Cushing’s disease and Addison’s disease are often mentioned together because they are, in effect, opposites. Both involve the adrenal glands and the hormone cortisol — but in completely different directions. Knowing the distinction helps you understand your dog’s treatment and why monitoring matters.
Cushing's Disease: Too Much Cortisol
Cushing’s disease (hyperadrenocorticism) is a state of excess cortisol. As described throughout this guide, it typically produces increased thirst and urination, a bigger appetite, a pot-bellied look, hair loss and panting. It usually develops gradually in middle-aged and older dogs.
Addison's Disease: Too Little Cortisol
Addison’s disease (hypoadrenocorticism) is the reverse — the adrenal glands produce too little cortisol (and often too little of a second hormone, aldosterone). Its signs can be vague and come and go: low energy, poor appetite, vomiting, diarrhea and weakness. Addison’s more often affects young to middle-aged dogs and can, in a crisis, become a medical emergency. Because its signs are so non-specific, it is sometimes called “the great pretender.”
Why the Two Are Linked in Treatment
Here is the crucial connection: the medications used to treat Cushing’s disease work by lowering cortisol. If cortisol is lowered too far, a dog can develop signs that look just like Addison’s disease — a situation sometimes called iatrogenic (treatment-induced) hypoadrenocorticism. This is precisely why dogs being treated for Cushing’s need regular monitoring tests, and why owners are told to watch for weakness, loss of appetite, vomiting or diarrhea and to call the vet promptly if they appear.
Telling Them Apart
Only your veterinarian can distinguish these conditions, because it requires specific hormone testing — the same categories of tests, such as the ACTH stimulation test, are used to assess both. The two diseases share the adrenal glands as their stage, but the script is opposite, and the treatments are opposite too.
The Takeaway for Owners
You do not need to become an endocrinologist — but understanding that Cushing’s means “too much” and Addison’s means “too little” helps you appreciate why your vet fine-tunes medication so carefully and asks you to report certain symptoms without delay. For reliable background on both conditions, see the Merck Veterinary Manual and your veterinary team. Return to our treatment overview for more on safe monitoring.