Biochem/physiol Actions L-Thyroxine (T4) and triiodo-L-thyronine (T3) are iodine-containing hormones produced from thyroglobulin in the thyroid follicular cells. The stimulation of metabolic rate and regulation of growth and development by these hormones appear to be due to their effects on DNA transcription and, thus, protein synthesis. Legal Information BCR is a registered trademark of European Commission