Total Thyroid hormone levels include protein bound levels and are affected by abnormalitiesin thyroid hormone binding protein like Thyroid Binding Globulin (TBG). TBG is increased due to effect of estrogens like pregnancy, oral contraceptives & Tamoxifen therapy. TBG is decreased due to effect of excessive androgens and Nephrotic syndrome. In addition, genetic disorders, acute illness, various drugs can all affect thyroid hormone binding. Hence assays that measure unbound or free hormone are preferable than total thyroid hormones.