Hyperthyroidism is one of the most commonly diagnosed endocrine diseases affecting older cats (Wakeling et al, 2011; Graves, 2017). The clinical signs of hyperthyroidism have now been well described (Capen and Martin, 2003; Peterson, 2012) with an increase in incidence of cats being diagnosed with hyperthyroidism since the late 1970s. According to Capen and Martin (2003) this is the result of a larger number of cats receiving veterinary medical care, an improvement in assays for thyroid hormones, detailed characteristics of hyperthyroidism and increased awareness of its occurrence in cats by veterinary surgeons (VS).
Research indicates that there has been a change in emphasis from simply confirming a diagnosis in a cat presenting with classical clinical signs to diagnosing hyperthyroidism in cats with no clinical signs (Graves, 2017). In light of this, there has been a direct improvement in the efficacy of the diagnostic tests used to confirm hyperthyroidism with the opportunity to routinely test those cats with early or mild signs of the disease. More veterinary practitioners have improved their resources and are now able to run inhouse blood tests that previously would have been sent to an external laboratory. This, in turn, allows more tests to be performed easily producing immediate results with a huge increase in offering clients the option of pre-anaesthetic blood tests. Consequently, testing for hyperthyroidism has become easier and this paper recommends it to be considered as part of the routine blood tests that are run on a daily basis within the practice.
The majority of cats diagnosed with hyperthyroidism are middle to old aged (Figure 1) with a reported age range between 4–22 years (Gordon et al, 2003), yet only 5% of cats are younger than 10 years of age at time of diagnosis (Peterson, 2006). This may be the result of practices only testing for the disease when clinical signs are present, thus allowing the disease to progress far enough for signs to be present years after the onset of hyperthyroidism. To minimise this, routine testing for hyperthyroidism could become standard practice in all cats, before the onset of clinical signs.
This study aimed to provide answers as to whether it is necessary to routinely test all cats for hyperthyroidism by examining serum total thyroxine (T4) concentration levels in a population of clinically normal cats. Specific objectives were to determine the range of T4 levels in clinically normal cats, to estimate the prevalence of elevated T4 levels above the clinical reference range in clinically normal cats and to identify risk factors for elevated T4 levels in clinically normal cats tested for hyperthyroidism and are diagnosed, e.g. gender, neutered status, age, breed, colour and weight. For the purposes of this study, cats were described as being clinically normal if they did not show any clinical signs of hyperthyroidism.
Literature review
Hyperthyroidism
Hyperthyroidism is a multisystemic disorder and is a very common endocrinopathy affecting older cats, typically older than 8 years of age (Ward, 2007; Peterson, 2012). It is usually caused by the increased production and secretion of thyroid hormones (thyroxine (T4) and triiodothyronine (T3)) from the thyroid glands located in the neck (Graves, 2017). Thyroid hormones play an important role in controlling the body's metabolism and thus the general level of activity in cats (Peterson, 2012). According to the Feline Advisory Bureau (FAB) (2019) increased T4 levels in the majority of cases are due to a benign change. Peterson and Ward (2007) reported benign changes in 98% of cats with the remaining 2% having a thyroid carcinoma.
Functional adenoma is the most common pathologic abnormality associated with hyperthyroidism in cats (Peterson, 2012). Peterson (2012) suggested that adenomas are bilateral in more than 70% of cases and unilateral in fewer than 30%. In bilateral cases, both thyroid glands are affected, although one may be more affected than the other (Peterson, 2012). The abnormal thyroid tissue becomes enlarged, although the underlying cause of this change is unknown with further research being required (Peterson and Ward, 2007).
If hyperthyroidism is diagnosed early and treated appropriately then the outlook for the affected cat is usually very good. Trepanier (2007) described that the best management options for hyperthyroidism, included radioiodine therapy, thyroidectomy or medical treatment with drugs that inhibit synthesis of thyroid hormone in the thyroid gland such as methimazole/thiamazole (Figure 2).
Clinical signs
Clinical signs can be extreme and cats can become critically ill with hyperthyroidism if left untreated. However, in most cases, hyperthyroidism can be manageable if diagnosed early enough. According to Mooney and Peterson (2004) common signs of hyperthyroidism are usually quite subtle at first and can often go unrecognised, but can become severe as the disease progresses. Signs include weight loss despite a normal or increased appetite, hyperactivity, intermittent gastrointestinal disorders, polydipsia and polyuria, tachycardia, cardiac murmur and palpable goitre (Taylor et al, 1989; Graves, 2017). However, it is important to note that almost 30% of cats that suffer from hyperthyroidism may never present with clinical signs (Feldman and Nelson, 2004) thus making a diagnosis more difficult.
Thyroid hormones affect most organs in the body and hyperthyroidism can occasionally cause secondary problems that may lead to further treatment being required (Graves, 2017). FAB (2019) described the effect of elevated thyroid hormones on the heart which caused an increased heart rate and a stronger contraction of the heart muscle. Over time, with untreated hyperthyroidism, the muscle of the left ventricle of the heart enlarges and thickens (left ventricular hypertrophy). In a study conducted by Weichselbaum et al (2003), it was reported that of 149 cats with hyperthyroidism, 38% were receiving cardiac-related medication. If left untreated and unmanaged these changes eventually compromise the normal heart function and can lead to heart failure. However, once the underlying hyperthyroidism has been controlled, the heart changes will often improve or resolve simultaneously (Weichselbaum et al, 2003). Recent recommendations in feline cardiac care suggest not to treat cardiac problems unless the patient is in heart failure, has an atrial thromboembolism (ATE) episode or has enlarged left atria with spontaneous echo contrast (Sangster et al, 2014).
A study conducted by Blois et al (2010) found that 1.8–5.5% of cats diagnosed with hyperthyroidism also had diabetes mellitus. However further research is needed to investigate whether there is a direct link between hyperthyroidism and diabetes mellitus.
Diagnosis
The diagnosis of hyperthyroidism is considered routine by most small animal practices and includes the assessment of patient history, clinical signs, physical examination including thyroid palpation and routine laboratory thyroid functioning tests (Peterson et al, 2001).
Boretti et al (2009) discussed previous case studies and found that 38% of euthyroid cats examined had a palpable thyroid gland, thus highlighting the need to conduct further diagnostic tests to confirm hyperthyroidism.
The most commonly used laboratory test to confirm a diagnosis is the determination of serum total T4 concentration. Free T4 testing has also been used, but is less suited as a screening test due to elevated levels in some euthyroid cats giving a false positive reading (Norsworthy et al, 2002). Thus free T4 levels can only be interpreted in conjunction with total T4 levels that are above the reference range to confirm hyperthyroidism (Scott-Moncrieff, 2012).
The most reliable test for confirming hyperthyroidism is a demonstration of an elevated basal T4 or free T4 concentration (generally considered to be >65 nmol/litre of T4 or free T4 >15 pmol/litre) (Quantum Veterinary Diagnostics, 2019).
Unlike human diagnosis, there is usually no advantage in determining T3 concentration (which occasionally remains within a normal range) or performing a thyroid stimulating hormone (TSH) or thyrotrophin releasing hormone (TRH) stimulation test according to Broome et al (1988). However, significant daily variations have been shown to occur in T4 concentration in hyperthyroid cats, and in occasional cases the level may at times fall within the normal range (Peterson et al, 1987; Broome et al, 1988). If hyperthyroidism is suspected on clinical grounds but cannot be confirmed by repeated basal T4 estimations, a T3 suppression test can be performed.
Quantum Veterinary Diagnostics (2019) sets the range of normal serum T4 levels between 13–48 nmol/litre. T4 levels greater than 48 nmol/litre indicated inadequate treatment or untreated hyperthyroidism, T4 levels ranging between 35–45 nmol/litre required clinical assessments, while T4 levels less than 13 nmol/litre indicated hypothyroidism or over dosing of medication (Quantum Vet Diagnostics, 2019).
Although diagnosis in most cats is relatively straightforward, studies have shown that several cats suspected of having hyperthyroidism do not present with clinical signs or a palpable goitre. There were no published reports on the prevalence of a palpable goitre in cats and their relationship to T4 levels (Norsworthy et al, 2002). There was also increasing evidence that the presence of a goitre alone was a poor indicator of clinical hyperthyroidism (Boretti et al, 2009) and that suspected patients would therefore require a measurement of T4 and T3 biochemical blood test to confirm diagnosis (Peterson, 2006).
Due to the evidence of non-clinical hyperthyroidism within cats, a key importance of this study was to determine whether there was any value in routinely blood testing those cats not presenting with clinical signs. It is important to acknowledge that cats with hyperthyroidism can have normal T4 levels despite having the disease thus care had to be taken when interpreting laboratory results (Graves, 2011). Overall, in order to confirm a diagnosis of hyperthyroidism, Panciera et al (2004) suggested the use of several other screening tests in conjunction with serum T4 levels, such as a complete blood count, biochemical analysis, urinalysis, radiography and cardiac ultrasonography.
Subclinical hyperthyroidism
Scott-Moncrieff (2012) explained that cats with early hyperthyroidism or concurrent non-thyroidal illness may have a T4 concentration within the upper half of the reference range. If the T4 level is high normal or borderline, the measurement should be repeated after concurrent diseases have been treated or after a period of 4 to 8 weeks, because hyperthyroidism is a chronic progressive disease and T4 levels increase over time. The current study is designed to test whether it was worthwhile routinely testing those clinically normal cats in the high region of T4 levels for a definitive diagnosis of early hyperthyroidism.
There has been a steady increase in the incidence of hyperthyroidism in recent years (Taylor et al, 1989) where the signalment of hyperthyroidism is cats generally over the age of 8 years old (Wakeling et al, 2011). Previously, there have been no reports in the literature of hyperthyroidism in cats less than 4 years old. However Gordon et al (2003) documented a case report on juvenile hyperthyroidism in an 8-month-old kitten. Hyperthyroidism was considered very unlikely due to the patient's age, however, T4 levels were evaluated at a laboratory on three occasions as a comprehensive feline blood panel, in which a definitive diagnosis of hyperthyroidism was made. Similarly, a case report conducted by Quante et al (2010) reported hypothyroidism in a 7-month-old kitten, demonstrating that although congenital hypothyroidism is a rare disease, it can be diagnosed outside of the common age range.
A study conducted by Wakeling et al (2011) found that there was a higher prevalence of hyperthyroidism in euthyroid geriatric cats with undetectable TSH compared with those with detectable TSH concentrations. This suggests that subclinical hyperthyroidism also exists in cats and might be diagnosed and treated before overt signs develop; 30% of cats suffering from hyperthyroidism do not present with clinical signs (Blaxter and Gruffydd-Jones, 1994), therefore routine testing could diagnose those cats that do not present with clinical signs as well as establishing evidence in veterinary medicine that early diagnosis improves the patient's outcome and that subclinical hyperthyroidism has a negative effect on the patient's health. Clients are becoming more willing to diagnose and have routine blood tests carried out on their pets as well as to continually assess their pet's performance, thus making routine testing possible. It could be argued that there is no need to treat the condition if there are no clinical signs of the disease. However, thyroid hormones are vital in regulating important body functions including metabolism, body temperature, gastrointestinal function, heart rate and blood pressure. If the body is producing too much thyroid hormone and is left untreated then there can be serious damage to vital organs like the heart and kidneys. Diagnosis and treatment could therefore be valuable even in subclinical cats.
Elderly human patients with subclinical hyperthyroidism have been shown to develop obvious hyperthyroidism at a rate of 2–9% per year, and if left untreated can have effects on the cardiovascular function with an increased risk of developing atrial fibrillation as well as affecting bone density (Wakeling et al, 2011). If applied to veterinary medicine it would emphasise the potential importance of treating those cats that do not show any clinical signs of hyperthyroidism yet suffer from the disease.
This research has found that no studies have been conducted in regards to breed or gender that are particularly prone to developing hyperthyroidism. Peterson et al (1987) did not identify sex or breed predispositions of hyperthyroidism although their findings showed a relatively low prevalence in pedigree cats. There was also some evidence to suggest it was less common in Siamese cats (Mooney and Peterson, 2004).
The pathogenesis of feline hyperthyroidism is poorly understood. However, Gordon et al (2003) discussed several studies that have evaluated risk factors of hyperthyroidism which suggested that canned cat foods, the use of cat litter; the use of topical ectoparasitic preparations and living indoors were associated with the development of hyperthyroidism. Further studies are required to advance the level of knowledge in risk factors of the disease however Peterson (2012) suggested measures to help minimise the risk of hyperthyroidism.
As such, the current study built on previous studies of diagnosed hyperthyroidism and estimated the prevalence of the disease and identified risk factors associated with hyperthyroidism within subclinical cats.
None of the studies to date have singled out a dominant risk factor of the development of the disease or suggested early testing in those cats that appeared to be clinically normal. The impact of the current study could change the workings of small animal practices and the standard protocols put in place for routine blood testing to include T4 for cats.
Methods
The research was conducted in a small-animal practice in Glasgow that routinely tested all cats for hyperthyroidism. The aim was to diagnose subclinical patients with T4 levels that were beginning to rise and to potentially alleviate changes to the cardiac muscle by treating those cases diagnosed before the onset of clinical signs presented.
In total, 202 clinical total T4 blood test results from the practice were analysed, together with an analysis of risk factors using Statistical Package for Social Sciences (SPSS). This provided reasons for the routine testing of all cats for hyperthyroidism, and highlighted a group to prioritise testing on.
Records from cats tested for routine pre anaesthetic bloods (including T4) over a year's period that did not show signs of hyperthyroidism were extracted and retrospectively analysed. Each blood sample was taken from the jugular vein of each patient and decanted into a serum blood tube for analysis. The level of T4 for each patient was measured in-house using a Quantum Saturno 100 Vet wet chemistry analyser with a normal range being between 13–48 nmol/litre. Any result above or below the normal reference range was classified as being hypo/hyperthyroid. Each test that came back as being hyperthyroid was then sent to an external laboratory for confirmation of diagnosis. The same external laboratory was used each time.
Data collection
202 clinical records from the practice were tabulated into a Microsoft excel spreadsheet for analysis where information was gathered regarding gender, age, breed, colour, weight, T4 result and T4 description (hyperthyroid/euthyroid) of each patient case.
Due to the large range of breeds recorded (Abyssinian, Bengal, Birman, Bombay, British Shorthair (BSH), Burmese, Domestic Longhair (DLH), Domestic Shorthair (DSH), Exotic Shorthair, Main Coon, Persian, Ragdoll, Russian and Siamese) they were grouped into statistically significant groups that included crossbreed v purebreed and DSH v other. Similarly to breed, colour was another risk factor that was grouped into measurable categories due to the wide range documented. The final nine categories of colour included (black, black and white, blue, brown, ginger, grey, tabby, tortoiseshell and white). In order to estimate the prevalence of elevated T4 levels in a sample of clinically normal cats, each case was determined as either having a low, normal or high T4 level depending on where it was in comparison to the normal reference range (13–48 nmol/litre). They were then filtered into being either hyperthyroid or euthyroid.
Each risk factor was then analysed individually by looking at numbers and percentages for categorical data and a level of deviation and mean/median for continuous variables. Histograms of all continuous variables were examined to determine whether or not they were normally distributed. An analysis of any associations between each factor was then conducted with the appropriate statistical test being carried out in SPSS.
A variety of tests were performed which included a non-parametric Mann-Whitney test for an association between a continuous variable and a binary categorical variable; a Chi-square test for an association between two categorical variables; a non-parametric Spearman test for an association between two continuous variables, as well as a non-parametric Kruskal-Wallis test for an association between a continuous variable and a categorical variable with more than two categories. The level of significance was set at p<0.05 with a 95% confidence interval.
Results
202 clinically normal cat results were obtained, of which, just over half (111; 55%) were female and 91 (45%) were male with 13 (6.4%) being entire and 189 (93.6%) being neutered. The median age was 11.4 years with a range between 0.4–22.6 years, and the median weight of the cats analysed was 4.4 kg with a range between 1.9–8.4 kg. The majority of cats recorded were crossbreeds (170; 84.2%). The majority 152 (75.2%) were DSH.
The median T4 result was 45 nmol/litre with a range between 8–153 nmol/litre as seen in Figure 3.
The prevalence of hyperthyroidism (elevated T4 levels) was 76 (37.6%) cats and the remaining 126 (62.4%) were euthyroid. A summary of all categorical variables can be seen in Table 1 with all numerical variables summarised in Table 2.
| Variable | Number (202) | Percentage (%) |
|---|---|---|
|
Neutered status
|
189/13 | 93.6/6.4 |
|
Breed
|
|
|
|
T4 description
|
|
|
|
Gender
|
|
|
| Variable | Median | Range |
|---|---|---|
| T4 Result | 45 nmol/litre | 8–153 nmol/litre |
| Age | 11.4 years | 0.4–22.6 years |
| Weight | 4.4 kg | 1.9–8.4kg |
Risk factors for elevated T4 levels
Breed and T4 result
The association between breed and T4 result was statistically significant as the Mann-Whitney p=0.002. The median T4 result for crossbreeds was 43.5 nmol/litre against that of purebreeds which was higher at 55 nmol/litre. The range of T4 results in crossbreeds was between 8–153 nmol/litre whereas purebreeds had a range between 25–103 nmol/litre.
Of the 170 crossbreed cats, 152 were DSH with a median T4 result of 44 nmol/litre compared with the remaining 50 cats that had a median T4 result of 51 nmol/litre, which was slightly higher. DSH had a range of T4 results between 8–153 nmol/litre and the remaining cats had a range between 25–103 nmol/litre. This again was a significant difference as p=0.009.
Breed and hyperthyroidism
The association between breed and hyperthyroidism was statistically significant as the Chi-Sqaure p-value=0.002. From all the purebreed cats analysed, 20 (62.5%) had elevated T4 results compared to 56 (32.9%) which were crossbreeds. This can also be confirmed by comparing DSH against all other cats (which included all purebreed cats). From all the DSH cats, 48 (31.6%) were hyperthyroid while 28 (56.0%) of all the other breeds were hyperthyroid. This was statistically significant with p=0.002.
Gender
The association between gender and T4 result and hyperthyroidism was not statistically significant. The median T4 result in female cats was 46 nmol/litre which was slightly higher than that of males which had a median T4 result of 44 nmol/litre. The range of T4 results for female cats was between 8–153 nmol/litre, while males had a range of 14–102 nmol/litre. The Mann-Whitney p-value=0.63. To reinforce this, 45 (40.5%) of all the female cats were hyperthyroid while 31 (34.1%) of all the male cats were hyperthyroid, which showed hyperthyroidism was more prevalent in female cats, but again this was not statistically significant with the Chi-Square p=0.38.
Neutered status
The association between neutered status and T4 result and hyperthyroidism was not statistically significant. The median T4 result of entire cats was 48 nmol/litre with a range between 31–118 nmol/litre compared with neutered cats with a median T4 result of 45 nmol/litre and a range of 8–153 nmol/litre. The Mann-Whitney p value= 0.33. From all the neutered cats recorded, 70 (37%) were hyperthyroid while 6 (46.2%) of all the entire cats were hyperthyroid. However this was again not statistically significant with the Chi-Square p-value=0.56 and can be explained by the few cases recorded as being entire.
Age
The association between age and T4 result and hyperthyroidism was not statistically significant. The median age of cats diagnosed with hyperthyroidism was 10.8 years with an overall sample range of 0.4–20.7 years. The median age of euthyroid cats was 11.7 years with a range between 0.4–22.6 years. Although there was a slight difference between the results, there was no statistically significant difference as the Mann-Whitney p-value=0.42. Again this reinforced the fact that age was not statistically associated with hyperthyroidism in clinically normal cats and there was no correlation between age and T4 result as shown in Figure 4 with the Spearman's Non-Parametric p value=0.08.
Colour
The association between colour and T4 result and hyperthyroidism was not statistically significant. The relationship between coat colour and both hyperthyroidism and T4 result is summarised in Table 3. White cats had the highest median T4 result and tabby cats had the lowest result. White cats also had the highest percentage prevalence of hyperthyroidism of 75%, brown being 57.1% and blue 50%, which can commonly be associated with purebreed colours; thus purebreeds showed a higher prevalence of hyperthyroidism. However there was no statistically significant relationship between either hyperthyroidism or T4 result and coat colour.
| Colour | Frequency (n/202) | Percentage (%) | Prevalence of hyperthyroidism n (%) | T4 median nmol/litre |
|---|---|---|---|---|
| White | 4 | 2 | 3 (75.0) | 60.0 |
| Brown | 7 | 3.5 | 4 (57.1) | 50.0 |
| Blue | 8 | 4 | 4 (50.0) | 45.0 |
| Grey | 23 | 11.4 | 10 (43.5) | 45.0 |
| Black | 22 | 10.9 | 9 (40.9) | 46.5 |
| Tortoiseshell | 26 | 12 | 10 (38.5) | 45.0 |
| Black and White | 39 | 19.3 | 14 (35.9) | 44.0 |
| Ginger | 31 | 15.3 | 11 (35.5) | 45.0 |
| Tabby | 42 | 20.8 | 11 (26.2) | 42.0 |
Weight
The association between weight and T4 result and hyperthyroidism was not statistically significant. The median weight of hyperthyroid cats was 4.3 kg with a range of 2.4–8.4 kg. The median weight of euthryoid cats was 4.4 kg with a range of 1.9–6.7 kg. This was not statistically significant as the Mann-Whitney p-value=0.68. There was therefore no correlation between weight and T4 result with the Spearman's Non-Parametric p-value=0.86 as shown in Figure 5.
A summary of risk factors associated with T4 results and hyperthyroidism can be seen in Tables 4 and 5.
| Relationship between variable and T4 result | Statistical test | Level of significance p<0.05 |
|---|---|---|
| Breed | Mann-Whitney Test | 0.002 |
| Age | Spearman's Non-Parametric Test | 0.08 |
| Neutered status | Mann-Whitney Test | 0.33 |
| Gender | Mann-Whitney Test | 0.63 |
| Colour | Kruskal-Wallis Test | 0.74 |
| Weight | Spearman's Non-Parametric Test | 0.86 |
| Relationship between variable and hyperthyroidism | Statistical test | Level of Significance p<0.05 |
|---|---|---|
| Breed | Chi Square Test (Fishers Exact Test) | 0.002 |
| Gender | Chi Square Test (Fishers Exact Test) | 0.38 |
| Age | Mann-Whitney Test | 0.42 |
| Colour | Chi Square Test | 0.54 |
| Neutered Status | Chi Square Test (Fishers Exact Test) | 0.56 |
| Weight | Mann-Whitney Test | 0.68 |
Discussion
This study demonstrated there was a statistically significant association between purity of breed and T4 result. To date, there have been no studies conducted in regards to breed that may be prone to hyperthyroidism, however Mooney and Peterson (2004) reported it being less common in Siamese cats. In contrast, this study demonstrated a much higher prevalence of hyperthyroidism and higher median T4 result in purebreed cats compared with crossbreed cats. Similarly, the most common crossbreed cat (DSH) was found to have a much lower prevalence of hyperthyroidism than cats categorised as ‘other’ (i.e. any breed other than DSH). This suggests that routine testing on purebreed cats could be worthwhile.
Gordon et al (2003) described certain risk factors related to hyperthyroidism that included living indoors, canned cat foods, topical ectoparasitic preparations and the use of litter trays. It is possible that these factors are more common in highly prized purebreed cats than in common crossbreeds which could be evidence for why there was a higher prevalence of the disease amongst this group.
Although hyperthyroidism is reported to be more common in older cats (Peterson, 2006) and rarely seen in cats less than 7 years of age, there was no significant correlation between T4 result and age in the current study of clinically normal cats. This current study concludes that the association between hyperthyroidism and age is not statistically significant and therefore routine testing of hyperthyroidism is not recommended. A possible explanation for hyperthyroid cats being diagnosed middle to old aged, generally older than 10 years old (Peterson, 2006), could be that cats are currently only tested for hyperthyroidism once clinical signs are present, possibly years after the onset of the disease, thus increasing the apparent age of onset of hyperthyroidism compared with the actual age of onset of the disease subclinically. This research cannot recommend an age to routinely test for hyperthyroidism due to the results not being statistically significant with Spearman's Non-Parametric p-value=0.08. However, further studies could look into this using a larger sample.
There was no significant association between hyperthyroidism and T4 result in either gender or neutered status in this study. Research showed male and females are likely to be affected by hyperthyroidism with an equal frequency and the current study could not offer any further evidence to contradict this finding. It can be concluded that there is no indication for preferential screening of cats according to gender or neutered status.
Colour was interesting to examine however there was no statistically significant association to prove hyperthyroidism affects a certain colour of cat. Looking at the results however, it was interesting to note that white, brown, blue and grey cats had the highest prevalence of hyperthyroidism, which can be more commonly associated with purebreed cats, which reinforced the results of the current study that there was a statistically significant association between hyperthyroidism and purebreed cats. To reinforce this, tabby, which can be more commonly associated with crossbreed cats, had a much lower prevalence of hyperthyroidism.
Finally, weight was not statistically associated with hyperthyroidism and therefore no conclusions were drawn to suggest any specific weight range to routine test cats for hyperthyroidism.
Recommendations for clinical practice
Currently, most small animal practices are able to run inhouse blood tests with the option to add on a T4 test when required. There has been no requirement to routinely test all cats for hyperthyroidism to date. In the current study, the practice in Glasgow routinely tested T4 levels in all cats and not only those that presented with clinical signs of the disease over a period of time. From the 202 clinically normal cats reviewed in this study, 76 were found to have elevated T4 levels without recognition from the owners, or without showing common clinical signs of the disease. This shows that 37.6% were subclinical cases picked up on routine bloods and suggests that routine and pre-emptive sampling should be considered. There was not enough evidence to confirm the necessity to perform the test on every single cat, however it should be an option discussed with owners.
A further recommendation for the practice can be seen in research conducted by Scott-Moncrieff (2012) who suggested that for those cats in the upper half of the normal reference range, further observation and work up should be conducted with a repeat test of T4 after a period of 4 to 8 weeks. This can track the progress of the disease as well as provide the opportunity to diagnose early stages of the disease.
From a practice point of view, protocols should look at educating owners of the early signs of hyperthyroidism as well as encouraging frequent health check-ups and not by an age cut-off point as, from this research, there was no association between age and hyperthyroidism amongst clinically normal cats. Therefore practices cannot suggest a recommended age at which to start testing for hyperthyroidism.
Limitations
Routine testing all cats for hyperthyroidism is not a common concept, it is under researched and difficult to measure. Results of the current study have relied solely on one practice's clinical records.
While carrying out the study, research and diagnosis was limited to one in-house T4 blood test result run on a Quantum Saturno 100 VET wet chemistry analyser with a normal range of T4 levels being between 13–48 nmol/litre. A wider study might be limited to only those practices with similar equipment. A confirmation test for those results that came back as being hyperthyroid was then sent to the same external laboratory for a further diagnostic confirmation test. A greater variety of tests may be required to confirm and get a definitive diagnosis as cats may be tested before a serum total T4 concentration exceeds the reference range, as well as there being an increasing number of cats with clinical signs of hyperthyroidism and palpably large thyroid glands whose baseline serum total thyroid hormone concentrations are within the normal or borderline range, making diagnosis difficult.
Suggestions for further research
Future studies should look at conducting multivariable statistics, specifically looking at age in purebreed cats diagnosed as being hyperthyroid in order to find any association along with other risk factors within the purebreed category. It would also be important to examine any significance between the median ages of purebreed cats versus DSH cats.
Colour may relate to breed. If there proves to be a statistical significance to individual breeds then future studies should again conduct multivariable statistics and look at individual colours and their relationship to purebreed or crossbreed cats, and then compare the relationship to T4 results.
Conclusion
This study aimed to gain an understanding of whether routine testing of all cats for hyperthyroidism should be carried out in small animal practice. Previous research indicated that hyperthyroidism was less likely to affect purebreed cats such as Siamese, however, the current study showed that in a sample of 202 clinically normal cats tested for hyperthyroidism, purebreed cats had a much higher prevalence of the disease. As a reult the author recommends that practices should routinely test T4 levels in purebred cats.
This research also provided an understanding that, within the small sample of cats tested, routine testing of all cats for hyperthyroidism did indeed cause an overall improvement in diagnosing the disease as it diagnosed 37.6% of cats as being hyperthyroid without showing any signs of the disease, which would not have been recognised until a much later date, if at all. Again, looking at the sample tested, there was not an increase in diagnosis due to an increase in the number of cats tested but it did highlight the argument for routine testing as 76 out of the 202 clinically normal cats, did, in fact, have hyperthyroidism.