Im an avid exerciser and most of my lifts are well into the advanced or elite level. Having said that I found that adding Iodine to my routine really hasnt made a noticeable impact on my workouts, either good or bad.
Since starting Iodine however I have had significant fat loss without actual loss of body weight so I have no doubt I was deficient.
Acclimating to exercise can take a while, especially if you dont have a long standing history. All of your vitamins and minerals will have an impact on your ability to perform and recover.
How long have you been taking D and at what dose?
How much potassium/magnesium/calcium do you get daily from food and supplements?
Are you taking any vit A or E at all?
When you say you have low end magnesium levels, how was that tested?