Figure 1. Timeline for the Major Technical Advances in Thyroid Testing. The figure shows the development of increasingly more sensitive TSH tests: first generation, (1G), second generation (2G), and third generation (3G), and advances in the methodologies used to measure total thyroid hormones (TT4 and TT3), indirectly estimate free thyroid hormones (FT4 and FT3), directly measure FT4, and measure the thyroid autoantibodies TPOAb, TgAb, and TRAb and Thyroglobulin (Tg). From reference 1.