complicated but I will try to explain it as
a thyroid hormone. First the body produces T4, which is biologically inactive (unusable); then, in various parts of the body, T4 is converted into T3, the biologically active (usable) form. T3 is not just a hormone but also functions as a neurotransmitter in the brain, like serotonin, dopamine, etc. T3 is connected to mood/emotion, cognitive function, metabolism, etc.