Japanese cuisine-induced hyperthyroidism

We present a 46-year-old man with subclinical hyperthyroidism and low iodine 123 uptake. Three months later, all symptoms resolved on beta blockers only; iodine 123 uptake and hormone levels returned to normal. We suspect that hyperthyroidism was secondary to consumption of kombu, a long dark brown to grayish-black seaweed, in a soup. With the current popularity of Japanese restaurants, this possibility should be kept in mind when investigating the causes of iodine overload.