Q & A: How to Calculate the Maximal Dose of ACTH (Cortrosyn) for Large-Breed Dogs

I have a 180 pound (82 kg) St. Bernard that was referred to me for supportive care overnight and to have an ACTH stimulation test done to rule out Addison's disease.

In the past, the maximal dose of synthetic cosyntropin (Cortrosyn) I've used in these large-breed dose is 250 μg per dog, or the whole vial of Cortrosyn. But there seems to be differing opinions on whether to give this dose or to use a 5-μg/kg dose calculated based upon the dog's body weight. Since this is such a large dog, should I administer 1 vial (250 μg), or should I give the 5 μg/kg-dose? In this dog, that would amount to giving 1.6 vials of the Cortrosyn!

Thanks so much for helping with my dilemma.

My Response:

The human dose for cosyntropin (Cortrosyn), no matter what the body weight, is a total dose of 0.25 mg (250 μg). So I would never administer more than 1 entire vial to any large dog, even if the calculated dosage turned out to be less than 5 μg/kg. For more information on the best protocol for ACTH stimulation testing in dogs and cats, see my previous blog post on the topic.

Studies (1,2) have reported that there is actually little difference in the serum cortisol responses in dogs when tested with the standard dose of Cortrosyn (5 μg/kg) and compared to a much lower dosage (0.5-1 μg/kg). So even with the low-dose protocol we now recommend 5 μg/kg), we already know that we are administering supra-physiological doses of ACTH to these dogs.

The bottom line about dosing with Cortrosyn for ACTH stimulation testing: 

• The maximal dose of Cortrosyn for any sized dog is a total dose of 250 μg.
• Even if a dog gets a bit less than the calculated 5 μg/kg-dose (no matter what the body weight), this will still produce a maximal serum cortisol response and the ACTH stimulation test results will still be completely valid.

References: 

1. Kerl ME, Peterson ME, Wallace MS, Melián C, Kemppainen RJ. Evaluation of a low-dose synthetic adrenocorticotropic hormone stimulation test in clinically normal dogs and dogs with naturally developing hyperadrenocorticism. J Am Vet Med Assoc 1999,214:1497-501.
2. Martin LG, Behrend EN, Mealey KL, Carpenter DM, Hickey KC. Effect of low doses of cosyntropin on serum cortisol concentrations in clinically normal dogs. Am J Vet Res 2007;68:555-60.