The relation between plasma inorganic iodine level, thyroid clearance and absolute iodine uptake of the thyroid gland has been studied in Iceland and compared with results obtained in Glasgow using identical methods. In Iceland the plasma inorganic iodine (PII) is higher than in Glasgow due to the high iodine content of the diet and the thyroid clearance lower. This adjustment is, however, incomplete, relatively high PII levels being associated with a low thyroid clearance and a raised absolute uptake by the thyroid. There was no evidence of increased hormone production as reflected in the serum PBI.