Reference range of serum thyroid hormones profile in domestic cats

Introduction

Thyroid hormones are essential regulators of basal metabolism, growth, thermoregulation, and energy balance in cats. Among feline endocrine disorders, hyperthyroidism is one of the most commonly reported conditions, particularly in middle-aged and geriatric cats (Peterson et al., 1983; Peterson, 2015) [4,8] . Thyroxine (T 4 ), the principal hormone secreted by the thyroid gland, functions largely as a prohormone and is converted peripherally to the biologically active triiodothyronine (T 3 ) (Thoday et al., 1984) [9] . Only the free, non-proteinbound fraction of thyroid hormones is metabolically active and reflects true thyroid status (Thoday et al., 1984; Wakeling et al., 2020) [9,10] . Cat population in India is steadily increasing in recent years. Also the endocrine related disorders reported by the private veterinary clinics are increasing, considering the demand from veterinarians it is necessary to establish physiological reference of various hormone levels in cats for proper interpretation of laboratory reports. Interpretation of thyroid function tests in cats is complicated by age-related variation, hormone fluctuation, and the suppressive effects of non-thyroidal illness (Peterson et al., 1987; Peterson et al., 1990) [5,6] . Moreover, population-specific reference ranges are essential, as environmental, dietary, and methodological differences influence hormone concentrations (Naik, 2016; Alborough et al., 2022) [3,2] . Information on physiological reference ranges for thyroid hormones in domestic cats in india is limited. Hence, the present study was conducted to establish reference ranges for serum TT 3 , TT 4 , and fT 4 in clinically healthy domestic cats.

Materials and Methods

The study was conducted at the Department of Veterinary Biochemistry, Mumbai Veterinary College, Parel, Mumbai, with radioimmunoassay facilities at the Radiation Medicine Centre, Bhabha Atomic Research Centre, Mumbai (INDIA). A total of 300 clinically healthy cats of either sex were selected during June-December 2025 from veterinary hospitals, NGOs (Non-Government Organization), private practitioners, and pet owners. Cats were categorized into three age groups: 1-5 years, 5-10 years, and above 10 years. Animals showing clinical or laboratory evidence of systemic or endocrine disease were excluded. Approximately 3 mL of blood was collected aseptically from the jugular vein. Serum was separated by centrifugation at 3000 rpm for 15 minutes and stored at -20 °C until analysis. Serum TT 3 , TT 4 , and fT 4 concentrations were estimated using BRIT-BARC radioimmunoassay kits following standard protocols. Reference ranges were established using the non-parametric percentile (25 th -75 th percentile) method. Age-wise comparison was carried out using one-way ANOVA.

Results and Discussion

As depicted in fig. 1 [9] ; Peterson et al. (1987) [5] in healthy cats. There was no significant difference (P > 0.05) in TT 3 levels among different age groups of cats. Although younger cats (0-5 years) showed slightly higher values, age did not significantly influence circulating TT 3 levels in clinically healthy cats. As depicted in fig. 2 the Mean± SE of serum TT 4 concentration was 2.06 ± 0.05 µg/dL, with an interquartile range of 1.33-2.81 µg/dL and T-bars extending from 0.40 to 4.13 µg/dL & median as 2.03µg/dl. TT 4 concentrations increased significantly with advancing age (P < 0.05). As depicted in Table 1. The Mean ± SE levels of TT 4 (µg/dl) in cats aged 1-5 years, 5-10 years, and more than 10 years were 1.95 ± 0.06, 2.26 ± 0.10, and 2.51 ± 0.21, respectively. Similar age-associated alterations in TT 4 have been reported previously and may reflect subclinical thyroid stimulation or early hyper functional changes in older cats. The levels of TT 4 in present study are within the range reported by Thoday et al. (1984) [9] , Peterson et al.( 1990) [6] , Naik,(2016) [3] in healthy cats. As depicted in fig. 3 the Mean± SE of serum fT 4 concentration was 1.30 ± 0.01 ng/dL, with an interquartile range of 1.09-1.50 ng/dL and T-bars ranging from 0.63 to 1.94 ng/dL & median as 1.30ng/dl. As depicted in Table 1. The Mean ± SE values of serum fT 4 (µg/dl) in cats aged 1-5 years, 5-10 years, and more than 10 years were 1.28 ± 0.02, 1.31 ± 0.02, and 1.34 ± 0.06, respectively. Free thyroxine values did not show significant age-related variation, supporting its stability as a diagnostic marker. Previous studies have emphasized the diagnostic superiority of fT 4 in differentiating euthyroid and thyroid-diseased cats, particularly in borderline cases. The levels of fT 4 in present study are within the range reported by Peterson et al. (1994) [7] & Wakeling et al. (2020) [10] in healthy cats. Overall, TT 4 emerged as the most age-responsive thyroid hormone, whereas TT 3 and fT 4 remained relatively stable across age groups. These findings are consistent with earlier reports describing hormonal fluctuation, overlap, and the importance of population-specific reference intervals for accurate diagnosis of feline thyroid disorders (Peterson et al., 1987; Aldridge et al., 2015) [5,1] .

Summary and Conclusion

The present study established reference ranges for serum thyroid hormones in domestic cats in India as 48.67-72.81 ng/dL, 1.33-2.81 µg/dL, and 1.09-1.50 ng/dL for TT 3, TT 4, fT 4 respectively. Among the thyroid parameters evaluated, TT 4 exhibited significant age-related variation, whereas TT 3 and fT 4 remained comparatively stable. These reference ranges provide reliable baseline data for clinical evaluation and diagnosis of thyroid dysfunction in cats under Indian conditions.

References

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