapy, which targets and destroys only the tumour cells in the thyroid, or daily medication to reduce the levels of T4 in the blood

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

Dogs

In dogs the most common thyroid problem is 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
• Idiopathic thyroid gland atrophy which is a gradual shrinking and wasting of the thyroid gland due to unknown cause.

These two conditions are responsible for approximately 95% of cases of canine hypothyroidism.  The remaining 5% are very rare.  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.

Treatment involves medication with a synthetic thyroid hormone replacement, in the form of tablets.

How are these conditions diagnosed in cats and dogs?

Due to the widespread effects of thyroid hormones on the body any imbalance is often reflected in changes in the results of blood and serum biochemistry tests.  Please see handouts - Routine blood tests and Routine laboratory tests for cats and dogs 1 & Routine laboratory tests for cats and dogs 2.   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.

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

Hypothyroidism usually results in an increase in cholesterol (hypercholesterolemia).  This is seen in over 75% of hypothyroid dogs.  Urine analysis tests are also used to support or refute the 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 via TSH.  T3 has a somewhat lesser role.  Although produced in the thyroid it is also produced by breakdown of T4 hormone in the tissues. All that is necessary initially 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 the assessment of T3 and TSH levels.

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 may have to be undertaken.

Please ask us if you wish to discuss this further.