Thyroid hormone T3, molecular model. Triiodothyronine, T3, is the true thyroid hormone involved in the regulation of metabolism. The dominant thyroid hormone is thyroxine (T4), which is converted to T3 by an iodinase enzyme. T3 has a shorter half-life than T4 but is several times more potent. Both are used to treat thyroid disorders. Atoms are colour-coded: carbon is black; hydrogen is white; iodine is purple; nitrogen is blue; oxygen is red.