Concurrent measurement of fT4 and TSH is helpful, especially in cats with equivocal tT4; to date, however, only a canine TSH assay has been evaluated in cats, and most hyperthyroid cats have TSH levels below the assay detection threshold.15 Findings of suppressed TSH in combination with increased fT4 and tT4 in the upper one-third of the reference range are consistent with early hyperthyroidism. Of note, up to 2% of hyperthyroid cats may have normal TSH from incomplete suppression, and ≈30% of elderly euthyroid cats have undetectable TSH.8,15,21 Consequently, the canine TSH assay cannot distinguish low-normal TSH in euthyroid cats from truly low TSH in hyperthyroid cats. Like fT4, TSH test results should be interpreted in combination with other test results, and the TSH tests should not be the sole screening test used.
The T3 suppression test can be expensive and may have limited availability in some countries.2,7-9,15,16 To conduct this test, T3 is given orally q8h for 3 days, then both T3 and tT4 are measured. In a euthyroid cat, tT4 will decrease as a consequence of negative feedback from T3 administration, whereas a hyperthyroid cat’s tT4 will not.9,15,16 This test is generally reliable if T3 is administered properly, but practical difficulties can arise when owners attempt to give multiple oral doses over several days. Poor compliance with T3 dosing can lead to euthyroid cats with tT4 results similar to those of hyperthyroid cats and an incorrect diagnosis. Therefore, it is important to assess owner compliance by measuring T3 along with tT4.9,15,16