Seasonal Influence on Insulin and Cortisol Results from Overnight Dexamethasone Suppression Tests (DST) in Normal and Previously Laminitic Ponies

Article Outline

• Introduction
• Materials & Methods
• Results
• Discussion
• Conclusion, Clinical Relevance & Take Home Message
• Copyright

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Introduction 

DSTs can be used to aid identification of ponies susceptible to laminitis. Cortisol responses are affected by season, being higher in Summer/Autumn. Seasonal influence on insulin responses has not been previously reported in ponies.

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Materials & Methods 

Standard 19 hour overnight DSTs were performed in April, July and December in six normal (NP) and six previously-laminitic ponies (PLP, although not laminitic at the time of testing) maintained at pasture. Serum cortisol analysis was by chemiluminescent assay at a commercial laboratory (Beaufort Cottage Laboratory, UK). Insulin was analyzed by radioimmunoassay (Coat-A-Count, Siemens). Results were analyzed using a 2-way ANOVA.

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Results 

Basal cortisol in NP was significantly higher in July (112±59nmol/l; mean ± SD) than December (62±7nmol/l; p<0.05). Post-injection cortisol in NP (45±83nmol/l) and PLP (30±32nmol/l) tested in July were above the reference range (<25nmol/l) but within the reference range for both groups in April and December. Post-injection insulin was significantly (p<0.01) higher in PLP in April (966±488μIU/ml) and July (844±820μIU/ml) than in December (26±16μIU/ml) and compared to NP at all seasons (April 246±146μIU/ml; July 206±204μIU/ml; December 28±16μIU/ml). There were no significant differences in basal insulin between groups at any season (April NP 98±94μIU/ml vs. PLP 240±160μIU/ml; July NP 22±20μIU/ml vs. PLP 166±223μIU/ml; December NP 10±5μIU/ml vs. PLP 13±10μIU/ml).

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Discussion 

In July, PLP had abnormal cortisol and insulin responses to DST. In April, PLP had normal cortisol results but exaggerated insulin responses. All results were normal in December. NP had abnormal cortisol results in July.

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Conclusion, Clinical Relevance & Take Home Message 

DSTs should be performed in spring to optimize diagnosis of PPID and laminitis susceptibility.