Why do I feel weak?

Usually, lifestyle factors are the cause. There are many reasons why you might feel weak, including physical and mental causes: Physical causes: Dehydration: Not drinking enough water can make you feel tired and low on energy. Low levels of potassium or sodium: Infections: Such as a urinary tract infection or a respiratory infection: Anemia: Not having enough healthy red blood cells or hemoglobin to carry oxygen to the body's tissues: Thyroid problems: Such as an underactive thyroid, which is when you have too little of the thyroid hormone thyroxine Mental causes: Stress: Emotional stress from many situations, such as work, financial problems, relationship issues, major life events, and upheavals: Lack of sleep: Not getting enough sleep or finding it hard to get to sleep (insomnia). Doing too much activity at one time can make you feel weak.