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Equine Health Resource

Glucose Tolerance Tests



This test is performed to evaluate the horse’s ability to metabolize glucose normally. Insulin is a hormone that is critically involved in blood sugar (glucose) regulation. Increases in insulin cause a decrease in levels of blood sugar. Insulin resistance is the body’s failure to respond to insulin in a normal way and is an extremely common condition in domestic horses. Type 1 diabetes is the lack of insulin, and is very rare in horses.

When a horse is presented with a sugar (glucose) load in the blood, it should produce a spike of insulin, which then causes the sugar to drop in a predictable way. The test requires fasting the horse for 8 hours before it is performed. Sugar (dextrose) is given to the horse intravenously. Blood samples are taken at specific times up to 4 hours post injection. Blood glucose levels should be normal in 3 hours after injection. High blood glucose remaining at 3 hours indicates insulin resistance or type 1 diabetes.


This is the best way to assess the ability of a horse to metabolize glucose, i.e. to test the horse's production of and response to insulin. It is a helpful test in further characterizing horses with insulin resistance.


This test does not necessarily differentiate between causes, it only provides information that a problem exists.

Author: Doug Thal DVM Dipl. ABVP


Lavoie JP, Hinchcliff KW eds. Blackwell's 5 Minute Vet Consult: Equine. 2nd Ed. Ames: Wiley Blackwell 2008.


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