Correct way to boost Vitamin D

Patient: My vitamin D levels are 16. My doctor prescribed 1000 IUs once a week for 8 weeks, but I already take a multivitamin 3-4 times a week with 700 Iu’s of D, yet my levels are 16 from this. What is the correct way to raise these levels? I think my Doctor is WAY off. Basically I already take more than she prescribed and its low.