Long term follow-up of the patients with delayed TSH elevation or subclinical hypothyroidism has been seldom reported. The purpose of this study was to explore the diagnostic criteria for subclinical hypothyroidism and the initial dosage of L-thyroxine through long-term follow up for infants with hyperthyrotropinemia，and evaluate the curative effect.All live-born neonates in province took part in the screening program from October, 1999 to September ,2006. Laboratory tests are performed as a time resolved fluoro-immunoassay (TRFIA). TSH is measured with a cut-off > 9 mU/L. The diagnostic standard for subclinical hypothyroidism was: TSH ≥ 20 mU/L, T3 and T4 normal or low-normal ,or TSH > 5.6 mU/L and < 20 mU/L on initial determination and on follow-up or TSH levels increase on follow-up and/or gradually declining T4 levels.L-Thyroxine was administered for substitution therapy. After 2 years therapy，allround evaluation was performed. Compare the correlation to effects with different level of TSH、starting time of therapy and environment. Compare the effects between subclinical、temporary and permanent hypothyroidism.