The biologically active thyroid hormone — one of the iodine-containing hormones made by the thyroid gland, also generated by converting thyroxine (T4) in peripheral tissues. Triiodothyronine (T3) regulates metabolic rate, growth, and development far more potently than T4. Iodine deficiency cripples its production, causing goitre and developmental harm — the rationale for iodized salt. In livestock, thyroid-active substances are banned as growth manipulators, since residues would mean both fraud and a health risk; dietary goitrogens in brassicas and cassava can also interfere with thyroid action where iodine nutrition is marginal.