This study was conducted to examine the usual iodine intake in patients with thyroid diseases and to compare iodine status with normal subjects. The dietary iodine intake was assessed using a semi-quantitative food frequency questionnaire, and urinary iodine excretion was measured in 184 patients diagnosed with thyroid diseases and 207 normal subjects. The average usual iodine intake of patients with thyroid diseases was 673.8 ± 794.9 ug/day and that of normal subjects was 468.9 ± 481.9 ug/day. Among the patients with thyroid diseases, higher values were found in the patients with thyroid cancer (1460.6 ± 1044.8 ug/day) and lower values were found in patients with simple goiter (443.5 ± 470.4 ug/day). The urinary iodine excretions of patients and normal subjects were 4.33 ± 5.70 mg/L and 2.11 ± 0.69 mg/L, respectively. The iodine intake and urinary iodine excretion of patients with thyroid diseases were significantly higher than those of normal subjects (p < 0.05). The dietary iodine intake and urinary excretion of patients with thyroid cancer were significantly higher than other patients with thyroid diseases and normal subjects because of the use of seaweed or seaweed-containing dietary supplements (p < 0.01). This study suggests that the habitual ingestion of seaweed-containing dietary supplements in addition to dietary iodine intake will have adverse effects due to its excessive iodine intake.