The question you should always ask before you pop any pill (apart from benefit versus risk) is "Can I make lifestyle changes instead of taking this?" If the answer is "yes," then do it! If not, lifestyle changes can make almost any pill work better. Recent research has shown that diet and exercise work as well as many prescription drugs.
I believe they work as well as supplements, too. Naturally, it's also true that the worse your lifestyle is, the less you can expect from your pills. Recently, cholesterol-lowering medications and Viagra were found to work much better in people who exercised compared to people who didn't exercise much.
Along these same lines, lifestyle can also affect blood tests. (For example, a big breakfast the day of your blood draw can reduce your testosterone temporarily, and weight gain can cause reductions in vitamin D levels.) So always check the latest information on how to take a blood test, especially if your doctor is using it to prescribe a supplement or drug.