Effect of Age, Season, Body Condition, and Endocrine Status on Serum Free Cortisol Fraction and Insulin Concentration in Horses

Abstract

Background: Increased free cortisol fraction is associated with insulin dysregulation (ID) in people with Metabolic Syndrome and Cushing's Disease. Free cortisol has not been investigated in equine endocrine disorders.

Hypotheses: (1) In healthy horses, sex, age, body condition score (BCS), and season impact free cortisol; (2) free cortisol is increased in horses with Pituitary Pars Intermedia Dysfunction (PPID) or Equine Metabolic Syndrome (EMS).

Animals: Fifty-seven healthy horses; 40 horses and ponies with PPID (n = 20) or EMS (n = 20).

Methods: Prospective study. Serum collected seasonally from healthy animals and archived serum from PPID and EMS animals was analyzed for insulin, total and free cortisol concentrations, and free cortisol fraction (FCF). Linear mixed models were used to determine effects of age, sex, season, and BCS on hormones in controls. Hormone measurements were compared between disease groups and age- and season-matched controls with t-tests. EMS and hyperinsulinemic PPID animals were combined in an ID (hyperinsulinemia) group.

Results: Free cortisol concentrations were increased in overweight/obese controls (0.3 ± 0.1 μg/dL) compared to lean controls (0.2 ± 0.1 μg/dL; P = .017). Mean FCF was significantly higher in animals with PPID (8.8 ± 5.8 μg/dL, P = .005) or ID (8.8 ± 10.2 μg/dL, P = .039) than controls (5.0 ± 0.9 μg/dL), but total cortisol concentrations were similar (P ≥ .350) (PPID: 4.2 ± 4.3 μg/dL; ID: 5.0 ± 4.5 μg/dL; controls: 4.6 ± 1.7 and 5.1 ± 2.1 μg/dL).

Conclusions and clinical importance: Increased FCF is associated with obesity in healthy horses and with ID (hyperinsulinemia) in horses and ponies with endocrine disease. Decreased plasma cortisol-binding capacity could be a component of these endocrine disorders in horses.

Keywords: Equine; Equine Metabolic Syndrome; Insulin resistance; Obesity; Pituitary pars intermedia dysfunction; Steroid.

Figure 1

Serum total cortisol concentration (A), serum free cortisol fraction (B), serum free cortisol concentration (C), and serum insulin concentration (D) in 57 healthy horses over the 9‐month sampling period. Each data point represents 1 animal, and animals are divided into 3 age groups: age group A, 1–6 years (open circles), age group B, 7–14 years (black squares), and age group C, ≥15 years (open triangles). The horizontal lines represent the mean for each age group during each season.

Figure 2

Serum total cortisol concentration (A), serum free cortisol fraction (B), serum free cortisol concentration (C), and serum insulin concentration (D) in horses and ponies with Pituitary Pars Intermedia Dysfunction (PPID, n = 20; X), Equine Metabolic Syndrome (EMS, n = 20, plus signs +), or PPID or EMS and concurrent hyperinsulinemia (n = 27, asterisks *) and in healthy age‐ and season‐matched control horses (n = 18–27, open circles). #Denotes significant (P < .05) difference between groups. Horizontal lines represent the mean for each group.