|Background: The prevalence rate of subclinical hypothyroidism increase as a screening and progress of sensitivity of a method of measurement. The management of this syndrome is controversial and important problem due to risk of progression to overt hypothyroidism and risk of dyslipidemia, cardiac dysfunction and neurophyshiatris symptoms. We analyzed the natural history and ultrasonographic findings of 177 patients of subclinical hypothyroidism over 5-year. Methods: We reviewed prospectively total 177 patients showed elevated TSH secretion in the presence of normal concentrations. The auto-antibody(anti-TPO antibody, anti-Tg antibody), lipid profile were assessed initially. TFT, thyroid USG followed up over 5-years. Results: On baseline characteristics, age is 51± 12 years, female sex are total 131 patients(74%). glucose (98.5±30.3 mg/dL), total cholesterol 208.9±72.8 mg/dL), LDL (131.8±105.7 mg/dL), triglyceride(168.4±262.0 mg/dL) are checked. After 5 years, 80 of 177 patients(45.2%) normalized of TSH. 62 patients(35%) remained persistence of TSH, 19 patients(10.7%) developed overt hypothyroidism. thyroid cancer(4.5%), painless thyroiditis(2.8%), hyperthyroidism(1.7%) were developed. Ultrasonographic findings show that homogenous echogenecity(55.4%) is more than heterogenous echogenecity. Thyroid nodule is absent at 50.8% patients. In overt hypothyroidism patients, heterogenous echogenecity is more than homogenous(76.5% vs 23.5%, p=0.048) and negative thyroid nodule finding is more than positive(88.2% vs 11.8% p=0.041). As initial presence of auto-antibody, in normalized of TSH group, negative anti-Tg is more than positive(81.7% vs 18.3% p=0.041). But in overt hypothyroidism group, they are similar each other(53.3% vs 46.7%). And about anti-TPO antibody, in normalized of TSH group, negative anti-TPO is more than positive, too(83.3% vs 16.7%, p=0.016). Conclusion: There are limited data of natural course of subclinical hypthyroidism. Our data show that after 5 years 10.7% develop overt hypothyroidism over time, 35% remain in the subclinical stage, and 45.2% become normal. Also, we respect use of ultrasonographic finding and auto antibody state as prognostic factor that developed overt hypothyroidism.