Hypercholesterolemia is one of the most representative disorders of the common diseases. To evaluate the prevalence of hypothyroidism in the population of adult hypercholesterolemia, we prospectively examined the thyroid function in patients with untreated or treated hypercholesterolemia as a multi-center survey. Subjects were the patients who were treated with some antilipemic agents or the untreated patients whose total cholesterol (TC) was over 220 mg/dL and/or LDL-cholesterol (LDL-C) over

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... 0 mg/dL. Among 737 cases recruited, 725 cases (300 males and 425 females) participated in the survey including the thyroid function test. The patient's backgrounds include hypertension (51%), diabetes mellitus (49%), fatty liver (17%), smoking (15%), and habitual drinking (10%). The 72% of the patients were treated with some antilipemic agents and the mean values of TC, LDL-C, triglyceride (TG), HDL-cholesterol (HDL-C), and LDL-C/HDL-C ratio were 204.5 mg/dL, 119.6 mg/dL, 144.4 mg/dL, 60.7 mg/dL and 2.25, respectively. The primary hypothyroidism was seen in 27 cases (3.7%) (11 males, 16 females) with subclinical hypothyroidism in 17 cases (2.4%) and overt hypothyroidism in 10 cases (1.4%). The central hypothyroidism was seen in 4 cases (0.6%). The prevalence of hypothyroidism was 4.3% in patients with hypercholesterolemia. Taking account of the large number of patients with dyslipidemia and importance of avoiding unnecessary administration and associated adverse effects, evaluation of the thyroid function could be warranted in patients with dyslipidemia although cost-benefit issues waits further investigation.