Comparison of thyrotropin concentrations measured by bulk acoustic wave technology and chemiluminescence in hyperthyroid cats and cats with non-thyroidal illness

ObjectivesA thyrotropin (also known as thyroid-stimulating hormone [TSH]) assay using bulk acoustic wave (TSH-BAW) technology is a sensitive and specific test for diagnosing hyperthyroidism; however, the effect of various types of non-thyroidal illness (NTI) have not been evaluated with this assay. The objectives of this study were to compare serum TSH concentrations using the TSH-BAW and a currently available TSH chemiluminescent immunoassay (TSH-CLIA) in hyperthyroid cats, cats with NTI and healthy cats, as well as to compare sensitivity and specificity for diagnosing hyperthyroidism.MethodsA prospective cross-sectional study was conducted comparing the TSH concentration of 37 hyperthyroid, 32 healthy and 32 NTI cats using the TSH-CLIA and TSH-BAW assays. The effect of disease severity was evaluated with hyperthyroidism and NTI.ResultsThe TSH-BAW had a lower sensitivity (78%, 95% confidence interval [CI] 62-90) and negative predictive value (89%, 95% CI 79-95) but higher specificity (97%, 95% CI 89-100) and positive predictive value (94%, 95% CI 79-99) than the TSH-CLIA. The median serum TSH concentration was significantly different between hyperthyroid cats and both healthy and NTI cats with both assays (<i>P</i> <0.01) but was not different between NTI and healthy cats (TSH-CLIA <i>P</i> = 0.168, TSH-BAW <i>P</i> = 0.673). Eight (21.6%) hyperthyroid cats had a detectable TSH-BAW but undetectable TSH-CLIA concentration, with seven (18.9%) having a TSH-BAW within the reference interval. A total of 12 (18.8%) non-hyperthyroid cats (four [12.5%] healthy cats and eight [25%] NTI cats) had an undetectable TSH-CLIA compared with only two (6%) cats (one [3%] healthy cat and one [3%] NTI cat) with the TSH-BAW assay. The proportion of cats with an undetectable serum TSH concentration was significantly higher with the TSH-CLIA than the TSH-BAW in NTI cats (<i>P</i> = 0.008). This was especially evident in NTI cats suffering from moderate to severe illnesses (<i>P</i> = 0.025).Conclusions and relevanceThe TSH-BAW has a high specificity for detecting hyperthyroidism and identifies a normal serum TSH concentration in non-hyperthyroid cats more often than the TSH-CLIA. However, a normal result cannot be used to rule out hyperthyroidism.