Many with mercury toxicity have trouble converting t4 to t3. part of this may be due to a selenium deficiency. Selenium is necessary for the conversion, but mercury binds to selenium in the body and makes it unusable. Mercury can also clog up the iodine binding sites on the thyroid. One good test for hypothyroidism to to take your temperature as soon as you wake up before you start moving around. If it is low, that is a sign of hypothyroidism.