The impact of thyroid dysfunction in subclinical ranges on metabolic syndrome (MetS) is not well known. The aim of the present study is to evaluate the association of thyroid dysfunction with MetS and its components. In the cross-sectional population-based Tehran Thyroid Study, out of 5 786 randomly selected participants, aged >= 20 years, subjects with thyroid nodules and cancer or any severe systemic disease, those who were pregnant and those using thyroid medication were excluded, leaving 5422 subjects to be investigated. Body weight, waist circumference, and blood pressure were measured. Fasting blood glucose and concentrations of lipids and lipoproteins, free T4, and TSH were assayed. Mean age of the participants was 40.3 +/- 14.4 of whom 101 (2 %) had overt hypothyroidism, 294 (5 %) subclinical hypothyroidism, 82 (2 %) overt hyperthyroidism, and 178 (3 %) had subclinical hyperthyroidism; 1 704 (32 %) had MetS. Clinically hypothyroid subjects had the highest prevalence of MetS (41.6 %), abdominal obesity (45 %), and hypertriglyceridemia (58 %) compared to other groups (p50. Overt and subclinical hyperthyroidism had significantly higher odds of hyperglycemia in men and women after full adjustment for age, smoking, and BMI. Overt hypothyroidism and subclinical hypothyroidism especially in the elderly could be associated with MetS. Hyperthyroidism may induce hyperglycemia.