Thus, during fasting, disease, or stress, the
conversion of T4 to reverse-T3 increases. At these times,
conversion of T4 to T3 decreases about 50%, and conversion of T4 to reverse-T3 increases about 50%. Under normal, non-stressful conditions, different enzymes convert some T4 to T3 and some to reverse-T3. The same is true during fasting, illness, or stress; only the percentages change--less T4 is converted to T3 and more is converted to reverse-T3.