Introduction: The aim of this study was to determine the incidence of thyroid dysfunction and the natural course of subclinical thyroid disorders in the Tehranian community. Materials and Methods: All individuals ≥20 years, who participated during the first to third phases (6 years, 7 months), of the "Tehran Lipid and Glucose Study" and provided the relevant data were included in this study. Both Tpo-Ab and TSH were measured. In 808 TPO-Ab negative individuals who were not taking any thyroid & anti-thyroid drugs and without a history of thyroid disease, thyroid surgery, goiter and thyroid nodules, mean, median, 2.5, 5, 95, 97.5 percentile TSH were determined. On the basis of 2.5 and 97.5 percentile, normal reference range for TSH was 0.4-5.8 μu/mL. In those, whose TSH fell outside the reference range, T3, T4, and T3 uptake were measured and FTI was calculated. Results: In the first stage, 1065 women and 693 men had normal thyroid tests. After 6.7 years the incidence of clinical hypothyroidism was 0.28 in 1000 women and 0. 21 in 1000 men, subclinical hypothyroidism was 11.59 in 1000 women and 4. 69 in 1000 men, clinical hyperthyroidism was 1.4 in 1000 women and 0.21 in 1000 men, subclinical hyperthyroidism was 5.72 in 1000 women and 3.62 in 1000 men. In this period, increasing positivity of TPO-Ab from 15.9 to 17.7% in women was significant. (P=0.006) In the first stage 8 women had subclinical hypothyroidism, 5 still did on follow-up, one was normal, and one was diagnosed with clinical hypothyroidism. The remaining one was hyperthyroid on levothyroxin. Two women with subclinical hyperthyroidism in the first stage were normal in follow-up, without any treatment. In the first stage 2 men had subclinical hypothyroidism, and in follow-up, one was same, while the other was diagnosed with clinical hypothyroidism. Conclusion: Normal range of TSH was 0.4-5.8 μu/mL in the Tehranian community. There was significant increase of the frequency of subclinical thyroid disorders in both genders and frequency of clinical hyperthyroidism and TPO-Ab positive in women. Compared to clinical thyroid disorders, the incidence of subclinical thyroid disorders, was more significant.