Selenium can help lower thyroid peroxide antibodies (TPOAb) antibodies. I prefer to eat Brazil nuts (two a day) than take selenium tablets. Four minerals that support healthy blood sugar (and are low in diabetics) are chromium, magnesium, zinc, selenium. I have read diabetes also lowers vitamin C.
Can't really answer that without more information. First on what basis were you diagnosed as needing Levothyroxin? If you have had thyroid related tests from before and after starting the Levo, please post results and reference ranges shown on the lab report. If tested for Vitamin D, B12 and ferritin, please post those as well. What, if any symptoms are you having? Symptoms are even more important to know than test results. Why do you think you want to take selenium?