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Thyroid stimulating hormone (TSH) measurement is routinely used for the diagnosis of thyroid disease in humans and dogs, but measurement of feline TSH is not routinely performed because a commercial feline TSH assay is not available. Research has been somewhat frustrated by the poor sensitivity of the canine TSH assay which is a long way from achieving the sensitivity seen in the newer generations of human TSH assays. However there are certain situations for which I believe the canine TSH assay has a useful role in the diagnosis of thyroid disease in cats, as long as the limitations of the assay are fully understood. Recent studies have used the DPC canine TSH assay for the measurement of feline TSH (1–4).
Measurement of feline TSH is not routine at most veterinary laboratories; therefore, it is necessary to discuss this measurement with the laboratory manager before requesting a feline TSH measurement locally. The limit of quantification of the canine DPC TSH assay is 0.03 ng/mL, and during my PhD studies, we determined a reference range for feline TSH in 90 apparently healthy older cats (> 8 y) of < 0.03–0.15 ng/mL (1,2,4).
The prevalence of chemical hypothyroidism following I-131 therapy has been reported to be as high as 30% (5), but the existence of clinically significant hypothyroidism following I-131 treatment is still controversial. The prevalence of clinically significant hypothyroidism is thought to be low and very little data have been published describing the diagnosis and treatment of hypothyroidism in cats following I-131 treatment. Many radioactive iodine units in both North America and Europe, however, are routinely recommending thyroxine supplementation of cats with persistently low total thyroxine (tT4) concentrations (< 5 nmol/L) and compatible clinical signs (lethargy, obesity, hair loss). Therefore, anecdotally, many specialists in the field believe that clinically significant hypothyroidism does exist in cats following the treatment of hyperthyroidism, despite the lack of published peer-reviewed data to this effect. As cats are just as susceptible to sick euthyroidism as dogs (6,7), it is desirable to add a TSH measurement to support the diagnosis of hypothyroidism. An elevated TSH is almost invariably found in I-131 treated cats with persistently low tT4, indeed many such cats have a TSH concentration that is > 0.50 ng/mL 3 to 6 mo following treatment.
As TSH concentrations are invariably < 0.03 ng/mL in hyper-thyroid cats, there are 2 situations in which the TSH assay can assist with the diagnosis of hyperthyroidism in cats. It is important to note here that data are only available for cats > 8 y.
The diagnosis of hyperthyroidism in cats with concurrent disease can be complicated by the suppression of tT4 into the upper half of the reference range (a false negative result) despite a clinical suspicion of hyperthyroidism. This is termed occult hyperthyroidism. Conversely, in cats with concurrent disease but without evidence of hyperthyroidism, free T4 (fT4) can be elevated (a false positive) but in this case the concurrent disease should suppress the total thyroxine into the bottom half of the reference range. It has therefore been proposed (8) that diagnosis of hyperthyroidism in cats with concurrent disease could be based on a high fT4 in conjunction with tT4 in the upper half of the reference range (> 25 nmol/L).
In a small retrospective study (2) of cats that had evidence of mild azotemia and clinical signs of hyperthyroidism, it was shown that the combination of an upper half reference range tT4 (> 30 nmol/L) and a TSH < 0.03 ng/mL had similar sensitivity and specificity compared to tT4 (> 30 nmol/L) and fT4 (> 40 pmol/L) for the diagnosis of occult hyperthyroidism. Although I usually do reach for the fT4 first after measuring a high normal tT4 in such a cat, if I have a cat that is borderline on all counts (for example only mild concurrent disease, minimal clinical signs of hyperthyroidism, tT4 30–40 nmol/L and/or borderline high fT4 (40–55 pmol/L), I like to add TSH < 0.03 ng/mL to the diagnostic criteria (or consider a T3 suppression test).
The second example is a cat that has had a routine annual blood panel, that is not clinically suspected to be hyperthyroid but that has a high normal tT4. These cats are often in the lower age bracket for hyperthyroidism (8 to 10-years-old) and overweight or obese. Rather than reaching for a free T4 in such a cat I usually recommend a TSH and use this test to rule out hyperthyroidism. If the TSH is > 0.03 ng/mL it is not only unlikely that the cat is hyperthyroid but also unlikely that the cat will be diagnosed with hyperthyroidism in the next 12 mo. This is because cats have a subclinical phase to their hyperthyroidism that evidence suggests is usually around 1 to 3 y in length (3,9–11). Although in my opinion most cats > 8 years old with a total T4 in the 40–55 nmol/L range are either occult or early hyperthyroid, there is a minority of cats with high normal tT4s that are often obese, in which a TSH concentration can definitively confirm that hyperthyroidism is not present. These obese cats could have a form of thyroid resistance akin to insulin resistance (4).
Most cats that become hyperthyroid have around a 1 to 3-year subclinical phase to their disease (3,9–12), with a persistently low (< 0.03 ng/mL) TSH on repeat measurement and tT4 well within the reference range prior to diagnosis of hyperthyroidism. Unfortunately due to the poor sensitivity of the assay there is also a number of healthy older cats that intermittently have a TSH below the limit of quantification of the assay without evidence of progression to hyperthyroidism. A single low TSH measurement, therefore, cannot in isolation be used to determine thyroid status. In addition, some would argue that as published data using dynamic thyroid testing (such as TSH and TRH suppression testing) are not available, a strong case cannot yet be made for the use of the TSH assay to diagnose either clinical or subclinical thyroid disease. However, a prospective study showed that around 30% of apparently euthyroid cats over 8-years-old with a single unmeasurable TSH concentration (< 0.03 ng/mL) became hyperthyroid within 14 months; therefore, there may be future merit in including TSH as a routine part of the senior health screen.
As always with veterinary medicine, further studies need to be done in this area with a strong need for a highly sensitive feline specific TSH assay that would eliminate some of the problems associated with using the canine TSH assay. However based on the data published to date, I believe there is a role for measurement of feline TSH for the diagnosis of thyroid disease in cats using the DPC canine TSH assay.