Many different conditions can lead to fatigue and so it can be difficult using this to make a diagnosis. However, if you feel unusually tired all of the time then this could very possibly be due to issues with your kidneys. When your kidneys are functioning properly, they will create a hormone called erythropoietin aka EPO.
This tells the body to create the oxygen carrying red blood cells that are needed to carry energy and nutrients around the body. When this stops, you might start to notice that you feel very tired.
Less oxygen to the brain means you might also notice the symptoms of ‘brain fog’.