Monitoring should be considered in the following settings: Patients taking medications which either bind vitamin D in the gut or accelerate the breakdown of vitamin D (e.g. cholestyramine; inducers of the cytochrome P450 pathway such as phenytoin and phenobarbital) Obesity BMI >30 Kg/m2 or high body mass- >90 Kg* Malabsorption syndromes Patients with osteoporosis