The Journal of Nutrition Vol. 128 No. 12 December 1998, pp. 2636S-2638S

Thyroid Hormone Levels in Cats: Colony Average and the Decrease with Age¹

Nicola D. Skinner

Waltham Centre for Pet Nutrition, Waltham-on-the-Wolds, Melton Mowbray, UK, LE14 4RT

KEY WORDS: cats · thyroid · age

	INTRODUCTION

Introduction References

Hyperthyroidism, a multisystemic disorder resulting from the excessive production and secretion of thyroid hormones, is considered to be the most common endocrine disease of middle-aged and old cats, with cats over the age of 6 y at risk. Most cases can be diagnosed by measuring serum thyroid hormone levels together with observation of clinical signs. Therefore, understanding those factors that have the potential to influence thyroid hormone levels is important in establishing normal values to which potentially hyperthyroid cats can be compared and in contributing to a more complete understanding of the aging cat. A number of factors known to influence the serum concentrations of thyroid hormones have been well documented, e.g., nonthyroidal illness (Peterson et al. 1983). However, thyroid hormone levels have not yet been determined in large numbers of cats maintained in the same environment. Published data have indicated a significant similarity between the thyroid hormone levels of cats housed together, compared with values from individually housed cats, although it has been difficult to distinguish between environmental and genetic influences (Thoday et al. 1984). The purpose of this study therefore was to examine thyroid hormone levels of healthy euthyroid cats in different cat populations and establish whether cats housed in the same environment, i.e., colony cats, require specific normal ranges for each colony for assessment of health status. The effect of age on thyroid hormone concentrations in euthyroid cats was also investigated because conflicting results exist for the question whether thyroid hormone levels alter with age (Peterson and Gamble 1990, Thoday et al. 1984).

Materials and methods.  To establish the thyroid hormone levels in colony cats, blood samples were taken from the cephalic vein from two different colony populations of domestic short-haired cats. Food was withheld from the cats for ~16 h before sampling. A group of 117 cats (50 neutered males, 67 entire females), ranging in age from 6 mo to 10 y, were sampled from the Waltham Centre for Pet Nutrition, UK. Thirty-seven cats (19 neutered males, 18 neutered females), ranging in age from 3 to 14 y were sampled from Wodonga cattery, Uncle Ben's of Australia. All cats had been maintained on a variety of nutritionally complete commercial cat foods. The serum concentrations of triiodothyronine (T3),² tetraiodothyronine (T4) and free tetraiodothyronine (FT4) were determined by SCL Bioscience Services Ltd., Cambridge, UK and the Central Diagnostic Laboratory, Victoria, Australia, using the following commercial radioimmunoassay kits: T3, Amersham/Kodak (Buckinghamshire, UK) IM 3001/3004; T4, DPC (Wales, UK) double antibody method KT 4D1/4D5; FT4, Amersham/Kodak monoclonal method, IM 5051/5054 validated for cat serum.

View larger version (12K): [in this window] [in a new window]   Fig 1. The relationship between serum triiodothyronine concentration (T3) and age in cats (panel A); n = 52 with 14 males and 38 females, with a mean age of 6.78 ± 3.62 y. Between the ages of 1.25 and 5.7 y (panel B), there was a significant negative correlation between serum T3 and age using simple regression; n = 22 with 7 males and 15 females. P < 0.005, r2 = 38%. Values are means (n = 2).

View larger version (8K): [in this window] [in a new window]   Fig 2. The significant negative correlation between serum tetraidodothyronine (T4) concentration and age in female cats (n = 38) using simple regression; P < 0.001, r2 = 33%. Values are means (n = 2).

To examine the effect of age on serum thyroid hormone concentrations, a study was conducted only with cats from the Waltham Centre, thereby reducing possible environmental factors that might influence variability of results. To achieve a balanced trial design, 52 cats with ages evenly distributed between 1.25 and 14 y (mean age 6.78 ± 3.62 y) were used for the study. The age and sex distribution of the cats are presented in Table 1. To ensure that all of the cats were of equivalent health status, every cat was given a full veterinary examination and samples were taken for routine blood biochemistry and hematology, feline immumodeficiency virus and feline leukemia virus. Although all the cats at Waltham Centre are virus free, these assays were carried out for the purpose of the completeness of the study. The levels of serum T3, T4 and FT4 were determined as previously described. Samples were taken from each cat for serum thyroid hormone analysis at two intervals 9 wk apart. The two values for each cat were combined and the mean value obtained for comparative purposes. Data were compared by ANOVA to determine any differences between age and sex for all thyroid parameters. Differences were then characterized by using simple regression (Rice 1995) with actual age as the independent variable and thyroid hormone values as the dependent variables. The arbitrary age ranges established (Table 1) to achieve a balanced trial design were not used in statistical analysis.

Results.  The mean serum T3, T4 and FT4 hormone concentrations (± SD), determined for the two colony populations of cats at the Waltham Centre and Wodonga, Australia to establish a normal reference range for each colony, are presented in Table 2. The results from the two different cat populations were comparable.

The health parameters assessed for the 52 Waltham Centre cats used in the study investigating the effect of age on thyroid status indicated that all cats were clinically healthy. All serum thyroid hormone concentrations of these cats were within the normal reference ranges as seen in Table 2 for this cat population. Data were analyzed for the effect of age as well as sex for each hormone level determined for these cats. The serum T3 concentrations are presented in Figure 1 (panel A) for all 52 cats. Multifactor ANOVA determined that only age, and not sex, was a significant influencing factor for T3 values. Regression analysis showed a significant negative correlation between age and T3 levels in the age range 1.25 to 5.7 y (P < 0.005, r² = 38%), with a mean value of 0.53 ± 0.15 nmol/L (Fig. 1, panel B). No correlation was found, however, between age and T3 serum concentrations in older cats, in the range of 6 to 13.6 y. The mean T3 value was 0.46 ± 0.08 nmol/L; within this age range, T3 values appeared to reach a plateau at this lower level after the initial decrease.

Sex and age were found to have significant effects on feline serum T4 concentrations (P < 0.05) in the 52 Waltham Centre cats investigated. The male cats (neutered) had significantly higher T4 values compared with the female cats (entire); the mean values were 41.9 ± 7.6 nmol/L and 37.7 ± 6.2 nmol/L, respectively. To eliminate the effect of sex, regression analysis was carried out separately for male and female data to examine solely the influence of age on serum T4 values. Male serum T4 concentrations showed no significant correlation with age. For female data, Figure 2, a significant negative correlation between age and T4 values was determined (P < 0.001, r² = 33%). The mean value for serum FT4 for all 52 cats was determined as 9.3 ± 1.7 pmol/L. Statistical analysis indicated no effect of age or sex on the serum FT4 concentrations in cats.

Discussion.  Much of the variation in published reference ranges for feline thyroid hormone levels is due to different techniques, assays or cat populations. However despite these differences, the normal ranges of the serum thyroid hormone concentrations, T3, T4 and FT4, of the two cat colony populations used in this study compared well with published data (Jones 1993, Peterson et al. 1983). It does not appear, therefore, that cats housed in similar environments (such as colony cats) require specific reference ranges. These data confirm that the two cat populations used in this study, maintained on commercially prepared pet food, had a healthy thyroid status and are representative of the general pet population in terms of thyroid status.

Interestingly, data from this study showed sex to have a significant effect on serum T4 concentrations, in that female cats exhibited lower values compared with male cats, although data for both groups were within the normal range. No such effect was observed for serum T3 or FT4 concentrations. However, there are many interactions between reproductive function and thyroid function that may affect normal thyroid hormone levels (Johnson 1994), the mechanisms of which are not yet fully understood. Neutering may be an influencing factor. Because the cats used in this study did not include entire male and neutered female cats, it is not possible to distinguish between the possible influence of sex per se or sexual entirety; therefore further studies are required to confirm these findings.

There appeared to be an age effect for both T3 and T4 serum concentrations, whereby the thyroid hormone levels gradually decreased with age, but still remained within the normal expected range. A significant age-related decline for T3 values was observed up to ~5-6 y of age, followed by a plateau, with no correlation between age and T3 values in older cats (>6 y). Serum T4 concentrations for female cats also showed an age-related decline. A significant decline was not observed for male cats, although the smaller data set for male cats may account for the lack of significance. Published data are conflicting regarding the effects of age on the thyroid hormone levels in euthyroid cats. Peterson and Gamble (1990) found no difference in T4 levels between young and old cats, whereas a study carried out by Thoday and colleagues (1984) showed that both T4 and T3 varied with age. The hormone T4 tended to decrease with age up to 5 y, then subsequently rise, although the variability above 9 y of age was greater compared with the lower age groups. T3 levels exhibited an initial decrease to ~5 y followed by a plateau. The T3 data in this study reflected those found by Thoday et al. (1984), but we observed no subsequent rise in T4 values after the age of 5 y. The apparent age-related gradual decline of thyroid hormone levels in healthy cats should be considered when evaluating their thyroid status.

	FOOTNOTES

	LITERATURE CITED

Introduction References

• Johnson C. A. Reproductive manifestations of thyroid disease. Vet. Clin. N. Am. Small Anim. Pract. 1994; 24:509-514[Medline]
• Jones B. Thyroid disease in the cat: diagnosis and treatment. Waltham Int. Focus 1993; 3:2-8
• Peterson M. E., Kintzer P. P., Cavanagh P. G., Fox P. R., Ferguson D. C., Johnson G. E., Becker D. V. Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. J. Am. Vet. Med. Assoc. 1983; 183:103-110[Medline]
• Peterson M. E., Gamble D. A. Effect of nonthyroidial illness on serum thyroxine concentrations in cats: 494 cases. J. Am. Vet. Med. Assoc. 1990; 197:1203-1208[Medline]
• Rice, J. (1995) Linear least squares. In: Mathematical Statistics and Data Analysis (Rice, J., ed.), 2nd ed., pp. 163-176. Duxbury Press, Belmont, CA.
• Thoday, K. L., Seth, J. & Elton, R. A. (1984) Radioimmunoassay of serum total thyroxine and triiodothyronine in healthy cats: assay methodology and effects of age, sex, breed, heredity and environment. J. Small Anim. Pract. 25, 457-472.

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