Summary Administration of triiodothyronine (liothyronine, 15 μg, q 8 h, for 6 treatments) caused marked decrease in serum concentration of thyroxine (T 4 ) and estimates of free T 4 (fT 4 ) concentration in clinically normal cats. A prospective clinical study was done to evaluate the use of this suppression test for diagnosis of hyperthyroidism in cats with clinical signs suggestive of the disease, but lacking high serum concentration of iodothyronines. Twenty-three cats were confirmed as hyperthyroid on the basis of histologic changes in the thyroid gland or clinical improvement in response to administration of methimazole. Mean ± sd serum concentration of T 4 (34.3 ± 12.7 to 31.3 ± 11.5 nmol/L) and estimate of fT 4 concentration (26.6 ± 6.4 to 25.6 ± 6.9 pmol/L) did not change after administration of liothyronine to these cats. Twenty-three cats were classified as nonhyperthyroid by histologic confirmation of other disease, abnormal results of other diagnostic tests that strongly supported primary disease other than hyperthyroidism, or spontaneous remission of weight loss without treatment. Mean ± sd serum concentration of T 4 (27.9 ± 10.3 to 11.7 ± 6.4 nmol/L) and estimate of fT 4 concentration (21.7 ± 5.4 to 10.4 ± 4.4 pmol/L) decreased significantly ( P < 0.001) in response to administration of liothyronine. Discriminant analysis was used to identify variables from iodothyronine assays (eg, absolute concentration of T 4 or absolute estimate of fT 4 concentration, or changes of T 4 or fT 4 concentration) that provided the best diagnostic sensitivity and specificity. The endocrine end points that best differentiated hyperthyroid vs nonhyperthyroid cats were the concentration of T 4 or estimate of fT 4 concentration in serum obtained after liothyronine administration and predictive values ues calculated from postliothyronine serum concentration of T 4 or fT 4 and percentage decrease of T 4 concentration. Difference in diagnostic sensitivity among endocrine end points compared was not apparent. Use of postliothyronine estimate of fT 4 concentration alone or as part of a predictive value improved diagnostic specificity for differentiation of hyperthyroid vs non-hyperthyroid cats ( P ≤ 0.081). Results of this study further confirm existence of hyperthyroidism in cats that do not have high serum concentration of iodothyronines. We concluded that the triiodothyronine suppression test is a safe and accurate test for diagnosis of hyperthyroidism in cats with suggestive clinical signs of the disease, but lacking high serum concentration of iodothyronines.
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Refsal et al. (1991) studied this question.