Thyroid Hormone Testing in Cats

What is the thyroid gland?

The thyroid gland is located near the trachea (windpipe), just below the larynx (voice box). It is a paired gland that is responsible for the production of thyroid hormones. 

The major thyroid hormone produced by the thyroid gland is thyroxine (T4). A small amount of another thyroid hormone, triiodothyronine (T3), is also made by the thyroid gland. Thyroid hormones have far-reaching effects on the body, in essence governing the body's metabolic rate.

The function of the thyroid gland is controlled by the pituitary gland at the base of the brain, through a hormone called thyroid stimulating hormone (TSH).

What types of thyroid gland disorders occur in cats?

The most common thyroid gland disorder in cats is excessive production of thyroid hormones. This condition is called hyperthyroidism and is due to a benign (non-cancerous) increase in the size and function of one or both thyroid glands. Less than 2% of the hyperthyroid conditions in cats are due to malignant (cancerous) tumors of the thyroid glands. See handout "Hyperthyroidism in Cats" for more information.

Hypothyroidism (a decreased production of thyroid hormones) is uncommon in cats and is usually related to previous therapy for a hyperthyroid condition, or due to a congenital condition (originating from birth).

What initial screening tests are used to evaluate hyperthyroidism in cats?

Thyroid hormones have widespread effects on the body. They are involved in the metabolism of food as well as the daily functions of most of the body's tissues and organs. Therefore, the effects of increased thyroid hormone concentrations are often seen in initial blood and serum screening tests. Changes in the initial screening tests may suggest the presence of hyperthyroidism, and may uncover other conditions, some of which may be related to increased thyroid hormone production, while others may indicate other, non-related problems.

Complete Blood Count (CBC)

The white blood cells, red blood cells, and platelets (cellular components involved in the clotting process) are evaluated in this test. Hyperthyroidism does not usually cause significant changes in the blood cell profile, apart from possibly a mild increase in red blood cell numbers. Therefore, the presence of significant alterations in any of these blood parameters may alert your veterinarian to the existence of other disease conditions.

Serum Biochemistry Profile

Serum (the liquid portion of blood) contains enzymes, proteins, lipids (fats), glucose (sugar), electrolytes, and other metabolites. A serum biochemistry profile allows your veterinarian to measure these components and provides specific values for enzymes related to the liver, kidneys, and pancreas, as well as an evaluation of the electrolytes, such as sodium and potassium.

The major change, found in approximately 75% of hyperthyroid cats, is related to mild increases in the liver-related enzymes (alkaline phosphatase [ALP], alanine aminotransferase [ALT], and aspartate aminotransferase [AST]). Excess thyroid hormones may have a direct, toxic effect on the liver, or the increase may be related to increased blood flow through the liver resulting from the hyperthyroid state. These changes are typically mild and should resolve with treatment of the hyperthyroid condition. If a more significant increase in the liver enzymes is noted in the biochemistry profile, then such an increase suggests the possibility of other problems.

Many hyperthyroid cats have increases in the kidney-related parameters (blood urea nitrogen [BUN], SDMA, and creatinine). These three parameters indicate how well the kidneys are filtering out metabolic waste products from the body. Increases in these enzymes may be seen with kidney disease, or with dehydration or decreased blood flow to the kidneys.

It can be difficult to determine whether increases are related to primary kidney disease or related to the effects of the thyroid hormones on the kidneys. Both conditions may be present simultaneously in older cats. Similarly, it is impossible to accurately predict if these parameters will increase or decrease with treatment of hyperthyroidism. Therefore, it is very important to document whether these changes are present at the outset, and then to follow these parameters as your cat is being treated for hyperthyroidism.

Hyperthyroid cats may have a history of excessive drinking, eating, and urination. These signs can be seen with other conditions, especially diabetes. Therefore, it is important to evaluate your cat's blood sugar concentration to help rule out diabetes as a possible cause of these symptoms.

Urinalysis

Evaluation of urine is important in any sick animal. In the case of hyperthyroidism, a urinalysis may help your veterinarian to determine if changes in the kidney tests are related to dehydration or to kidney disease. A urine sample will also aid in the diagnosis of diabetes.

Are there any specific diagnostic tests for hyperthyroidism?

Most cases of hyperthyroidism can be diagnosed with a single blood test that measures the total thyroxine (T4) concentration. Because hyperthyroidism in cats is due to the excess production of thyroid hormones (primarily T4), the blood T4 concentration is usually noticeably elevated in cats with the disease.

Is the thyroxine concentration always increased in hyperthyroid cats?

No, the thyroxine concentration may not always be increased in hyperthyroid cats. However, their T4 values are usually at the upper end of the reference range. There are several potential reasons for this. 

• Thyroxine levels in the blood naturally fluctuate throughout the day, depending on the metabolic needs of the cat. If your cat has a mild or early case of hyperthyroidism, the thyroxine concentration may occasionally dip into the normal range.
• Drugs administered for other medical conditions may affect the concentration of thyroxine. 
• The presence of other illnesses may cause the thyroxine concentration to be reduced.

What if the thyroxine concentration is decreased below the reference range?

Since hypothyroidism (decreased thyroid function due to disease of the thyroid glands) is very rare in cats, such a decrease is usually due to the presence of an illness other than thyroid disease. Cats with such a result do not have hyperthyroidism.

What is free thyroxine concentration?

Free thyroxine concentration (fT4) is the portion of the thyroxine hormone that is not bound to any blood proteins. Determination of fT4 involves a single blood sample that is sent to a referral laboratory for a special test called free T4 by equilibrium dialysis (fT4ED).

"Free thyroxine concentration (fT4) is the portion of the thyroxine hormone 
that is not bound to any blood proteins."

The concentration of fT4 is less affected by factors such as coexisting illness or medication. However, a small proportion of normal cats are known to have fT4 concentrations above the reference range. Therefore, determination of fT4 is usually reserved for those cats with strong clinical signs of hyperthyroidism, whose initial screening tests supported hyperthyroidism, but whose total thyroxine concentrations were not high enough to be diagnostic.

Are there any other tests for the diagnosis of hyperthyroidism?

Most cases of hyperthyroidism are diagnosed by a combination of clinical history, initial laboratory screening tests, and determination of T4 or fT4 blood concentrations. On rare occasions, additional tests such as the T3 suppression test or advanced imaging studies of the thyroid gland (thyroid scintigraphy) may need to be performed. Imaging studies are usually done at a referral center. Cats undergoing treatment for hyperthyroidism should have their T4, as well as kidney and liver parameters, monitored regularly to determine if there is adequate control of the disease.