Re: What exactly does Selenium do?
Selenium is the component of an enzyme that converts T4 into T3 at the cell level. That's why it's vital for thyroid health. Another thing it can do is reduce TPO antibodies by up to 49%, according to a large study.
Beyond thyroid health, it has antioxidant properties that help "mop up" free radicals which contribute to cancer. It may help in preventing heart disease. It's really great stuff!
Whether it's making you feel better is hard to say. Only way to know would be to have a trial period without it.
I have found that when I miss a few days without my calcium/magnesium, I get bad sugar cravings. That isn't an effect that I've read anything about, but it seems true for me. Very odd, I think.
The amount you're taking is good. You can assume you'll get at least a little from your diet... but probably nowhere near enough. If you take a multivitamin, check the ingredients to see if any is included in the formula. Just remember that 400 mcgs is the absolute upper limit to take, after which it becomes toxic.
And finally, for what it's worth, selenium from a yeast source is more absorbable than the other types. The label will give you that information.