Thyroid symptoms can come on suddenly, but I think for most they come one gradually. If you want to see people's histories, read through some of this thread:
I have had (maybe) slight symptoms over my life. Not really sure. I bicycle a lot, and was gearing up this spring, biking almost every evening about 20 miles or so, and many mornings, about 15-20 miles - and biking with Triathletes.
Then one day, suddenly, after biking 18 very agressive miles, I stayed exhausted the whole day. And that never quit.
Now a full three months of meds later, I'm feeling about 85%.
So yes, the symptoms can come on suddenly. But probably not the norm.
Maybe someone else can respond about what tests to ask about.
My opinions only, not medical advice. Always follow advice of your Doctor, not what I say here.