1 Daily secretion patterns of thyroxine, insulin, and cortisol in non-working horses on pasture when receiving one concentrate meal per day vs. no daily concentrate

Abstract

Testing of metabolic hormones such as insulin (I), thyroxine (T 4 ), and cortisol (C) has become common in equine veterinary practice as the prevalence of obesity and insulin resistance has increased. There are few reports of daily secretion patterns of these hormones, as well as how diet may impact their secretion; especially in horses on pasture. Therefore, the objective of this study was to assess the daily secretion patterns of I, T 4 , and C in non-working horses on pasture receiving 0 or 1 concentrate meals/d. Sixteen horses (13 geldings and 3 mares; 6 to 22 yr of age; no diagnosed endocrine conditions) were used in a static group comparison. Horses were housed in 1 of 2 large outdoor pens with pasture access 24 h/d. Each pen of horses had ad libitum access to Teff hay and water. One pen of horses received no concentrate [no grain (NG), n = 8] and 1 pen received 2 kg Hi-Pro WTAMU Pellet (Friona, TX) once/d [grain (G), n = 8]. The meal was fed daily at 0700 except on sample collection days, when fed at 1100. The 42-d experiment consisted of 6 7-d periods with 1 sample collection d/period. On each sample collection day, blood samples were collected via jugular venipuncture between 0800 and 1000 (AM), 1200 and 1400 (MD), 1600 and 1800 (PM) h. Data were analyzed using the MIXED procedure of SAS v. 9.4 (SAS Institute, Inc., Cary, NC). No diurnal patterns were observed for T 4 or I, but C exhibited an expected circadian secretion pattern with the greatest concentration observed in the AM and decreasing throughout the day. There was an effect of treatment on plasma T 4 , with greater ( P < 0.001) overall mean T 4 observed in NG (23.2 ± 1.3 ng/mL) compared with G (16.0 ± 1.2 ng/mL). There was an effect of the interaction of treatment, period, and time of day on plasma I ( P < 0.001), with a spike in I during the MD collection following the 1100 meal in G, while NG horses remained relatively low across all 3 collections. There was an effect of the interaction of treatment and time of day ( P = 0.036) on plasma C. Both groups of horses exhibited expected diurnal secretion patterns of C in multiple periods. Horses in the G group appeared to have a greater decrease in C from the AM to MD collection than NG horses in several periods. Results from this experiment indicate that there is no diurnal secretion pattern for T 4 or I but support the presence of the circadian rhythm of C secretion. These results also highlight the influence concentrate meals may have on the outcomes of diagnostic tests and the importance of following a standardized fasting protocol during diagnostic testing.