T4 is converted in the brain, thyroid, bloodstream, and other body tissues to the more active form of thyroid hormone — T3. (coachcalorie.com)
A study showed that even modest weight loss leads to a decreased conversion of the less active form of thyroid hormone, T4, to the active form. (cathe.com)
This is a problem because T3 is the more active form of thyroid hormone. (holtorfmed.com)