What Causes Fluctuating Energy Levels?

Hi, I m a 53 year old female with great general health apart from extreme fluctuations of fatigue and energy. There could be a week with great levels of energy but the next week is completely lacking in energy resulting in long periods of lethargy and fatigue. I m awaiting a blood test result but I m wondering if hormonal imbalance could be the culprit. Thank you. Orla