Alkaline phosphatase is an enzyme in our body. A normal level in adults is 30-140 IU/L. When a blood test shows a level lower than this, it is called hypophosphatasemia. At OneHowTo we will look at causes low alkaline phosphatase. Main symptoms of low alkaline phosphatase. Low alkaline phosphatase can produce symptoms such as fatigue, tachycardia (a pulse of more than 120 beats per minute), shortness of breath, intolerance to cold, constipation or major weight loss.