Before you do anything more with your selenium, You should have your Iodine levels tested. Here is an article explaining (It is done by ingesting 50 mg of Iodine and collecting 24 hours of urine. By measuring the amount of Iodine excreted in the urine, you can ascertain the iodine status of the body.)
Studies have shown that if iodine is low, selenium must also be kept low to prevent the hypothyroidism from becoming worse (from increased DI-I and T4 depletion, as explained above.) So if both minerals are low, then the person is hypo and gets a goiter, but the damage to the thyroid is kept to a minimum. More severe problems happen when either selenium or iodine is high and the other is low. If selenium is high and iodine low, then T4 to T3 to T2 conversion is accelerated without T4 being replenished, leading to a worsening of hypothyroidism. See article here... http://www.greenwillowtree.com/Page.bok?file=selenium.iodine.html