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.
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
- Baral R, Peterson ME: Thyroid Diseases, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press.
- Graves TK, Peterson ME. Diagnostic tests for feline hyperthyroidism. The Veterinary Clinics of North America: Small Animal Practice 1994;24:567-576.
- 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.
- Peterson ME. Diagnostic tests for hyperthyroidism in cats. Clinical Techniques in Small Animal Practice 2006;21:2-9.
- 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
- Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, in press.
- 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)