I have had symptoms of both hypo *and* hyperthyroid going on 20 years now. It fluctuates between the two. Sometimes I'll go through months (or even years) with a super slow heartbeat, weight gain, sluggishness, low body temp, etc. Other times I'll drop weight like mad without trying, I have a lot of insomnia, hot flashes (even when in my 20s) and my heart beat is rapid. What's more years ago I was on a med that is notorious for trashing your thyroid. For that reason, I get the full panel work up every once in awhile... However, my blood tests always come back perfectly normal.
What my doctor says: thyroid levels are highly unique to the individual, so what may be normal for most people may not be normal for me. But, we know there's nothing seriously wrong (or else I'd have had a crisis by now). The fluctuations I have are mild. It's more annoying than anything else.