200mcg Selenium: Also, adding in selenium which essential for the conversion of T4 to T3, which is the active form of thyroid hormone. During pregnancy, the immune system starts to change and circulate to protect the fetus. Once the fetus is born, the mother develops antibodies against her thyroid. In this case, adding 200mcg selenium while pregnant is not only very safe for the mother and fetus, it also lowers the autoimmune response after giving birth. Also lowers risk of cancer.