When I'm not sure and the fatigue is really wearing on me I get labs done. I have hypothyroid and a history of really low Vit. D levels so those 2 could be off again and my family doctor always checks everything else too. Usually we go into it with my saying it's just in case and I don't necessarily expect anything but don't want to miss anything. He's completely understanding about that and has never had any problem checking things out. Every so often it will be my thyroid and a med adjustment will help; usually we find out I'm depressed.