Confirming the Diagnosis of Hyperthyroidism in Cats: Serum Free T4 (Part 2)

As I discussed in a recent blog post, determining a cat's serum free T4 concentration is a very useful test in confirming the diagnosis of hyperthyroidism, especially when run together with a total T4 concentration (1-6).

Physiology of T4, Total T4, and Free T4 in the Body

Again, what's the difference between total and free T4? When we measure a serum T4 level, 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 in the circulation is “bound,” meaning that it is attached to thyroid-binding proteins in the bloodstream. When bound, this T4-thyroid binding protein complex is too large to enter the body's tissues (Figure 1).

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 be converted to T3 to have its effect on the body’s metabolism (Figure 1).

Serum free T4 represents the tiny fraction (less than 0.1%) of thyroxine hormone that is unbound and therefore is biologically active. It is important to realize that a dynamic equilibrium exists between free and protein-bound T4 that depends on the amount of thyroid-binding proteins in the blood stream, as well as the affinity of the thyroid binding proteins for T4. In other words, a T4 molecule circulating in the bloodstream may be free (unbound) one minute, protein-bound the next, and free again shortly thereafter.

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 (Figure 1).

Figure 1: T4 circulates mostly bound to thyroid binding proteins in the circulation. This bound T4 exists in a dynamic equilibrium with unbound or free T4, which accounts for less than 1% of the total T4 in the circulation. Only the free, nonprotein-bound moiety is able to pass into the body's cells, where it can be converted into T3, the most active thyroid hormone

Measuring Serum T4, T3, and Free T4 in Cats with Mild Hyperthyroidism

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 concentrations (or binding affinity) of the thyroid binding proteins does 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. However, the tests biggest advantage appears to be a more sensitive test for diagnosis of early or mild hyperthyroidism.

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 (for more information, see Figure 1 in my last blog post). This ability to confirm hyperthyroidism in cats with the disease was higher than the total T4 concentration, which was diagnostic in 91% of cats or T3, which was diagnostic on only 67%.

However, when we looked at the cats with mild hyperthyroidism in that study (3), the advantage of free T4 over of total T4 or T3 measurements was much more striking (Figure 2).

In that subgroup of 205 cats with mild hyperthyroidism, we found that determination of serum total T4 was diagnostic in 61% of the cases (left panel, yellow boxed data). Use of serum T3 concentrations were of almost no value in cats with mild disease, since they were diagnostic in only 20% of the cases (middle panel, blue boxed data). However, despite the poor test sensitivities of T4 and T3 measurements, determination of free T4 was still diagnostic for hyperthyroidism in 91% of these cats with mild hyperthyroidism (right panel, purple boxed data).

Figure 2: Box plots of serum total T4, T3, and free T4 concentrations in 205 cats with mild hyperthyroidism, defined as a serum T4 less than 5 μg/dl (66 nmol/L). The T-bars represent the main body of data. The box represents the interquartile range (25th percentile to 75th percentile or middle half of the data). The horizontal bar in the box is the median or 50% percentile. Outlying data points are represented by open circles. The green shaded area represents the reference interval (normal range). From reference 3.

Disadvantages of Free T4 as a Diagnostic Test

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 (for more information, see this blog post).

In contrast, cats with nonthyroidal illness will never have high total T4 values. Rather, the will have corresponding total T4 values in the low-normal or subnormal range. 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.

However, in cats in which hyperthyroidism is suspected based on clinical features (e.g., weight loss despite a good appetite, palpable thyroid tumor), the finding of a high free T4 concentration can be considered diagnostic. This is especially true when the total T4 values are in the upper half of the reference range (see Figure 2: left panel, yellow boxed data) and all other diseases have been excluded. 

If the corresponding total T4 is in the lower half of the reference range, however, nonthyroidal disease is very likely. In our study (3), all 5 of the cats that had total T4 concentrations in the low-normal range had severe concurrent illnesses (see Figure 2: left panel, yellow boxed data).

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.

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