This is a great question and too often over looked by guys what want to get into gear. Steroids have a bad reputation. There is some truth to the hype. These drugs are powerful hormones and can cause medical issues with misuse. Part of being a responsible, gear using athlete is to make sure that you are not doing harm to your body. The things I'm most concerned with are liver enzymes, kidney function and cholesterol. If you are under 35, or just want to avoid having to commit to a life of TRT, then you will also want to have your "free" and "total" test levels checked. Your best bet is to have your test levels taken before you ever use. That way, you know what your baseline normal level is, and you know when you have returned back to normal when you get rechecked after your cycle. I also get an EKG done once a year to monitor my heart and look for enlargement. If you are an athlete, you will have some heart enlargement. This can be totally normal. Your heart is a muscle and with intense physical activity, it will grow. My heart has slight enlargement of the left ventricle and is diagnosed as an "athletic heart". The tissue functions normally and just like any muscle, when I retire from the sport, it will go down in size. My doctor keeps an eye on it though, to make sure that we don't see signs of steroids effecting it.
For a lot of tests, a bodybuilder's results will look different than a normal person's, but the key is to keep an eye on things and make sure there are no radical changes from test to test. If I'm getting lab work done on cycle, I want to make sure that there are no major changes. A good example is creatinine. Creatinine is always higher in a bodybuilder than it is for a normal, sedentary person because a bodybuilder has a lot more muscle. Therefore, there is more muscle waste. On the opposite end of the spectrum, if a little old lady, with very little muscle, tested in the "normal" range for creatinine, her levels would probably be high. Tests are all relative to the patient. Anyway, I will always test out as "high" on my creatinine test, but my level should stay consistant. If it were all of a sudden up by 25% between one test to the next, thats a red flag that something is going on.
If you are using gear, your liver function will also be effected. We all know that orals strain the liver, but people don't really talk about the impact that injectables have. Injects effect the liver too. Just not as badly. If you get labs done while on steroids, your liver values may be high but they should stay pretty consistent. If you see a big jump from one test to another, once again, thats a red flag. Once you come off, a healthy liver should regenerate. If you are off cycle and want to get labs before you go back on, you'll want to check to make sure that your liver health has returned to normal.
Same goes with cholesterol. Gear, especially orals like winstrol, can destroy your HDL. This is a good reason not to run winstrol(or most orals) for extended periods. Once you come off gear, these levels should go back to normal. Before I go back on cycle, I want my HDL to be back up in the healthy range.
I get a full physical once a year. At this time, I also get my thyroid checked, along with the EKG I mentioned earlier. I've never had thyroid issues in the past, but it's good to stay aware of whats going on, across the board.