Thyroxine in a hormone produced in the body which affects the activity of mitochondria in cells by binding to DNA, it is produced to regulate the bodies overall metabolic rate.
Thyroxine is tetraiodothyronine, T4:
a closely related hormone of biological importance is triiodothyronine,T3.
Reverse T3 is an isomer of T3 but has no biological activity therefore is of no importance in the body.