My dad's calcium level is sometimes normal and sometimes slightly high. his vitamin d is always very low. can he safely take d3 supplements?

Vit D & parathyroid: You need vit D to absorb calcium in the gut. But if you're not absorbing calcium, why is it high? It could be an overactive parathyroid. Other causes can be certain medications, such as HCTZ. Your dad probably needs to take Vit D, but it should be done under the supervision of an MD who deals with calcium disorders, usually an Endocrinologist, or sometimes Nephrologist.