Vietnamese American here. I honestly don’t have much insight for you as for how to break the news, but I feel for you and you aren’t alone. Asian parents have a deep hatred of anything that could be seen and exploited by the gossip community as a flaw. Medical diagnoses and perfectly typical presentation be damned. I figured out I was lactose intolerant a couple of years ago and my mom wouldn’t have it. I’m 30. She kept saying, “you’ve never had a problem with milk!” Except all through childhood, if I ever had more than a small amount, I would spend entire evenings in the bathroom.
I equate this reaction to finding out that the perfect child you created and raised from your own DNA, parenting, and choices is broken. They see it as a personal failure. This is CLEARLY not true because you are a human and not a vase/ inanimate object and medical diagnoses are not flaws or failures in the least, but it stems from generations of perfectionism and being blamed by their parents for everything they ever did wrong.
I think once I became a parent myself I realized that our parents are still using us as a measure of their success in life. Anything that tarnishes this in their eyes in unacceptable. They have society induced tunnel vision.
Tell them. They won’t understand. Not for a long time, maybe never or at least not out loud. Just try not to take it so personally when they get that way. Do what makes you healthy and happy.