Can I eat or not? Blood work tomorrow.?

So I have to take a blood work for my thyroid tomorrow, they told me not to eat anything before the test which it's at 2 30 pm. Problem? I have my aerobics class around 8 am and it'll end at 11. After my class I'm so tired and hungry... if I eat at least an apple in the morning before class will that not affect me? I want to have some energy for class or I'll feel tired and not do my best.