Good evening, I am sorry about your dog's diagnosis. Unfortunately, pheochromocytoma is a difficult thing to deal with. May I ask how the diagnosis was made? Adrenal masses can be benign and specific testing can be performed to confirm the mass is functional and not incidental. If it has been confirmed to be functional (producing excess of what the adrenal gland already produces) with laboratory testing, then medical therapy involves controlling the side effects of excess adrenaline and possibly excess cortisol or aldosterone. Of these, the most common is excess adrenaline causing increases in blood pressure, heart rate and restlessness or agitation. These clinical signs are controlled by giving a medication to block the actions of adrenaline (and norepinephrine) on the body. Alpha blockers prevent increases in blood pressure and heart rate. If the tumor is functional and producing excess cortisol then medication given can prevent this by blocking conversion of the intermediates to the final product and reducing the overall amount of cortisol in the system.
Along with this there can be discomfort so you can use pain medications such as tramadol and gabapentin or other alternatives for pain management including supplements, anti-inflammatories, antioxidants, etc. I am not aware of any specific supplements for preventing or inhibiting adrenal function, most of them are aimed at optimizing or increasing adrenal function.
Standard Process is a reputable company that makes supplements for pets and people which are safe, high quality, and readily available by ordering online.
Surgery is the treatment of choice when it comes to pheochromocytoma but there are significant risks of surgery in addition to the cost of surgery as it needs to be performed by a board certified surgeon because of where the adrenal glands are and the mayor vessels that can be involved.
Does this help at all with your questions? If you were looking for more specific information or need clarification, please let me know and I am more than happy to do what it takes to get 200% satisfaction.