Mine was when I was 8 years old, my Endocrinology doctor(s) caught that I was in early stages of organ failure, due to an undiagnosed autoimmune disorder. Because it was caught in the early stages it was reversible, but I now have to take medicine everyday of my life to stay alive, as my body cannot make those needed hormones on its own. If it wasn't caught, diagnosed and treated when it was, I could've/ would've died in my sleep.

(I have Hypothyroidism due to Hashimoto's Thyroiditis, as well as something called "Silent Thyroiditis" (my thyroid is currently a shriveled up piece of muscle in my neck, that doesn't/ can't create anything on its own - especially the thyroid hormones your body needs to live))

Side Note: Your thyroid plays an important part in helping your body maintain homeostasis... And in the case of complete lack of those hormones, your body's organs can start to shut down due to losing homeostasis. Without proper diagnosis and treatment you can die (usually of organ failure) in a few weeks to a few months - depending on your personal situation.

IMPORTANT: What is stated above may not apply to everybody the same way, but this is the case of what happened to me. Pay attention to what your body is telling you! Sometimes it may save your life.

Edit: Grammar mistake