Relationship between total thyroxine,thyroid palpation and a clinical index in hyperthyroid and healthy cats and cats with other diseases

Abstract

Objectives

Present-day diagnosis of hyperthyroidism is often established in the early stages where clinical signs and physical examination findings typically associated with the disease may not yet be present. The purpose of this study was to investigate thyroid palpation score (TPS), total thyroxine (T4), body weight, body condition score and a clinical scoring index, which assesses severity of illness and quality of life, in untreated hyperthyroid cats, healthy cats and cats with non-thyroidal illness.

Methods

Fifty-five cats with hyperthyroidism, 45 healthy cats and 327 euthyroid cats with non-thyroidal disease were prospectively enrolled. A Kruskal–Wallis test was used to determine any differences between metric data. A χ² test was applied to compare nominal data between the three subgroups. Correlation between two variables was analysed using Spearman’s correlation coefficient.

Results

Almost 80% of the hyperthyroid cats and up to 20% of the healthy cats and cats with non-thyroidal illness had a palpable thyroid gland. Median TPS in hyperthyroid cats was 2, which was significantly higher compared with the other groups. Although there was a significant correlation between TPS and T4 when evaluating all cats, this could not be demonstrated in the three subgroups. Hyperthyroid cats with larger thyroid glands (TPS >3) did not have higher T4 levels. Hyperthyroid cats with lower clinical scores had significantly higher T4 levels compared with hyperthyroid cats with higher scores. Hyperthyroid cats were older and had lower body weights and body condition scores than other cats.

Conclusions and relevance

Hyperthyroid cats were commonly found to have palpable thyroid glands, but they were smaller than previously reported.

Keywords

Hyperthyroidism thyroxine thyroid gland palpation Karnofsky score body condition score

Introduction

Feline hyperthyroidism is a common disease in older cats.^1–7 It is defined as a multisystemic disorder caused from the autonomous production of L-tri-iodothyronine and L-thyroxine (T4) in the thyroid gland. Excessive circulating concentrations of these hormones leads to catabolic metabolism.^2,8–10 This will induce weight loss that, in the earlier stages of the disease, is primarily characterised by muscle wasting and not by a loss of tissue fat. Body weight does not adequately assess body composition and should be accompanied by assessment of the body condition score (BCS), which evaluates the number of fat depots and reflects the weight of the cat in proportion to its body size.¹¹

Autonomous function of the thyroid is usually caused by benign adenomas or adenomatous hyperplasia leading to an increased thyroid gland size.¹² Organomegaly of the thyroid gland can by assessed by palpation.^13,14 There is evidence that the likelihood of hyperthyroidism increases with increasing thyroid size and that most hyperthyroid cats have a palpable goiter.^14–16 In hyperthyroid cats, the size of the thyroid gland and T4 concentrations will increase over time.¹⁷ Previous studies reported a palpable thyroid gland in >90% of hyperthyroid cats,^8,14 and in up to 38–60% of euthyroid cats.^8,18

Owing to the greater awareness of clinicians of this disease and the inclusion of T4 concentration into feline geriatric blood profiles, hyperthyroidism is now diagnosed early before a goitre is present on physical examination. It is currently unknown whether severity of the disease and its impact on the cat’s quality of life are also linked to increasing T4 concentrations.

Therefore, the aim of this study was to determine the thyroid palpation score (TPS), T4 concentration, body weight, BCS and a clinical scoring index (Karnofsky score [KS]) that assesses severity of illness and quality of life in untreated hyperthyroid cats, healthy cats and cats with other diseases (non-thyroidal illness), and to compare results between the three groups and assess possible relationships between these factors.

Materials and methods

Study design and selection of animals

Client-owned cats that were presented to the Clinic of Small Animal Medicine, Ludwig Maximilian University, Munich, between September 2010 and April 2013 were included in this prospective study. Inclusion criteria were cats aged 8 years or older and cats presenting for any indication for blood sampling. Cats were presented with various diseases or for preventive health care. Consent forms were signed by all owners before their cats entered the study. The study was approved by the ‘Regierung von Oberbayern’, Germany.

All cats were then assigned to one of three groups: cats with newly diagnosed hyperthyroidism (n = 55; group 1), healthy cats (n = 45; group 2), and euthyroid cats with other diseases (also addressed as non-thyroidal illness [n = 327; group 3]).

Hyperthyroid cats showed clinical signs of hyperthyroidism, such as weight loss, polyphagia, polydipsia, polyuria, diarrhoea or vomiting, and had an elevated T4 level (>4.7 µg/dl). All cats were newly diagnosed and had not been treated for hyperthyroidism. Hyperthyroid cats with significant concurrent disease (eg, neoplasia, chronic renal disease) were excluded from the study as these diseases could suppress T4 levels.

Healthy cats showed no history or clinical signs of illness, their physical examination was unremarkable, their results of total blood count and biochemistry profile were within normal limits, and their T4 levels were not elevated.

Ill cats that were euthyroid were diagnosed with diseases other than hyperthyroidism. Hyperthyroidism was excluded in this group of cats based on establishment of another diagnosis and T4 values within or below the normal interval. Cats in this group had gastrointestinal disease (n = 70), urogenital disease (n = 64), respiratory disease (n = 19), systemic infections (n = 18), endocrine disease other than hyperthyroidism (n = 4), cardiac disease (n = 24), neoplasia (n = 60), neurological disease (n = 20), ophthalmological disease (n = 3), and orthopaedic (n = 23) or dermatological disease (n = 2).

Physical examination and assessment of body weight, BCS and TPS

Physical examination was performed on the basis of a standardised protocol. Cats were weighed on a calibrated digital scale (Digital Scale for Small Animals, Model MBT 20; Adam Equipment). For each cat, BCS and TPS were determined. For determination of the BCS, a semi-quantitative method for evaluating body composition using a 9 point scale (1–3 = under ideal, 4–5 = ideal, 6–9 = over ideal) was utilised.^11,19,20

Thyroid gland palpation was performed using the classic technique with the cat in a sitting position, the neck extended and the clinician’s thumb and forefinger sweeping downwards to the trachea on each side from the larynx to the sternal manubrium.^15,21,22 The presence of a goitre was demonstrated if a mobile subcutaneous nodule was palpated. The thyroid gland size was scored using a semi-quantitative estimation, as previously described,¹⁶ being a refinement of an earlier description.¹⁴ The scoring system ranged from 0 (non-palpable) to a maximum of 6 (nodule >25 mm) with score 1 = 1–3 mm, score 2 = 3–5 mm, score 3 = 5–8 mm, score 4 = 8–12 mm and score 5 = 12–25 mm. If the thyroid glands were not equal in size, thyroid size was recorded based on the size of the larger gland, as suggested previously.¹⁶

Assessment of KS

In order to assess the wellbeing of all cats, the KS modified for cats was calculated. The KS originally determined the ability of a human patient with cancer to carry on normal activities in life during and after chemotherapy by using a scale from 0–100%.²³ It was modified for cats by Hartmann and Kuffer.²⁴ The index enables judgement of quality of life and rates health status in cats. The score consists of two parts. One score was given by one clinician, determining the cat’s physical condition. A second score was given after the owner had completed a questionnaire with behaviour-based questions, such as eating, playing, sleeping, grooming and social behaviour. This second part evaluated the degree of quality of life. Both parts of the score accounted for 50% of the total score. The total score was expressed in a percentage ranging from 100% (healthy cat with normal behaviour) to 0% (death). The score was calculated for 16 hyperthyroid, 18 healthy and 18 euthyroid, ill cats. The score was always assessed by the same clinician.

Analysis of thyroid hormones

Baseline serum total T4 were determined in all cats. Serum total T4 concentration was measured by a homogenous enzyme immunoassay, which has been validated previously by an external laboratory (Vet Med Labor GmbH Division of IDEXX Laboratories).²⁵ The reference interval for T4 was 0.8–4.7 µg/dl (10.3–60.5 nmol/l).

Statistical analysis

All statistical analyses were performed using proprietary statistical software (MS Excel), GraphPad Prism Version 5.04 and MedCalc Version 15.

Data were not normally distributed; therefore, all analyses used non-parametric tests. Column statistics were prepared for age, weight, BCS, TPS, KS and T4 for all cats and in the three subgroups. Results are reported as median and interquartile range (IQR; 25th–75th percentile).

The Kruskal–Wallis test, followed by the Dunn multiple comparisons test, was used to determine whether age, weight, BCS, TPS, KS and T4 differed among cats with hyperthyroidism, healthy cats and cats with non-thyroidal illness. A χ² test was performed to compare BCS, breed and sex between the different study groups. Correlation between two variables was analysed using Spearman’s correlation coefficient. These results were regarded as meaningful if significance was demonstrated and if a moderate (r >0.5) or strong (r >0.7) correlation coefficient was present. The Mann–Whitney test was used to determine if T4 concentrations differed between hyperthyroid cats with smaller and larger thyroid glands. It was also applied to analyse if euthyroid ill cats and hyperthyroid cats with more severe illness (lower KS) had lower or higher T4 concentrations. Statistical significance was defined as P ⩽0.05. For the correlation analysis, the level of the P values was adjusted according to the Bonferroni–Holm method.

Results

Column statistics and assessment of differences between groups

Column statistics of age, sex, breed, weight, BCS, TPS, KS and T4 of all cats, of group 1, group 2 and group 3, and potential significant differences between the different subgroups are given in Table 1. Hyperthyroid cats were older and had lower body weights and BCSs than other cats. Figure 1 shows box and whisker plots of the KS in hyperthyroid cats, healthy cats and cats with non-thyroidal illness.

Table 1

Column statistics for signalment, weight, body condition score (BCS), thyroid palpation score (TPS), Karnofsky score (KS) and thyroxine (T4) in all cats, hyperthyroid cats (group 1), healthy cats (group 2) and cats with non-thyroidal illness (group 3)

	All cats	Group 1hyperthyroid	Group 2healthy	Group 3non-thyroidal illness	Comparisonbetween groups (P value)
Median (IQR) age (years)	13 (11–15) (n = 427)	15 (13–17) (n = 55)	11 (9–14) n = 45	13 (11–14) n = 327	1 vs 2: <0.001 1 vs 3: <0.001 2 vs 3: <0.05
Sex (% male)	54.1 (n = 231/427) MC: 213 FS: 173	45.5 (n = 25/55) MC: 25 FS: 26	44.4 (n = 20/45) MC: 20 FS: 20	56.9 (n = 186/327) MC: 168 FS: 127	1 vs 2: NS 1 vs 3: NS 2 vs 3: NS
Breed (% DSH)	78.7 (n = 336/427)	89.1 (n = 49/55)	66.7 (n = 30/45)	78.6 (n = 257/327)	1 vs 2: <0.01 1 vs 3: NS 2 vs 3: <0.05
Median (IQR) weight (kg)	4.3 (3.36–5.27) (n = 386)	3.51 (2.5–4.6) (n = 54)	4.75 (3.9–5.6) (n = 39)	4.5 (3.46–5.34) (n = 293)	1 vs 2: <0.001 1 vs 3: <0.001 2 vs 3: NS
Median (IQR) BCS (1–9)	4 (3–6) (n = 353) Under ideal: 102 (28.9%) Ideal: 159 (45.0%) Over ideal: 92 (26.1%)	4 (2.5–4) (n = 49) Under ideal: 24 (49.0%) Ideal: 18 (36.7%) Over ideal: 7 (14.3%)	5 (4–5) (n = 37) Under ideal: 4 (10.8%) Ideal: 26 (70.3%) Over ideal: 7 (18.9%)	4 (3–6) (n = 267) Under ideal: 74 (27.7%) Ideal: 115 (43.1%) Over ideal: 78 (29.2%)	1 vs 2: <0.01 1 vs 3: <0.01 2 vs 3: NS
TPS (0–6)	PNP = 48 (12.6%) Median = 0 IQR = 0–0.75 (n = 332) (87.4%) score 0 = 249 (75%) score 1 = 32 (9.6%) score 2 = 22 (6.6%) score 3 = 17 (5.1%) score 4 = 7 (2.1%) score 5 = 4 (1.2%) score 6 = 1 (0.3%)	PNP = 8 (14.6%) Median = 2 IQR = 1–3 (n = 47) (85.5%) score 0 = 10 (21.3%) score 1 = 10 (21.3%) score 2 = 11 (23.4%) score 3 = 6 (12.8%) score 4 = 6 (12.8%) score 5 = 3 (6.4%) score 6 = 1 (2.1%)	PNP = 5 (11.1%) Median = 0 IQR = 0–0 (n = 40) (88.9%) score 0 = 32 (80%) score 1 = 3 (7.5%) score 2 = 3 (7.5%) score 3 = 2 (5%) score 4 = 0 score 5 = 0 score 6 = 0	PNP = 35 (12.5%) Median = 0 IQR = 0–0 (n = 245) (87.5%) score 0 = 207 (84.5%) score 1 = 19 (7.8%) score 2 = 8 (3.3%) score 3 = 9 (3.7%) score 4 = 1 (0.4%) score 5 = 1 (0.4%) score 6 = 0	1 vs 2: <0.001 1 vs 3: <0.001 2 vs 3: NS
Median (IQR) KS (0–100%)	91.62 (71.43–100) (n = 52)	71.95 (52.3–92.29) (n = 16)	100 (100–100) (n = 18)	89.53 (69.66–91.62) (n = 18)	1 vs 2: <0.001 1 vs 3: NS 2 vs 3: <0.001
Median (IQR) T4 (RI = 0.8–4.7 µg/dl)	1.9 (1.4–2.5) (n = 427)	7.6 (6.1–11) (n = 55) Above RI: 55 (100%)	2 (1.65–2.7) (n = 45) Below RI: 1 (2.2%) Within RI: 44 (97.8%)	1.7 (1.3–2.2) (n = 327) Below RI: 33 (10.1%) Within RI: 294 (89.9%)	1 vs 2: <0.001 1 vs 3: <0.001 2 vs 3: <0.05

Comparison between groups was performed using the Kruskal–Wallis test with Dunn’s post-hoc test (age, weight, BCS, TPS, KS and T4) or the χ² test (sex, breed)

IQR = interquartile range; MC = male castrated; FS = female spayed; NS = not significant; DSH = domestic shorthair; PNP = palpation not possible; RI = reference interval

Figure 1

Box and whisker plots of the Karnofsky score in hyperthyroid cats, healthy cats and cats with non-thyroidal illness. There was a significant difference (P <0.001) between hyperthyroid and healthy cats, and healthy cats and cats with non-thyroidal illness

Assessment of TPS

A palpable thyroid was present in 78.7% of hyperthyroid cats, in 20% of healthy cats and in 15.5% of cats with non-thyroidal illness. Overall, palpation of the thyroid was possible in 85.5–88.9% of cats. No adverse reactions to palpation were noted. Median TPS in hyperthyroid cats was 2 (3–5 mm), which was significantly higher compared with the other groups (Table 1).

Relationship between age, weight, BCS, TPS, KS and T4 within the different groups

After the correction of P values, several significant correlations for the entire cat population and within the different groups were found (Table 2 and Supplementary Tables 1–4). Although there was a significant correlation between TPS and T4 when evaluating all cats, this was not demonstrated for hyperthyroid cats or healthy cats.

Table 2

Spearman’s correlation coefficient between age, weight, body condition score (BCS), thyroid palpation score (TPS), Karnfosky score (KS) and thyroxine (T4) in all cats (All), hyperthyroid cats (HT), healthy cats (HC) and cats with non-thyroidal illness (NTI)

		Age (years)	Weight (kg)	BCS (1–9)	TPS (0–6)	KS (0–100)
Weight (kg)	All	−0.28* (386)
	HT	−0.60 * (54)
	HC	−0.04(39)
	NTI	−0.18* (293)
BCS(1–9)	All	−0.22* (353)	0.75 * (349)
	HT	−0.43* (49)	0.85 * (49)
	HC	−0.05(37)	0.65 * (35)
	NTI	−0.12(267)	0.74 * (265)
TPS(0–6)	All	0.20* (332)	−0.22* (315)	−0.24* (299)
	HT	0.03(47)	−0.05(46)	−0.17(44)
	HC	0.07(40)	−0.26(37)	−0.4(37)
	NTI	0.08(245)	−0.10(232)	−0.11(218)
KS(0–100%)	All	−0.62 * (52)	0.46* (52)	0.59 * (52)	−0.46* (52)
	HT	−0.64 * (16)	0.31(16)	0.50(16)	−0.44(16)
	HC	−0.36(18)	−0.31(18)	−0.44(18)	0.13(18)
	NTI	−0.13(18)	0.30(18)	0.34(18)	−0.17(18)
T4(RI = 0.8–4.7 µg/dl)	All	0.19* (427)	−0.11* (386)	−0.09(353)	0.44* (332)	−0.36* (52)
	HT	0.05(55)	−0.32(54)	−0.27(49)	0.10(47)	−0.36(16)
	HC	0.32(45)	0.25(39)	0.15(37)	0.05(40)	−0.26(18)
	NTI	0.02(327)	0.01(293)	0.08(267)	0.23* (245)	−0.05(18)

Number of cats is given in brackets. Bold = r ⩾0.5

Correlation is significant after Bonferroni–Holm correction

RI = reference interval

Difference between T4 concentrations in hyperthyroid cats with smaller and larger thyroid glands

Hyperthyroid cats were grouped based on the size of their thyroid palpation score (‘⩽2’ and ‘⩾3’, and ‘⩽1’ and ‘⩾2’) and their corresponding T4 concentrations were compared. Results are given in Table 3. Hyperthyroid cats with larger thyroid glands (TPS >3) did not have higher T4 levels.

Table 3

Difference between thyroxine (T4) concentrations in hyperthyroid (HT) cats with smaller and larger thyroid glands

	HTTPS ⩽1	HTTPS ⩾2	HTTPS ⩽2	HTTPS ⩾3	Comparison between groups (P value)
T4(RI = 0.8–4.7 µg/dl)	7.4(5.68–12.38)n = 20	8.8(6.3–12.2)n = 27	7.4(5.9–12.2)n = 31	9.3(6.33–12.2)n = 16	⩽1 vs ⩾2: NS⩽2 vs ⩾3: NS

HT cats were grouped based on the size of their thyroid palpation score (TPS) (‘⩽2’ and ‘⩾3’, and ‘⩽1’ and ‘⩾2’). Comparison between groups was performed using the Kruskal–Wallis test with Dunn’s post-hoc test. Data are median (interquartile range) unless otherwise indicated

NS = non-significant; RI = reference interval

Difference between T4 concentrations in hyperthyroid cats and cats with non-thyroidal illness with different quality of life grades

Hyperthyroid cats and cats with non-thyroidal illness were each grouped into two groups based on the severity of their underlying disease and impact on their quality of life (KS ‘<80’ and ‘⩾80’). Results are shown in Table 4. Hyperthyroid cats with lower clinical scores had significantly higher T4 levels compared with hyperthyroid cats with higher scores.

Table 4

Difference between T4 concentrations in hyperthyroid cats (HT) and cats with non-thyroidal illness (NTI) with different quality of life scores (Karnofsky score [KS])

	HTKS <80	HTKS ⩾80	NTIKS <80	NTIKS ⩾80	Comparison between groups (P value)
T4(RI = 0.8–4.7 µg/dl)	8.9(7.75–11.8)n = 9	6.2(4.9–7.4)n = 7	1.5(1.3–1.8)n = 7	1.7(1.2–2.3)n = 11	HT: <80 vs ⩾80: <0.05NTI: <80 vs ⩾80: NS

Hyperthyroid cats and cats with non-thyroidal illness were each grouped into two groups based on the severity of their underlying disease and their quality of life (KS ‘<80’ and ‘⩾80’). Comparison between groups was performed using the Kruskal–Wallis test with Dunn’s post-hoc test. Data are median (interquartile range) unless otherwise indicated

NS = not significant; RI = reference interval

Discussion

Palpable thyroid glands are suggestive of clinical hyperthyroidism, with previous studies reporting a palpable thyroid gland in >90% of hyperthyroid cats,^8,14 and their palpation scores were often high (⩾5–8 mm).¹⁴ Results of the present study showed that 80% of hyperthyroid cats had a palpable thyroid gland vs healthy cats and cats with non-thyroidal illness, in which the thyroid was palpable in only 20% and 16% of cases, respectively. Hyperthyroid cats also had a significantly higher TPS score compared with the other groups. However, in most hyperthyroid cats the TPS was ⩽2 (⩽5 mm). If thyroid palpation is not thoroughly performed, the small nodule can be missed in many cats. None of the healthy cats had a score >3 and only 2/245 cats with non-thyroidal illness had a score >3, illustrating the high specificity for diagnosing hyperthyroidism if a TPS of >3 is found.

Palpation of the thyroid was possible in 85–90% of all cats examined. In accordance with two previous studies, the majority of hyperthyroid cats have a palpable goitre, although the percentage was lower in the present study (78.7% vs 97.6% and 95%, respectively),^8,14 which might indicate that cats are diagnosed nowadays at an earlier disease stage in which the thyroid gland is not enlarged in every affected cat. However, ectopic thyroid tissue or thyroid gland slipping into the thoracic cavity must also be considered for hyperthyroid cats with non-palpable thyroid glands.²⁶ In contrast to the studies by Chaitman et al and Norsworthy et al,^14,18 in which 38–60% of euthyroid cats had a palpable thyroid, this could be demonstrated in the present study in only 20% of healthy cats and in 15.5% of cats with other diseases. It is possible that some of the euthyroid cats in the previous studies suffered from hyperthyroidism but their T4 levels were within the reference interval, owing to an early stage of the disease or concurrent illness.

The likelihood of hyperthyroidism increases with the presence of a palpable goitre, and thyroid gland palpation should be performed in every cat suspected of having hyperthyroidism. However, the present study also shows that hyperthyroidism should not be excluded based on non-palpable or small thyroid gland size. A recent consensus statement reinforces the significance of a palpable goitre and recommends follow-up in cats with a palpable thyroid even if no signs compatible with hyperthyroidism are currently present and T4 is not elevated.27

In the present study, the diagnosis of hyperthyroidism was based on an elevated T4 concentration in combination with the presence of clinical signs. Although there were significant correlations between TPS and T4 in all cats, this could not be demonstrated in hyperthyroid cats, rendering this finding clinically unimportant.

There was also no difference between the T4 concentration in hyperthyroid cats with smaller and larger thyroid glands. Therefore, the magnitude of T4 secretion is not dependent on the size of abnormal thyroid tissue and is more dependent on the functional status of the tissue. Studies on thyroid scintigraphy found a correlation between the magnitude of technetium uptake and serum T4 and the number of hot foci were associated with an increased thyroid volume.28 –30 It is possible that the differences in thyroid volume in this study were too small to be appreciated during palpation, although it has been demonstrated that thyroid palpation correlates well with ultrasonography.¹⁵ It is also possible that in the present study many cats had small but very functional hot foci that did not increase thyroid volume significantly.

It is known that T4 concentrations increases with disease duration.²⁹ In the present study, only newly diagnosed untreated hyperthyroid cats with no evidence of concurrent disease were enrolled to exclude any impact on the T4 level by previous treatments or other diseases. It is therefore possible that the duration of disease was too short to demonstrate a correlation between thyroid size and serum T4.

Only in the subgroup of cats with non-thyroidal illness was a positive correlation between size of the thyroid gland and T4 observed. However, the degree of correlation was small, limiting its clinical significance. It could potentially indicate that larger thyroid glands produce more thyroid hormones; however, the correlation between thyroid gland size and serum T4 is difficult to interpret as these cats were ill and a suppression of their T4 concentrations was probably present. In a study by Boretti et al,¹⁶ a significant correlation between TPS and T4 was observed in the entire study population, which was composed of cats presented for clinical signs consistent with hyperthyroidism, but diagnosis was confirmed in only about 10% of cases. Correlation of TPS and T4 was not evaluated for the truly hyperthyroid cats.¹⁶

As expected, the mean KS was almost 100% in healthy cats, which indicates that physical examination results performed by the veterinarian and the assessment of the cats’ behaviour by the owner were unremarkable. Hyperthyroid cats and cats with non-thyroidal illness had significantly lower scores, and their underlying disease obviously affected their health status and quality of life. In the present study, hyperthyroid cats with lower KS had higher T4 levels than hyperthyroid cats with higher KS, although there was no linear relationship between T4 levels and the KS. However, this indicates that higher T4 levels have more of an impact on quality of life and health status than lower levels. Although this had been assumed previously, this is the first study to analyse T4 concentration in relation to the severity of illness in hyperthyroid cats. In the group of cats with non-thyroidal illness, there was no difference in T4 for higher or lower KS. The validity of the KS has been questioned lately owing to inter-observer differences when the physical condition of cats is assessed.³¹ It is a subjective measure, but its validity increases if the evaluation of the study population is always performed by the same veterinarian, as was the case in the present study.

Although only cats older than 8 years were included, hyperthyroid cats were still significantly older than healthy cats and cats with other diseases. This is an expected finding as hyperthyroidism occurs mainly in geriatric cats with most patients being aged 13 years and older.^2,5–8,13

Hyperthyroid cats weighed less and had lower BCSs than healthy cats and cats with other diseases. Although there was a significant correlation between weight and BCS in the entire study population and in all subgroups, weight and BCS cannot be used interchangeably. Body composition is not properly reflected by body weight as purebred cats might weigh more owing to their larger stature. The BCS allows assessment of an ideal, under-ideal or over-ideal body composition. The proportion of under-ideal cats was highest in the group of hyperthyroid cats (49%) vs healthy cats (11%) and cats with other diseases (30%). The proportion of under-ideal hyperthyroid cats in this study slightly exceeded the percentage of 30% that was described in a previous study.³² Interestingly the proportion of obese cats was highest in cats with non-thyroidal illness (15% in hyperthyroid cats, 20% in healthy cats and 30% in cats with non-thyroidal illness), which illustrates that obesity and being overweight might be general risk factors for the development of illness. There was an inverse relationship between age and weight in hyperthyroid cats. Older hyperthyroid cats weighed less than younger cats, which may be due to older cats being more affected by the catabolic impact of their disease or suffering from a longer duration of hyperthyroidism at the time of diagnosis.

The number of cats from different breeds was not equal between the groups as there were fewer pedigree cats in the hyperthyroid group. This supports other studies in which hyperthyroidism occurred mainly in domestic shorthair cats, and pedigree cats were less commonly affected.^3,6,33 There was an equal sex distribution in the three groups.

Some limitations of the present study need to be considered. Cats with an early stage or mild form of hyperthyroidism could have been overlooked if their T4 was not elevated. Establishment of the diagnosis was based on an elevated T4 value and not on thyroid scintigraphy, which remains the gold standard.^28,34,35 The assessment of BCS, TPS and KS are all subjective measures, and the reliability of any subjective measure can be questioned. However, all of these examinations were either assessed by the same or a very experienced clinician throughout the study.

Conclusions

It was demonstrated in the present study that thyroid gland enlargement can be found in up to 80% of cats with hyperthyroidism. However, in contrast to earlier reports, the thyroid glands in most of the affected cats were small (⩽5 mm length). This finding suggests that the disease is nowadays diagnosed earlier, before severe thyroid gland enlargement occurs. In contrast, only 15–20% of cats without hyperthyroidism were found to have a palpable thyroid gland. In hyperthyroid cats, the size of the thyroid gland was not found to correlate with T4 levels. However, the results showed that the level of T4 in hyperthyroid cats influenced their health status.

Supplemental Material

Supplementary Tables 1-4

Supplementary tables 1-4, which show the Spearman’s correlation coefficient between age, weight, BCS, TPS, KS and T4 for all cats, hyperthyroid cats, healthy cats and cats with non-thyroidal illnesses, respectively.

Footnotes

Accepted: 14 August 2018

Supplementary material

Supplementary tables 1–4, which show the Spearman’s correlation coefficient between age, weight, BCS, TPS, KS and T4 for all cats, hyperthyroid cats, healthy cats and cats with non-thyroidal illnesses, respectively.

Conflict of interest

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

This research was partially funded by MSD Animal Health.

ORCID iD

Astrid Wehner

References

Scarlett

JM.

Epidemiology of thyroid diseases of dogs and cats. Vet Clin North Am Small Anim Pract 1994; 24: 477–486.

Peterson

Kintzer

Cavanagh

et al . Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. J Am Vet Med Assoc 1983; 183: 103–110.

Edinboro

Scott-Moncrieff

Janovitz

et al . Epidemiologic study of relationships between consumption of commercial canned food and risk of hyperthyroidism in cats. J Am Vet Med Assoc 2004; 224: 879–886.

Martin

Rossing

Ryland

et al . Evaluation of dietary and environmental risk factors for hyperthyroidism in cats. J Am Vet Med Assoc 2000; 217: 853–856.

McLean

Lobetti

Mooney

et al . Prevalence of and risk factors for feline hyperthyroidism in South Africa. J Feline Med Surg 2017; 19: 1103–1109.

Kohler

Ballhausen

Stockhaus

et al . Prevalence of and risk factors for feline hyperthyroidism among a clinic population in Southern Germany. Tierarztl Prax Ausg K Kleintiere Heimtiere 2016; 44: 149–157.

Bree

Gallagher

Shiel

et al . Prevalence and risk factors for hyperthyroidism in Irish cats from the greater Dublin area. Ir Vet J 2018; 71: 2.

Thoday

Mooney

CT.

Historical, clinical and laboratory features of 126 hyperthyroid cats. Vet Rec 1992; 131: 257–264.

Hoenig

Goldschmidt

Ferguson

et al . Toxic nodular goitre in the cat. J Small Anim Pract 1982; 23: 1–12.

10.

Peterson

Ward

CR.

Etiopathologic findings of hyperthyroidism in cats. Vet Clin North Am Small Anim Pract 2007; 37: 633–645.

11.

LaFlamme

DP.

Development and validation of a body condition score system for cats: a clinical tool. Feline Practice 1997; 25: 13–18.

12.

Feldman

Nelson

RW.

Feline hyperthyroidism (thyrotoxicosis). In: Feldman

Nelson

(eds). Canine and feline endocrinologiy and reproduction. 3rd ed. Philadelphia, PA: Saunders, 2004, pp 152–218.

13.

Broussard

Peterson

Fox

PR.

Changes in clinical and laboratory findings in cats with hyperthyroidism from 1983 to 1993. J Am Vet Med Assoc 1995; 206: 302–305.

14.

Norsworthy

Adams

McElhaney

et al . Relationship between semi-quantitative thyroid palpation and total thyroxine concentration in cats with and without hyperthyroidism. J Feline Med Surg 2002; 4: 139–143.

15.

Paepe

Smets

van Hoek

et al . Within- and between-examiner agreement for two thyroid palpation techniques in healthy and hyperthyroid cats. J Feline Med Surg 2008; 10: 558–565.

16.

Boretti

Sieber-Ruckstuhl

Gerber

et al . Thyroid enlargement and its relationship to clinicopathological parameters and T(4) status in suspected hyperthyroid cats. J Feline Med Surg 2009; 11: 286–292.

17.

Peterson

Guterl

Nichols

et al . Evaluation of serum thyroid-stimulating hormone concentration as a diagnostic test for hyperthyroidism in cats. J Vet Intern Med 2015; 29: 1327–1334.

18.

Chaitman

Hess

Senz

et al . Thyroid adenomatous hyperplasia in euthyroid cats. J Vet Intern Med 1999; 13: 242.

19.

Bjornvad

Nielsen

Armstrong

et al . Evaluation of a nine-point body condition scoring system in physically inactive pet cats. Am J Vet Res 2011; 72: 433–437.

20.

German

Holden

Moxham

et al . A simple, reliable tool for owners to assess the body condition of their dog or cat. J Nutr 2006; 136: 2031S–2033S.

21.

Peterson

Randolph

Mooney

CT.

Endocrine diseases. In: Sherding

(ed). The cat: diseases and clinical management. 2nd ed. New York: Churchill Livingstone, 1994, pp 1404–1506.

22.

Baral

Peterson

. Thyroid gland disorders. In: Little

(ed). The cat: clinical medicine and management. St Louis, MO: Elsevier, 2012, pp 571–592.

23.

Timmermann

‘Just give me the best quality of life questionnaire’: the Karnofsky scale and the history of quality of life measurements in cancer trials. Chronic Illn 2013; 9: 179–190.

24.

Hartmann

Kuffer

Karnofsky’s score modified for cats. Eur J Med Res 1998; 3: 95–98.

25.

Horney

MacKenzie

Burton

et al . Evaluation of an automated, homogeneous enzyme immunoassay for serum thyroxine measurement in dog and cat serum. Vet Clin Pathol 1999; 28: 20–28.

26.

Naan

Kirpensteijn

Kooistra

et al . Results of thyroidectomy in 101 cats with hyperthyroidism. Vet Surg 2006; 35: 287–293.

27.

Carney

Ward

Bailey

et al . 2016 AAFP guidelines for the management of feline hyperthyroidism. J Feline Med Surg 2016; 18: 400–416.

28.

Bettencourt

Daniel

Panciera

et al . Evaluation of thyroid to background ratios and comparison of various scintigraphic measurements and their correlation to serum T4 in hyperthyroid cats. Vet Radiol Ultrasound 2016; 57: 290–298.

29.

Peterson

Broome

Rishniw

Prevalence and degree of thyroid pathology in hyperthyroid cats increases with disease duration: a cross-sectional analysis of 2096 cats referred for radioiodine therapy. J Feline Med Surg 2016; 18: 92–103.

30.

Volckaert

Vandermeulen

Dobbeleir

et al . Effect of thyroid volume on radioiodine therapy outcome in hyperthyroid cats. J Feline Med Surg 2016; 18: 144–149.

31.

Taffin

Paepe

Campos

et al . Evaluation of a modified Karnofsky score to assess physical and psychological wellbeing of cats in a hospital setting. J Feline Med Surg 2016; 18: 913–920.

32.

Peterson

Castellano

Rishniw

Evaluation of body weight, body condition, and muscle condition in cats with hyperthyroidism. J Vet Intern Med 2016; 30: 1780–1789.

33.

Kass

Peterson

Levy

et al . Evaluation of environmental, nutritional, and host factors in cats with hyperthyroidism. J Vet Intern Med 1999; 13: 323–329.

34.

Daniel

Sharp

Nieckarz

et al . Quantitative thyroid scintigraphy as a predictor of serum thyroxin concentration in normal and hyperthyroid cats. Vet Radiol Ultrasound 2002; 43: 374–382.

35.

Peterson

Guterl

Rishniw

et al . Evaluation of quantitative thyroid scintigraphy for diagnosis and staging of disease severity in cats with hyperthyroidism: comparison of the percent thyroidal uptake of pertechnetate to thyroid-to-salivary ratio and thyroid-to-background ratios. Vet Radiol Ultrasound 2016; 57: 427–440.

Supplementary Material

Please find the following supplemental material available below.

For Open Access articles published under a Creative Commons License, all supplemental material carries the same license as the article it is associated with.

For non-Open Access articles published, all supplemental material carries a non-exclusive license, and permission requests for re-use of supplemental material or any part of supplemental material shall be sent directly to the copyright owner as specified in the copyright notice associated with the article.

0.00 MB

0.02 MB