Serum TSH in hyperthyroid cats pre- and post therapy

Otero T, Archer J, Billings H, et al.

Proceedings of the European College of Veterinary Internal Medicine—Companion Animals Congress 2002;173.

Feline hyperthyroidism can be difficult to diagnose. Sensitive TSH assays are used to diagnose this condition in humans. The aim of this study was to assess if serum TSH could distinguish between normal and hyperthyroid cats and determine if therapy normalizes TSH levels. Endogenous TSH was measured using the ACS:180, Bayer chemiluminescent assay in 12 euthyroid and 22 hyperthyroid cats. Categorization of thyroid function was on the basis of clinical signs and total T4 measurements. TSH measurements were repeated 14-28 days after starting treatment with carbimazole (11 cats) or radioactive iodine (11 cats). Results pre- and post therapy were compared with a paired t-test. In euthyroid cats the median and (range) for T4 and TSH were 27.6 (19.8-42.8) nmol/l and 0.14 (0.05-0.28) mIU/l, respectively. In hyperthyroid cats T4 and TSH before therapy were 128.5 (63.7-913.8) nmol/l and 0 (0-0.03) mIU/l. There was no overlap in TSH values between euthyroid and hyperthyroid cats. Therapy with carbimazole caused a significant decrease in T4 to 6.6 (2-41.5) nmol/l, (p<0.001) and an insignificant change in TSH 0.05 (0-2.37) mIU/l, (p=0.097). Therapy with radioactive iodine caused a significant decrease in T4 to 12.2 (5.1-308) nmol/l and an insignificant change in TSH to 0.01 (0-0.08) nmol/l, (p=0.063). In conclusion TSH measured with the ACS:180, Bayer had a 100% sensitivity for the diagnosis of feline hyperthyroidism in this study, but 2-4 weeks of therapy was insufficient time to normalize TSH. This assay should now be tested in a larger group of cats, including animals with non-thyroidal illness, to assess its specificity.