Confirming the Diagnosis of Hyperthyroidism: The T3 Suppression Test

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In most cats with hyperthyroidism, the diagnosis is easily confirmed by measuring a single serum T4 concentration. Measuring serum free T4, T3, and TSH adds more diagnostic information that can be used to help confirm the diagnosis in cats with borderline hyperthyroidism.

Occasionally, cats suspected of having hyperthyroidism can be difficult to diagnose, even when repeated serum thyroid tests are run. Many of these cats have early or mild hyperthyroidism and show only mild clinical signs, whereas others appear to have more severe clinical features of hyperthyroidism but also have another obvious (or not so obvious) concurrent disease. The finding of concomitant disease (kidney, liver, or gastrointestinal disease) is relatively common in hyperthyroid cats, which is not surprising given the fact that many of these cats are middle-aged to older.

In the majority of these hyperthyroid cats in which a normal total T4 concentration is found, simply repeating the total T4 analysis with simultaneous measurement of free T4, as well as ruling out any concurrent disease, will easily confirm the diagnosis. Further diagnostic tests for hyperthyroidism are rarely required. However, if we still still suspect that a cat has mild hyperthyroid but serum concentrations of total and free T4 are either normal or equivocal, dynamic testing can be used to help make the diagnosis.

Dynamic Testing for Diagnosis of Hyperthyroidism

Two dynamic tests can be used in cats with mild or "occult" hyperthyroidism — both of these tests act on the hypothalamic-pituitary-thyroid axis (see Figure 1) to either suppress or stimulate pituitary TSH and thyroid hormone secretion.

Triiodothyronine (T3) suppression test
Thyrotropin-releasing hormone (TRH) stimulation test

Figure 1: Hypothalamic-Pituitary-Thyroid axis

Normally, TRH stimulates the secretion of TSH from the pituitary, which in turn, stimulates the thyroid to secrete T4 and T3. Both circulating T4 and T3 "feed-back" to the hypothalamus and pituitary to shut off the secretion of TRH and TSH respectively.

Triiodothyronine (T3) Suppression Test

The T3 suppression test operates on the principle that administration of relatively high doses of exogenous T3 will suppress (lower) the secretion of pituitary TSH in normal cats. Such inhibition of pituitary TSH secretion by high circulating concentrations of thyroid hormone is a characteristic feature of normal hypothalamic-pituitary-thyroid regulation (see Figure 1). The fall in circulating TSH levels results in lowered T4 secretion from the normal feline thyroid gland (see Figure 2, below, left panel).

Because resting serum TSH concentrations are already very low in normal cats (see my previous post on serum TSH measurements in cats), it would not be possible to use TSH measurement as an endpoint for interpretation of this test.

Figure 2: T3 suppression tests in normal cats (left) & cats with hyperthyroidism (right)

In contrast to the situation in normal cats, when thyroid function is autonomous — as it is in hyperthyroid cats, administration of large doses of exogenous T3 has little or no effect on T4 secretion. The reason for this that pituitary TSH secretion has already been completely suppressed by the higher-than-normal thyroid hormone secretion characteristic of hyperthyroidism (see Figure 2 above, right panel).

Testing Protocol for the T3 Suppression Test

To perform the T3 suppression test in cats, the following protocol is recommended:

One day 1, the veterinarian draws a blood sample is drawn for determination of baseline serum concentrations of total T4 and T3. This serum sample is not yet submitted to the laboratory but kept refrigerated (or frozen) until day 4.
Owners are instructed to give 7 doses of a T3 pill (liothyronine sodium; Cytomel) to their cat, beginning on the following morning.
On day 2 and 3, the owners administer the liothyronine at a dosage of 25 µg every 8 hours for 2 days (6 doses).
On the morning of day 4, a seventh 25-µg dose of liothyronine is given and the cat returned to the veterinary clinic within 2 to 4 hours.
The veterinarian again draws a blood sample for serum T4 and T3 determinations.
Both the basal (day 1) and post-liothyronine (day 4) serum samples are submitted to the laboratory together to eliminate the effect of between assay variation in hormone concentrations.

Interpretation of Results of T3 Suppression Testing

When the T3 suppression test is performed in normal cats and sick cats without hyperthyroidism, there is a marked fall in serum T4 concentrations after exogenous T3 administration (Figures 2 and 3). In contrast, when the test is performed in cats with hyperthyroidism, even in cats with only slightly high or high-normal resting serum T4 concentrations, minimal, if any, suppression of serum T4 concentrations is seen.

Figure 3: Box plots of the serum T4 concentrations before (A) and after (B) administration of liothyronine to 44 normal cats, 77 cats with hyperthyroidism, and 22 cats with nonthyroidal disease (from data in Reference no. 4). Data is plotted as box plots, in which the "box" represents the interquartile range from the 25th to 75th percentile (represents the middle one-half of the data). The horizontal bar through the box is the median. The "whiskers" represent the main body of data, which in most cases is equal to the range. Outlying data points are represented by open circles. To convert serum T4 concentrations from nmol/L to µg/dl, divide the given values by 12.87.

Regarding interpretation of T3 suppression test results, we find that the absolute serum T4 concentration after liothyronine administration is the best means of distinguishing hyperthyroid cats from normal cats or cats with nonthyroidal disease. Cats with hyperthyroidism have post-liothyronine serum T4 values greater than 20 nmol/L (greater than 1.5 μg/dl), whereas normal cats and cats with nonthyroidal disease have T4 values less than 20 nmol/L (Figure 3B). There may be a great deal of overlap of the per cent decrease in serum T4 concentrations after liothyronine administration between the three groups of cats, but suppression of 50 per cent or more only occurs in cats without hyperthyroidism.

Serum T3 concentrations, as part of the T3 suppression test, are not useful in the diagnosis of hyperthyroidism per se. However, these basal and post-liothyronine serum T3 determinations can be used to monitor owner compliance with giving the drug. If inadequate T4 suppression is found, but serum T3 values do not increase after treatment with liothyronine, problems with owner compliance should be suspected and the test result considered questionable.

Disadvantages of the T3 Suppression Test

Overall, the T3 suppression test is very useful for diagnosis of mild hyperthyroidism in cats, but the test does come with disadvantages:

T3 suppression testing is a relatively long test (3 days)
Owners are required to give multiple doses of liothyronine
Cats must swallow the tablets if the test is going to be meaningful.

If the liothyronine is not administered properly or the cat does not swallow the liothyronine tablet, circulating T3 concentrations will not rise to decrease pituitary TSH secretion, and the serum T4 value will not be suppressed, even if the pituitary-thyroid axis is normal. Failure of a cat to ingest the liothyronine could result in a false-positive diagnosis of hyperthyroidism in a normal cat or cat with nonthyroidal disease.

References:

Graves TK, Peterson ME: Occult hyperthyroidism in cats. In Kirk RW and Bonagura JD (eds): Current Veterinary Therapy XI, pp 334-337. Philadelphia, WB Saunders Co, 1992.
Peterson ME, Melián C, Nichols CE: Measurement of serum concentrations of total and free T4 in hyperthyroid cats and cats with nonthyroidal disease. Journal of Veterinary Internal Medicine 1998;12:211.
Utiger RD: Tests of thyroregulatory mechanisms. In Ingbar SH and Braverman LE (eds): The Thyroid: A Fundamental and Clinical Text, pp 511-523. Philadelphia, JB Lippincott, 1986.
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.
Refsal KR, Nachreiner RF, Stein BE, et al: Use of the triiodothyronine suppression test for diagnosis of hyperthyroidism in ill cats that have serum concentration of iodothyronines within normal range. J Am Vet Med Assoc 1991;199:1594-1601.
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.