After four weeks of LT4 withdrawal, a 123I thyroid scintigram, 123I uptake rate, saliva/serum iodine ratio, perchlorate discharge test, thyroid function tests (TSH, FT4, FT3, and thyroglobulin), and thyroid ultrasonography are performed to diagnose athyreosis, hypoplasia, ectopic thyroid gland, hormone organification defect, and iodine concentration deficiency. The gene discussed is TG; the disease is athyreosis.