If the only symptoms are panting and increased thirst then the preliminary diagnosis is probably a bit premature. Cushings in general can result in an intolerance to environmental temperatures. But usually more serious symptoms appear such as increased appetite leading to obesity or rapid gain of weight, water retention, digestive disturbances, skin dryness and pigmentation, alopecia or hair loss, and very frequently muscle weakness.

The increased thirst (and urination) is definitely a sign of Cushings. But also it's a sign of many other medical problems, and/or the result of a Summer environment. The later you can guage for yourself by asking if your dog previously had problems with the heat? My 2 yr old 1/2 GP female pants, while I and her little brother are quite comfortable. She just runs hot, and tends to drink a lot of water.

So what you need to do is to ask if the excessive drinking represents a change for your dog over past years? If so then a basic blood profile should be taken with the idea the problem could be anything. If you are unsure it might be wise to wait until the summer is over to see if they persist, or if other symptoms of a metabolic or disease problem appear. This is not to suggest you should wait for most or all of the symptoms to appear, as if it is Cushings then the sooner the treatment the better for your dog. The speculation here is that your Vet is adding the excessive drinking, plus age, plus the fact Poodles have a genetic tendency towards Cushings, and considering it a possibility. A blood panel may get supporting evidence, but a Dexamethasone loading test is required for diagnosis.