I got a blood test a year ago that verified I was extremely deficient in vitamin D. After taking it over the past year (on and off, admittedly), my vitamin D levels are now within normal levels according to my latest blood test. I was taking 5,000 IU for the majority of the past year, and started taking 10,000 IU about 2 months ago. I am feeling better and better everyday since increasing my dosage. Additionally, I've my hair has stopped thinning as much (hair thinning is a common side effect for those who are deficient in vitamin D), but I've also been using topical minoxidil (5%). I hope this is helpful!