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Feline Thyroid Disease: What’s the TSH?
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STEFANIE DEMONACO , DVM, MS, DACVIM Assistant Professor, Small Animal Internal Medicine Virginia-Maryland College of Veterinary Medicine
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with nonthyroidal illness. In sick cats, TSH generally remains within the reference interval, but up to one-third of cats were found to have undetectable cTSH concentrations attributed to the effects of severe NTI.1 The degree of suppression of serum cTSH concentrations in cats with NTI appears to be impacted mostly by severity of illness where TSH decreases as severity of illness increases. The category of disease has minimal impact on cTSH, similar to tT4 and fT4. Again, the main limiting factor of cTSH is the lower detection limit. In cats with NTI and undetectable cTSH concentrations, it is uncertain whether cTSH could be low-normal or actually suppressed in these cats. Clearly, a TSH assay optimized for cats with the ability differentiate low-normal vs. those below the reference range has the potential to improve diagnosis of hyperthyroid cats. A feline-optimized TSH (fTSH) assay has recently become available with a lowest limit of detection of 0.008 ng/mL and low-normal is 0.01 ng/mL. 2 Current studies have shown that this assay readily differentiates between healthy and hyperthyroid cats. When evaluating serum fTSH in clinically healthy cats compared to hyperthyroid cats, the majority of healthy cats had a serum fTSH above the reference interval (> 0.01 ng/mL) whereas the majority of hyperthyroid cats had an fTSH concentration below the reference interval (< 0.01 ng/mL). Current studies on how fTSH performs in the face of nonthyroidal illness as well as fTSH in cats with a various severity of hyperthyroidism are necessary. Use of TSH to diagnose hypothyroidism Iatrogenic hypothyroidism is the most common form of hypothyroidism in cats and occurs after treatment of hyperthyroidism with anti-thyroidal drugs, radioiodine therapy, and thyroidectomy. Based on current data, the prevalence of iatrogenic hypothyroidism in cats is approximately 10-25%. 1 Cats do not display overt signs of hypothyroidism and it can
be easily missed if serum TSH is not measured. Feline hypothyroidism is associated with decreased survival and kidney function, making an early and accurate diagnosis imperative. The screening test of choice for hypothyroidism in dogs is tT4; however, concurrent diseases (e.g., chronic kidney disease, or CKD, in cats) can falsely lower levels. The high prevalence of concurrent CKD in cats treated for hyperthyroidism could lead to overdiagnosis of hypothyroidism if tT4 is used alone. Luckily, serum cTSH performs well for the diagnosis of hypothyroidism and accurately differentiated azotemic iatrogenic hypothyroid cats from euthyroid azotemic cats. A diagnosis of hypothyroidism is supported by a low tT4 and an elevated TSH concentration. The current recommendation in monitoring cats post treatment of hyperthyroidism, particularly with I-131, is to routinely monitor both tT4 and TSH in order to catch the diagnosis of hypothyroidism as soon as possible and initiate treatment when necessary. Currently, the best use of serum cTSH in cats is for the diagnosis of iatrogenic hypothyroidism as there are limitations for its use in feline hyperthyroidism. The new fTSH assay has the potential to improve diagnosis and outcome in hyperthyroid cats given its ability to differentiate euthyroid from hyperthyroid cats. Further studies evaluating fTSH in cats are necessary, particularly those with NTI. Peterson, Mark. (2013). More Than Just T4: Diagnostic testing for hyperthyroidism in cats. Journal of Feline Medicine & Surgery. 15. 765-77. 2. Wood A, Ryder M, Harmon I. TRUFORMA™ Point-of-Care Canine and Feline Thyroid-Stimulating Hormone (TSH) assay. Zomedica website. https://zomedica.com/wp-content/uploads/2021/09/ TRUFORMA-TSH-White-Paper.pdf References: 1.
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