Synthetic adrenocorticotropic hormone stimulation tests in healthy neonatal foals

Background: Cosyntropin (adrenocorticotropic hormone [ACTH]) stimulation tests are used to evaluate adrenal function. Low-dose ACTH stimulation tests are the most accurate method for diagnosing relative adrenal insufficiency in critically ill humans but have not been evaluated in foals. Hypothesis: Peak serum cortisol concentrations in healthy foals will not be significantly different after intravenous administration of 1, 10, 100, and 250 μg of cosyntropin. Animals: 14 healthy neonatal foals, 3-4 days of age. Methods: A randomized cross-over model was used in which cosyntropin (1, 10, 100, or 250 μg) was administered intravenously on days 3 and 4 of life. Blood samples were collected before and 30, 60, 90, 120, and 150 minutes after administration of cosyntropin for determination of serum cortisol concentration. Results: Serum cortisol concentrations did not significantly increase after administration of 1 μg of cosyntropin. Cortisol concentration peaked 30 minutes after administration of 10 μg of cosyntropin and 90 minutes after 100 and 250 μg of cosyntropin. There was no relationship between cosyntropin dose and serum cortisol concentration at 30 minutes. Compared with the 10-μg dose, 100 and 250 μg of cosyntropin induced significantly greater cortisol concentrations at 90 minutes, at which point the 10-μg cosyntropin-dose cortisol values were indistinguishable from baseline. There was no significant difference in the area under the cortisol concentration curve between the 100- and 250-μg doses. No effect of day of testing or foal weight on peak Cortisol concentration was detected. Conclusions and Clinical Importance: The results of this study suggest that 10- and 100-μg doses of cosyntropin would be appropriate for evaluating adrenal function in neonatal foals.