Extreme fatigue (tiredness) is symptom of Iron Deficiency Anemia.

Iron is an essential nutrient, critical for producing hemoglobin, the substance in red blood cells that transports oxygen all over the body. If you don¹t have enough iron, your body has to work a whole lot harder to get the energy it needs, which can leave you feeling tired, weak, irritable and unable to focus.
Many equate iron deficiency with anemia, the condition in which your body has a lower than normal number of red blood cells. But research shows that there's a whole other category of iron deficiency that precedes anemia. To diagnose it, a blood test must measure ferritin levels, the iron stores in your blood.