Hyperthyroidism in cats, caused by nodular hyperplasia or adenomas, is clinically and histologically similar to toxic nodular goiter in humans. Subclinical hyperthyroidism in humans is defined as low thyrotropin (TSH) in conjunction with thyroid hormone concentrations within reference ranges, but has not previously been defined in cats.

A recent published study addressed whether euthyroid senior cats with low TSH have histological evidence of thyroid nodular hyperplasia and/or adenoma.

Cats with undetectable TSH had a higher frequency of thyroid nodules, a greater percentage of abnormal thyroid tissue, and a higher overall histopathological grade of thyroid pathology than did cats with detectable TSH.

Results supported the existence of subclinical hyperthyroidism in cats by providing evidence that euthyroid cats with undetectable TSH concentration are more likely to have evidence of thyroid nodular hyperplasia than euthyroid cats with measurable TSH.

The cats with subclinical hyperthyroidism were checked every 6–8 weeks for subtle clinical and biochemical changes typical of hyperthyroidism in cats, such as persistent weight loss with good appetite, increased heart rate, heart murmur, decreased creatinine concentration, and increasing liver enzymes. While measurement of a single freeT4 concentration has been shown to have good sensitivity and specificity for the diagnosis of hyperthyroidism in cats, the diagnosis is usually more reliable if both T4 and freeT4 are measured.

There is no feline–specific assay for feline TSH, although the genetic sequence of canine TSH has 96% homology with feline TSH. The canine TSH assay is therefore used, and despite suboptimal sensitivity, has good repeatability at the low end of its working range when used to measure feline TSH.

Based on the present study, the calculated TSH reference range for healthy senior cats (> 8 years) is < 0.03–0.15 ng/mL (n=90). This range also included those cats with TSH < 0.03 ng/mL that were found to be subclinically hyperthyroid. A follow up prospective study showed that senior cats with low TSH (< 0.03 ng/mL) have a high risk of progression to biochemical and/or clinical hyperthyroidism within 12 months.

Some young cats (6-24 months) and those with severe concurrent illness such as chronic renal failure can have TSH concentration < 0.03 ng/mL. In the present study, there was no significant difference in the degree of severity of kidney disease between the undetectable TSH and detectable TSH groups as determined by creatinine concentration. In humans, severe illness has also been shown to suppress TSH secretion.

Euthyroid senior cats with undetectable TSH are likely to have histological evidence of nodular thyroid disease, and such cats could be considered to be subclinically hyperthyroid. The cat appears to be a good model for subclinical hyperthyroidism associated with nodular goiter in people.

References: Wakeling, Smith, Scase et al. Thyroid 17:1201-1209, 2007.