Thyroid Hormone and Thyrotropin Concentrations and Responses to Thyrotropin Releasing Hormone in Ageing Horses

Reasons for performing study Thyroid hormones ( TH s) decrease with age in healthy dogs and cats, although they tend to remain within established reference ranges. Thyrotropin ( TSH ) is increased in elderly people, with or without mild alterations in TH s. Objectives To test the hypothesis that geriatric horses will have lower TH s and/or higher TSH compared with younger horses. Study design Cross‐sectional study. Methods Resting TH s and TSH , and responses to thyrotropin releasing hormone ( TRH ) were compared between young and old horses. Data from 71 normal, healthy horses that had participated in prior research projects were examined, and found to contain 42 horses 3–10 years of age, 16 horses >15 years and 10 horses >20 years. All samples had been assayed in the same previously validated radioimmunoassays. Statistical analysis was performed with commercial software. Results Although lower, TH s were not significantly different between young and old horses when compared with the Mann–Whitney rank sum test. However, TSH concentration was significantly higher in horses aged >20 (median 0.52 ng/ml) or >15 years (0.44 ng/ml) compared to younger horses (0.33 ng/ml). TRH stimulation tests were performed in 19 of the younger horses, 7 of the horses >20, and 11 of the horses >15. Two‐way repeated measures ANOVA on ranks revealed no significant differences in TH responses to TRH . The TSH response to TRH appeared to be slightly greater in the older horses, but did not quite reach significance (P = 0.06). Conclusion Similar to reports in elderly humans, aged horses have higher serum TSH concentrations than younger horses. Ethical animal research:  All horses in this study were part of a NCSU IACUC approved study. Owners gave informed consent for their horses' inclusion in the study. Sources of funding:  College of Veterinary Medicine North Carolina State University, USA Equestrian. Competing interests:  None declared.