Hyperthyroidism is a common endocrine disease of older cats. Finding a high serum total thyroxine (T4) concentration leads to a straightforward diagnosis in many cases where cats exhibit classic signs of the disease (weight loss despite a good appetite, palpable goiter). Approximately 10% or less of hyperthyroid cats, and over 30% of cases with a mild or early case of hyperthyroidism, have a T4 concentration remaining in a normal reference range. In addition, cats with concurrent nonthyroidal illness can have suppressed high serum T4 concentrations to within the normal reference range.
While in humans, the measurement of serum thyroid-stimulating hormone (TSH) concentration is used routinely as a front-line test of thyroid function, a similar test has not been determined for a corresponding use in cats. With recent reports showing canine TSH (cTSH) assays can be utilized to measure TSH in cats and these results can be used to include or exclude hyperthyroidism, the authors in this study investigated the usefulness of cTSH measurements as a discriminatory diagnostic test in cats with hyperthyroidism. A feline-specific TSH assay is not available currently. Physiologically, slight elevations in circulating T4 or T3 should suppress pituitary TSH secretion leading to low or undetectable serum TSH concentrations. The cTSH assay is not considered to accurately distinguish between low normal and subnormal concentrations in cats, though reports to date indicate all hyperthyroid cats have undetectable TSH concentrations with the cTSH assay yet most normal or sick euthyroid cats show measurable TSH concentrations.
Another thyroid function test, serum free T4 (fT4) concentration, is available and the value is high in approximately 95% of cats with hidden hyperthyroidism which can aid in diagnosis. Unfortunately, the test specificity is poor, with up to 20% of sick and clinically normal cats that are not hyperthyroid having increased serum fT4 concentrations. Thyroid scintigraphy (quantitative) is thought to be the most accurate test for these occult cases of hyperthyroidism. Unfortunately, only a small number of veterinarians have access to this specialized equipment to aid in this diagnosis.
This study was a prospective study involving a large number of cats: 917 cats with untreated hyperthyroidism, 32 euthyroid cats (normal thyroid levels) suspected of having hyperthyroidism, and 131 clinically normal cats. The cats were evaluated with serum T4, T3, fT4, and TSH concentrations. Thyroid scintigraphy was the gold standard used to confirm or exclude a diagnosis of hyperthyroidism. The ages of the cats ranged from 6 to 19 years of age (median=12 years).
The results indicate that only 18 (2%) of hyperthyroid cats had a measurable serum TSH concentration, yet the remaining 899 (98.0%) had serum TSH concentrations below the limit of detection. Serum TSH concentrations were detectable in 87 (66.4%) of clinically normal cats and in 27 (84.4%) of euthyroid suspect cats. The authors state that their results indicate measurement of serum TSH concentration is very sensitive but is a nonspecific diagnostic test for feline hyperthyroidism. While 98% of hyperthyroid cats had serum TSH concentrations below the quantification limit (<0.03 ng/mL), approximately 30% of older euthyroid cats also had undetectable serum TSH concentrations. This demonstrates that the cTSH assay used for this study cannot accurately measure serum TSH concentrations at a low enough level to distinguish those cats with occult hyperthyroidism from a low-normal concentration found in some euthyroid cats.
An additional finding indicates though that combining serum TSH concentrations with serum T4 or fT4 concentrations does improve the ability to differentiate occult or mild hyperthyroid cases from euthyroid cats suspected of having thyroid disease. Finding an undetectable serum TSH concentration with either a high T4 or fT4 concentration markedly improves diagnostic accuracy while finding measurable serum TSH concentration, even with high-normal T4 or fT4 concentrations makes hyperthyroidism very unlikely. Combination testing in cats should be considered when the hyperthyroid disease status of the cat is in question.
The total T4 test remains the best choice for an initial screening test for hyperthyroidism. Due to the low specificity of serum fT4 concentrations (20% of cats with fT4 in upper half of reference range were euthyroid), this test should not be used alone to confirm a diagnosis of hyperthyroidism. As a final point, the authors state the study proves a better TSH assay is needed to aid in diagnosis of cats with occult or mild hyperthyroidism.