Confirming the Diagnosis of Hyperthyroidism in Cats: Serum Free T4 Concentrations

Thyroxine (T4) is the main thyroid hormone secreted by the thyroid gland. As discussed in my recent post about T4 measurement, checking a cat’s serum T4 level is a very good screening test for hyperthyroidism (1-6). Over 90% of hyperthyroid cats will have a high serum T4 concentration, thereby confirming the diagnosis.

When we measure a serum T4 level, it is important to understand that we are checking the total amount of T4 hormone circulating in the blood—both the bound and unbound T4 molecules. More than 99% of T4 hormone is “bound,” meaning that it attaches to proteins in the blood. For circulating T4 to do its functions and regulate metabolism, the hormone must first break loose from its binding proteins (i.e., become “free” T4) in order to leave the bloodstream and enter the body’s tissues and cells. Only then can intracellular free T4 have its effect on the body’s metabolism.

Therefore, measuring a total T4 concentration by itself can sometimes be misleading, because the total T4 is affected by changes in the amount of circulating thyroid binding proteins or the binding affinity of these proteins for thyroid hormone. Such changes in the binding of T4 can occur in cats (and dogs) treated with certain drugs (e.g., glucocorticoids), as well as with many nonthyroidal illnesses (e.g., kidney disease, liver, disease, intestinal disease, or cancer).

In hyperthyroid cats, administration of drugs or other concurrent illness can lower the total T4 concentration. In some hyperthyroid cats, the high serum T4 concentration may be suppressed enough to actually fall into the normal range. In this scenario, use of a total T4 determination would fail to diagnose hyperthyroidism. In addition, some cats with mild or early hyperthyroidism will also have normal serum T4 concentrations and can be difficult to diagnose.

Physiology of Free T4 in the Body

Serum free T4 represents the tiny fraction (less than 0.1%) of thyroxine hormone that is unbound and therefore is biologically active. Again, it is only the free T4 portion of the total T4 measured in the blood that can pass into the cells and act on the body’s tissues to influence metabolism.

When we measure a serum free T4 concentration, we are checking only the free or unbound portion of the T4 hormone circulating in the blood. Since changes in the thyroid binding proteins in the blood do not affect the free T4 levels, this test is considered a more accurate test of true thyroid activity than determination of serum total T4. Free T4 is much less likely to be influenced by nonthyroidal illness or drugs. And finally, it appears to be a more sensitive test for diagnosis of early or mild hyperthyroidism.

Measuring the Serum Thyroxine (T4) Concentration: Advantages as a Diagnostic Test  

As might be expected, the free T4 test is a more sensitive diagnostic test for feline hyperthyroidism than is determination of the total T4 concentration.

In our study of 917 hyperthyroid cats (3), we found that determination of serum free T4 was diagnostic in 98.5% of the cases (see Figure below; middle blue boxed data). This ability to confirm hyperthyroidism in cats with the disease was higher than the total T4 concentration, which was diagnostic in 91% of cats (see Figure 3 in my previous blog on T4 measurement).

The free T4 was high in most cats with mild or early hyperthyroidism, even when the total T4 was within the established normal range. In addition, in hyperthyroid cats with concurrent diseases or in those treated with drugs, the free T4 is not suppressed and remains high even when the total T4 has been suppressed. Therefore, use of the free T4 is a very useful diagnostic test, especially in hyperthyroid cats in which total T4 values are within reference range limits.

Figure 1: Box plots of Free T4 concentrations in 172 clinically normal cats, 917 cats with hyperthyroidism, and 221 cats with nonthyroidal disease. The T-bars represent the main body of data. The box represents the interquartile range (25th percentile to 75th percentile range or the middle half of the data). The horizontal bar in the box is the median. Outlying data points are represented by open circles. The shaded area indicates the reference interval (normal range). From reference 3.

Measuring the Serum Thyroxine (T4) Concentration: Disadvantages as a Diagnostic Test  

So if determination of the free T4 level is more diagnostic, why bother with running the total T4 at all? Why not measure just measure free T4 concentrations as the main diagnostic test for hyperthyroidism?

It turns out that there are 2 good reasons why free T4 concentrations cannot replace total T4 as the main diagnostic test for hyperthyroid cats.

First of all, although the free T4 is a very sensitive diagnostic, the main problem with free T4 assays is that the test is less specific than the total T4 value. In other words, many cats suffering from other illnesses NOT associated with hyperthyroidism can have false-positive results with the free T4 test (see Figure above; right purple boxed data).

Of the 221 cats with nonthyroidal disease in my study, 14 cats (6.3%) had a falsely high free T4 concentration (see Figure above; right purple boxed data; Notice all of the outlying data points above the normal range). Therefore, calculated specificity of measuring serum free T4 concentration as a diagnostic test for hyperthyroidism was significantly lower than that of the total T4 concentration.

Remember that these cats with nonthyroidal illness will NEVER have high total T4 values — rather, as expected, the have corresponding total T4 values in the low-normal or subnormal range (see Figure 3 in my previous blog on T4 measurement).

For this reason, the total T4 test remains the diagnostic test of choice for cats with suspected hyperthyroidism because we almost never see false-positive results with the total T4 test. Measurement of free T4 alone can never be used to make a reliable diagnosis because of the chance that the result is not accurate.

The second reason why free T4 concentrations should not be used as the initial screening test for hyperthyroidism is that the free T4 test is generally about 2 to 3 times as expensive as a T4 alone.

While most endocrinologists favor the standard equilibrium dialysis radioimmunoassay method for measuring free T4, newer analog and chemiluminescence assays now offer alternative and accurate methodologies. These new assays are also faster to run and are less costly.

No matter how free T4 is determined, however, the free T4 test must be run with a total T4, adding to the expense of diagnostic testing for hyperthyroid cats.

Bottom Line:  Caution is advised in using serum free T4 measurements as the sole diagnostic test for hyperthyroidism. This test is associated with a high rate of false-positive results in sick cats without hyperthyroidism. This test should never be run by itself but it is more reliable if interpreted with a corresponding total T4 value:

• High-normal total and free T4 concentrations are generally consistent with hyperthyroidism (especially is clinical features of disease present— i.e., thyroid nodule, weight loss despite good appetite)
• Low to low-normal total T4 concentrations together with a high free T4 are usually associated with non-thyroidal illness

References:

1. Baral R, Peterson ME: Thyroid Diseases, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press.
2. Graves TK, Peterson ME. Diagnostic tests for feline hyperthyroidism. The Veterinary Clinics of North America: Small Animal Practice 1994;24:567-576.
3. Peterson ME, Melian C, Nichols R. Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. Journal of the American Veterinary Medical Association 2001;218:529-536.
4. Peterson ME. Diagnostic tests for hyperthyroidism in cats. Clinical Techniques in Small Animal Practice 2006;21:2-9.
5. Peterson ME: Diagnostic testing for feline hyper- and hypothyroidism. Proceedings of the 2011 American College of Veterinary Internal Medicine (ACVIM) Forum. pp. 95-97, 2011
6. Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, in press.
7. Peterson ME, Broome MR, Robertson J: Accuracy of serum free thyroxine concentrations determined by a new veterinary chemiluminescent immunoassay in euthyroid and hyperthyroid cats. Proceedings of 2011 European College of Veterinary Internal Medicine (in press)

Confirming the Diagnosis of Hyperthyroidism in Cats: Serum T3 Concentrations

As I discussed in my last post, the feline thyroid gland makes two active thyroid hormones, thyroxine (T4) and triiodothyronine (T3). T4 makes up nearly 90% of the circulating thyroid hormones, while T3 makes up less than 10%.

Although only 10% of the hormone produced in the thyroid gland is T3, this thyroid hormone is three to ten times more active than T4. In order for the body’s cells and tissues to use more of this active form of thyroid hormone, T4 (which contains four iodine molecules) is converted to T3 or (which contains three iodine molecules) by losing an iodine molecule. In cats, this function is performed in the liver and kidney by deiodinase enzymes that act to remove an iodine group from the thyroid hormone molecule (1).

The deiodination of T4 to T3, reverse T3, and other iodothyronines is an integral component of thyroid hormone metabolism. This deiodination, depending on whether it occurs at the outer or inner rings of the T4 molecule, serves to either activate or inactivate this hormone (see Figure above). Again, T3 is the most active thyroid hormone, whereas reverse T3 (rT3) is considered to be metabolically inactive.

There are 3 different thyroid deiodioinase enzymes:

• Iodothyronine deiodinase type I (IDI) is commonly found in the liver and kidney. It is also present in the thyroid of many species including dogs and humans, but IDI is not present in the feline thyroid gland (1).
• The type II enzyme (IDII) in skeletal muscle and thyroid gland (but again, cats do not appear to have IDII in their thyroid gland).
• Type III (IDIII) is found in the fetal tissue and brain matter.

IDII can only deiodinate the outer ring of T4 or rT3. IDIII can only deiodinate the inner ring of T4 or T3. IDI can deiodinate both the outer and inner rings.

But the real question is this: If T3 is three to ten times more active than T4, shouldn’t we be measuring serum T3 to better diagnose hyperthyroidism in cats?

Measuring the Serum Triiodothyronine (T3) Concentration

Although T3 is the important thyroid hormone for regulation of a cat's metabolic state, this hormone reflects tissue thyroid activity. Unfortunately, measuring the serum levels of T3 are a poor reflection of what is happening in the tissues.

Compared to the serum T4,  measuring the serum T3 concentration is much less useful as a diagnostic test for hyperthyroidism (2-7). Over 30% of hyperthyroid cats have normal circulating T3 concentrations (See Figure, below).

Box plots of total T3 concentrations in 172 clinically normal cats, 917 cats with hyperthyroidism, and 221 cats with nonthyroidal disease. The T-bars represent the main body of data. The box represents the interquartile range (25th percentile to 75th percentile range or the middle half of the data). The horizontal bar in the box is the median. Outlying data points are represented by open circles. The shaded area indicates the reference interval (normal range). From reference 3.

None of the cats with high serum T3 values have normal serum T4 concentrations, so measuring T3 test does not provide any additional information not gained from measuring T4 alone.

Bottom line: Measuring total T3 alone is not recommended for investigation of hyperthyroidism in cats.

The reason for this is unknown, but based on studies of normal cats (1), it is likely that most of the circulating T3 in these hyperthyroid cats comes from conversion from T4 in peripheral tissues, and is not secreted directly by the thyroid tumor.  That could explain why measurement of T4 is a better diagnostic test for hyperthyroidism than is T3.

In my next posts, we will discuss the use of free T4, then look at the use of serum TSH measurements. I’d end this section on diagnostic testing with a post on the value of thyroid scintigraphy (thyroid nuclear imaging or scanning) in the diagnosis of hyperthyroidism.

References:

1. Foster DJ, Thoday KL, Beckett GJ. Thyroid hormone deiodination in the domestic cat. Journal of Molecular Endocrinology 2000;24:119-126.
2. Baral R, Peterson ME: Thyroid Diseases, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press.
3. Graves TK, Peterson ME. Diagnostic tests for feline hyperthyroidism. The Veterinary Clinics of North America: Small Animal Practice 1994;24:567-576.
4. Peterson ME, Melian C, Nichols R. Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. Journal of the American Veterinary Medical Association 2001;218:529-536.
5. Peterson ME. Diagnostic tests for hyperthyroidism in cats. Clinical Techniques in Small Animal Practice 2006;21:2-9.
6. Peterson ME: Diagnostic testing for feline hyper- and hypothyroidism. Proceedings of the 2011 American College of Veterinary Internal Medicine (ACVIM) Forum. pp. 95-97, 2011
7. Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, in press.

Confirming the Diagnosis of Hyperthyroidism in Cats: Serum T4 Concentrations

Figure 1: Palpating a cat's neck
for a thyroid tumor

As I've discussed in my last few blood posts, we base our preliminary or initial diagnosis of hyperthyroidism on a combination of the three factors or findings:

• Your cat's clinical features (e.g., weight loss despite a good appetite).
• Your cat's physical examination findings (e.g., rapid heart rate, enlarged thyroid tumor).
• The findings on routine laboratory testing (i.e., complete blood count, chemistry panel, and urinalysis).

If, after reviewing the cat's history and exam finding, we still believe that your cat has hyperthyroidism, the next step in the workup is to do specific tests to confirm the diagnosis of hyperthyroidism. This allows the veterinarian to properly treat your cat for the correct problem.

We have three different groups of tests that can be used to confirm as diagnosis of hyperthyroidism:

• Basal or single blood tests, in which we measure one or more of the thyroid hormones themselves (e.g., T4 or T3), or the pituitary hormone called thyroid-stimulating hormone (TSH). As its name implied, TSH plays an important role in stimulating the thyroid gland to secrete the thyroid hormones.
• Dynamic testing, in which 2 or more blood samples are collected before and hours-to-days after administration of a thyroid-stimulating or suppressive agent. These tests check the integrity of the pituitary-thyroid axis, which is malfunctioning in cats with hyperthyroidism.
• Imaging techniques such as thyroid scintigraphy provide a visual display of the structure and function of the thyroid gland. This is considered by most to be the “gold standard” diagnostic test since thyroid scanning will allow the diagnosis of hyperthyroidism before laboratory tests are abnormal. Thyroid scanning can also exclude a misdiagnosis of hyperthyroidism in cats with false thyroid hormone elevations.

In this first blog post in this series of “Confirming the Diagnosis of Hyperthyroidism in Cats,” I'm going to discuss the use of serum thyroxine (T4)  concentrations to make a definitive diagnosis of hyperthyroidism in cats.

In my next posts, we will discuss the use of free T4, then look at the use of serum TSH measurements. I will then turn to when we do dynamic thyroid function testing. Finally, I’d end this section on diagnostic testing with a post on the value of thyroid scintigraphy (thyroid nuclear imaging or scanning) in the diagnosis of hyperthyroidism.

How the Thyroid Works —Production of Its Hormones

Figure 2: T4 with its
 four iodine groups

A unique role of the thyroid gland is to take iodine supplied in food and use it to produce the 2 major thyroid hormones, thyroxine (tetraiodothyronine or T4) and triiodothyronine (T3). Important components of both the T4 and T3 molecules include the amino acid tyrosine and iodine (see Figure 2).

Inside the thyroid gland, T4 and T4 are bound to a large storage protein called thyroglobulin. When the body requires thyroid hormone, the thyroid gland releases stored T4 and T3 into circulation. T4 makes up nearly 90% of the circulating thyroid hormones, while T3 makes up less than 10%.

Because thyroid hormone synthesis requires iodine, either a deficiency or an excess of iodine can profoundly affect thyroid function, and promote autoimmune thyroiditis.

Measuring the Serum Thyroxine (T4) Concentration

The test to measures the total amount of T4 in your cat’s blood should always be the first diagnostic test used if hyperthyroidism is suspected.

The finding of a high serum T4 concentration is the hallmark of hyperthyroidism. As a diagnostic test for hyperthyroidism, the total T4 is a highly specific test. Falsely high T4 values (false-positive test results) generally are not seen unless overt laboratory errors in measurement are responsible, and that is extremely rare. Methods for T4 measurement are readily accessible, relatively cheap, and only require a small blood sample.

Figure 3: Box plots of total T4 concentrations in normal cats, hyperthyroid cats, and cats with nonthyroidal disease. The T-bars represent the main body of data. The box represents the interquartile range (25th percentile to 75th percentile range or the middle half of the data). The horizontal bar in the box is the median. Outlying data points are represented by open circles. The shaded area indicates the reference interval (normal range). From reference 9.

Serum total T4 is a great screening test for hyperthyroidism. The total T4 assay has a test sensitivity of over 90%; in other words, 90% of all hyperthyroid cats can be diagnosed by measuring a T4 concentration (Figure 3, above).

However, this means that approximately 10% of cats with hyperthyroid will have serum total T4 concentration that are not high but remain within the normal, reference range limits. When “normal” T4 values are found, they are usually within the high-normal range.

Thus, a high total T4 value is diagnostic for hyperthyroidism in cats, especially if you suspect hyperthyroidism based upon the cat’s clinical features and physical exam findings. However, if hyperthyroidism is suspected but the serum T4 test is normal, it is still possible you’re your cat has the disease.

What how can the serum T4 be normal if a cat is hyperthyroid?

There are 2 main reasons or situations where this can occur:

1. In early or mildly affected hyperthyroid cats, serum total T4 concentrations can fluctuate in and out of the reference range. Such fluctuation occurs in all hyperthyroid cats, but the degree of fluctuation is of little diagnostic significance in cats with moderate to advanced hyperthyroidism and markedly high T4 concentrations.
2. Other illness or disease (not related to hyperthyroidism) is capable of suppressing serum total T4 from high concentrations to within the mid- to high-end of the reference range. Concurrent hyperthyroidism should always be suspected in severely ill cats with mid to high reference range serum total T4 concentrations.

What do we do if the serum T4 is normal and hyperthyroidism is still suspected?

In most cats with early or mildly hyperthyroidism, serum total T4 concentrations will eventually increase into the diagnostic hyperthyroid range with time. So one approach to cats like this is to simply wait and retest a few weeks later.

Similarly, after a hyperthyroid cat recovers from the nonthyroidal illness, the serum T4 values will again increase in the high range, confirming the diagnosis. So concurrent disease should always be excluded in any cat suspected of having hyperthyroidism when the serum T4 concentration is normal.

With time, almost all hyperthyroid cats will eventually develop high T4 concentrations.

Other diagnostic tests may also be used provide a means of diagnosing hyperthyroidism in cats that have a normal T4 test. These options include measurement of free T4, TSH, dynamic thyroid function testing, or thyroid scintigraphy (all discussed in my next upcoming blog posts).

References:

1. Baral R, Peterson ME: Thyroid Diseases, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press.
2. Broome MR. Thyroid scintigraphy in hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:10-16.
3. Graves TK, Peterson ME. Diagnostic tests for feline hyperthyroidism. The Veterinary Clinics of North America: Small Animal Practice 1994;24:567-576.
4. Kintzer PP, Peterson ME. Nuclear medicine of the thyroid gland. Scintigraphy and radioiodine therapy. The Veterinary Clinics of North America: Small Animal Practice 1994;24:587-605.
5. Peterson ME, Graves TK, Cavanagh I. Serum thyroid hormone concentrations fluctuate in cats with hyperthyroidism. Journal of Veterinary Internal Medicine 1987;1:142-146.
6. Peterson ME, Graves TK, Gamble DA. Triiodothyronine (T3) suppression test. An aid in the diagnosis of mild hyperthyroidism in cats. Journal of Veterinary Internal Medicine 1990;4:233-238.
7. Peterson ME, Gamble DA. Effect of nonthyroidal illness on serum thyroxine concentrations in cats: 494 cases (1988). Journal of the American Veterinary Medical Association 1990;197:1203-1208.
8. Peterson ME, Broussard JD, Gamble DA. Use of the thyrotropin releasing hormone stimulation test to diagnose mild hyperthyroidism in cats. Journal of Veterinary Internal Medicine 1994;8:279-286.
9. Peterson ME, Melian C, Nichols R. Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. Journal of the American Veterinary Medical Association 2001;218:529-536.
10. Peterson ME. Diagnostic tests for hyperthyroidism in cats. Clinical Techniques in Small Animal Practice 2006;21:2-9.
11. Peterson ME: Diagnostic testing for feline hyper- and hypothyroidism. Proceedings of the 2011 American College of Veterinary Internal Medicine (ACVIM) Forum. pp. 95-97, 2011
12. Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, in press.

The Animal Endocrine Clinic Announces Its New Monthly Newsletter

We've just emailed the first of our monthly Animal Endocrine Clinic (AEC) newsletters. A snapshot of the July 2011 AEC Newsletter appears above, and the full Newsletter can be read online here.

If you'd like to sign up for the AEC Newsletters, please click through to the sign-up page.

For more information about AEC visit www.animalendocrine.com.

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Check Out the Animal Endocrine Clinic's New Website

PRESS RELEASE: JUNE 30, 2011

The Animal Endocrine Clinic Announces the Launch of its Redesigned Website

Dr. Mark Peterson and the Animal Endocrine Clinic launch the most comprehensive, up-to-date website on the study of endocrinology in dogs and cats

The Animal Endocrine Clinic (AEC) has announced the launch of its totally redesigned website. The website was developed as a resource for pet owners and veterinarians to learn more about Dr. Peterson, his specialty veterinary clinic, and about endocrine disorders that commonly affect dogs and cats.

Featuring a new logo, the website combines AEC’s three divisions: Hypurrcat, the Endocrine Clinic, and Nuclear Imaging for Animals. The website also reflects the sentiments of dog and cat owners and veterinarians seeking more information about the diagnosis and treatment of endocrinology disorders.

Dr. Mark E. Peterson, owner and founder of the Animal Endocrine Clinic says, “As our practice has grown, and we have received calls and emails from both pet owners and veterinarians, we realized the need for an online resource for those who want to understand more about these common pet afflictions. Our new website does just that.”

Highlights of the Redesign Include:

Endocrine Resources — A comprehensive look at the study of endocrinology in dogs and cats including an overview of both animals’ endocrine systems, specific endocrine diseases that affect each animal, symptoms to look for, and the most comprehensive online glossary of terms available.

Also included are descriptions of the practice’s divisions and the cutting edge technology that AEC uses to diagnose and treat these disorders.

Publications — A world-renowned specialist in feline and canine endocrinology, Dr. Mark E. Peterson was the first veterinarian to document hyperthyroidism in cats as well as the first to treat hyperthyroid cats with radioiodine. He has also published extensively about diabetes mellitus and adrenal disease in dogs and cats. A list of his published writings have been archived on the website.

Events — Dr. Peterson continues to be very involved in organized veterinary medicine and veterinary continuing education events. A schedule of his current and past speaking engagements is maintained on the new site.

Integrated Social Media — The new website includes videos, testimonials and allows users to access all of the AEC social media directly from the AEC website.

“The new design provides a cleaner, user experience while integrating our work at AEC with the expanding social media market,” said Dr. Peterson. “We now have the opportunity to deepen existing relationships by further engaging our audience, while also providing access to timely and quality information which has become more important to pet owners and veterinary colleagues.”

About The Animal Endocrine Clinic

The Animal Endocrine Clinic (AEC) is a specialized veterinary practice that diagnoses and treats cats and dogs with endocrine (hormonal) disorders.

AEC has three divisions: Hypurrcat exclusively treats hyperthyroid cats with radioactive iodine; the Endocrine Clinic is dedicated to diagnosing and treating dogs and cats with endocrine disorders, such as diabetes or Cushing's disease; and Nuclear Imaging for Animals is a state-of-the-art medical imaging facility that performs nuclear scanning to diagnose thyroid, bone, liver, and kidney diseases in dogs and cats.

AEC has clinics in Manhattan and Westchester County and treats clients in New York City, Long Island, Westchester County, New Jersey and Connecticut. It is the only practice of its kind in the United States.

For more information about AEC visit www.animalendocrine.com.

Become a fan of AEC on Facebook (www.facebook.com/animalendocrine)
Follow AEC’s veterinary blog (http://endocrinevet.blogspot.com/)
Follow AEC on Twitter (http://twitter.com/#!/animalendocrine)
Follow AEC on YouTube: (http://www.youtube.com/animalendocrine)