這裏一個網站似乎是美國政府有關部門的推薦：

Normal Results

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The normal range of vitamin D is measured as nanograms per milliliter (ng/mL). Many experts recommend a level between 20 and 40 ng/mL. Others recommend a level between 30 and 50 ng/mL.

https://medlineplus.gov/ency/article/003569.htm