T3, also known as triiodothyronine, is the active form of thyroid hormone. It is available in pill form as liothyronine sodium (L-T3) or Cytomel (the only brand in North America). As discussed in Chapter 1, only 20 percent of the thyroid hormone made by the thyroid gland is T3, the rest being T4. Most of the T3 in each cell of the body comes from T4 that has been converted into T3 while inside that cell. This T3, inside of each cell, does essentially all of the things that thyroid hormone is known to do. When your thyroid hormone is provided as a T4 pill, each cell in your body is able to make its own T3, exercising some control over this by controlling the enzymes inside the cells that change T4 to T3. In some situations, you can substitute a pill providing T3 for your T4 pill. There are particular reasons why you would want to do this in special circumstances, but there are also reasons why you shouldn't do this.
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