A survey of goitre was made in the goitrous regions on the coast of Hokkaido, the northern island of Japan. Prevalence of goitre was confirmed in Hidaka coast and Rishiri Island. All goitrous patients were clinically euthyroid. The usual diet of the inhabitants of these districts consisted of a large quantity of iodine-rich seaweeds. Urinary excretion of iodine in five patients exceeded 20 mg per day. Studies of 131I and 127I metabolism were performed both during ingestion and after restriction of seaweed. When the patients were taking their usual diet, the mean thyroidal 131I uptake in 57 patients was 9.6% at 3 hours and 11.7% at 24 hours. In five of seven patients plasma inorganic iodine and thyroidal iodine space were markedly increased. Significant discharge of thyroidal 131I followed administration of thiocyanate. After withdrawal of seaweed from their usual diet, the plasma inorganic iodine was below 2 μg/100 ml but the thyroidal stable iodine uptake was higher than normal, depending on increase in thyroidal 131I clearance rate. No discharge was shown by thiocyanate block. Plasma PBI and thyronine-iodine level and serum T3 resin uptake were within the normal range. Radiochromatography of the thyroid tissue of the goitrous patients showed an increase in MIT/DIT ratio and a decrease in T3 + T4 proportion. No evidence for peripheral defect in DIT-131I deiodination was obtained. In a few patients restriction of seaweed induced a marked decrease in the size of goitre. The major cause of the endemic coast goitre seems to be excessive and longstanding intake of iodine from seaweed, and the similarities of iodine metabolism between the endemic coast goitre and iodide goitre arc discussed.

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