The Master of Science Seminar and Examination of Camille Brassard

Background:

About 10% of hyperthyroid cats have a normal total T4 (TT4), requiring further testing to make the diagnosis. In people, thyroid stimulating hormone (TSH) is used. TSH is measured using the canine assay (cTSH) as the only assay currently available. However, this assay cannot differentiate between subnormal and low-normal TSH concentrations in cats due to poor specificity (70-85%). A novel feline optimized TSH assay (fTSH, Truforma by Zomedica) was recently developed. It differentiates better between euthyroid and hyperthyroid cats compared to cTSH. However, the effect of non-thyroidal illness (NTI) on fTSH has not been evaluated.

Hypothesis/Objectives:

Comparison of fTSH and cTSH concentrations among hyperthyroid cats, cats with non-thyroidal illness (NTI), and healthy cats. Evaluation of sensitivity and specificity of fTSH to diagnose hyperthyroidism.

Animals:

The study was performed on 102 client-owned cats, including 37 hyperthyroid, 33 healthy, and 32 NTI cats.

Methods:

Prospective cross-sectional study. TT4, cTSH (Immulite 2000) and fTSH (Truforma) were measured in all cats. Hyperthyroidism was confirmed by thyroid scintigraphy. TT4 was repeated 3 months after enrollment if available in healthy and NTI cats to rule out subclinical hyperthyroidism. Euthyroid cats with NTI were divided between 3 groups based on the severity of their illness. Significance was set at P <0.05.

Results:

The sensitivity and specificity of fTSH are 78% (62-90%) and 97% (84-100%), respectively. There is a significant difference between hyperthyroid cats and healthy and NTI cats with both assays (P<0.01), and the latter euthyroid groups were not different. Nineteen (19) percent of cats had a suppressed cTSH but normal fTSH. Eight (21.6%) hyperthyroid cats have a normal fTSH but undetectable cTSH. Twelve (4 healthy, 8 NTI) euthyroid cats (18.5%) have an undetectable cTSH vs. only 2 (1 healthy, 1 NTI) (3%) have an undetectable fTSH. No statistical difference in TSH concentration was detected between the different severity groups of NTI. However, the cTSH (not the fTSH) was suppressed in twice as many cats with severe illnesses.

Conclusions:

The fTSH has a high specificity, identifies normal TSH in healthy cats more often, and appears to not be affected by NTI. It can be a useful tool for the diagnosis of feline hyperthyroidism.