The 25-hydroxy vitamin D ( 25(OH)D ) test is the most accurate way to measure how much vitamin D is in your body. In the kidney, 25-hydroxy vitamin D changes into an active form of the vitamin. The active form of vitamin D helps control calcium and phosphate levels in the body.

Other names include 25-hydroxyvitamin D, 25-OH vitamin D test, and Calcidiol 25-hydroxycholecalciferol test.

Based on research by GrassrootsHealth, it appears as though most adults need about 8,000 IU's of vitamin D a day in order to raise their serum levels above 40 ng/ml. For children, many experts agree they need about 35 IU's of vitamin D per pound of body weight.

At the time GrassrootsHealth performed the studies that resulted in this dosage recommendation, the optimal serum level was believed to be between 40 to 60 ng/ml. Since then, the optimal vitamin D level has been raised to 50-70 ng/ml, and when treating cancer or heart disease, as high as 70-100 ng/ml.

What this means is that even if you do not regularly monitor your vitamin D levels (which you should), your risk of overdosing is going to be fairly slim even if you take as much as 8,000 IU's a day. However, the only way to determine your optimal dose is to get your blood tested regularly, and adjust your dosage to maintain the desired level.