Is reactive hypoglycemia a "real" disorder?

No: In my 35 years of practice, I have never seen a case of reactive hypoglycemia. The disorder doesn't exist, if you're overweight the symptoms are a natural response to your glucose level returning to normal. A glucose tolerance test (if you've had one) is not and never will be a valid diagnostic test for hypoglycemia. Don't waste your money o GTT or any fad diets of treatments. Lose wt if heavy.

Yes: This is a tough question to answer as it depends what you mean by real. There are several things which cause reactive hypoglycemia but loosely this term refers to a style of digestion in which blood sugar suddenly drops to low enough levels to cause symptoms up to 4 hours after a meal. It is caused when the body is still releasing Insulin after a meal is digested resulting in a sugar drop.