Everyone should have his or her vitamin D level checked at least once a year (infants through the elderly).

Below are conditions that have been associated with vitamin D levels:
< 10 ng/mL
< 15 ng/mL < 20 ng/mL < 30 ng/mL
30–50 ng/mL
< 34 ng/mL < 36 ng/mL < 40 ng/mL 50–80 ng/mL > 50 ng/mL 80–100 ng/mL > 100 ng/mL

Severely deficient
Risk of rickets 75% greater risk of colon cancer Deficient Increased calcium loss from bones, osteoporosis Poor wound healing Increased muscle pain Increased joint and back pain Greater risk of depression Increased diabetes Increased schizophrenia Increased migraines Increased autoimmune disease (lupus, scleroderma) Increased allergies Increased preeclampsia Increased inflammation Suboptimal levels Twice the risk of heart attack Increased high blood pressure Three times the risk of multiple sclerosis Optimal levels 50% reduction in breast cancer, decreased risk of all solid cancers Slowing of cancer growth in patients with cancer Increased risk of toxic symptoms (hypercalcemia)

Natural Production of Vitamin D
Your skin makes vitamin D when it is exposed to a pinking dose of sunlight. How much vitamin D you make depends on your age, how much skin is uncovered, and your skin tone. Without sunblock and with arms and legs exposed, your skin will make 10,000 to 15,000 units of vitamin D in one pinking sun exposure, on average. (Sunblock with an SPF of more than 15 blocks 100% of vitamin D production in the skin.)

(From the Institute of Functional Medicine)