To explore the hypothesis that the iodine intake of vegans might be inadequate, thyroid stimulating hormone concentrations were measured in plasma samples from 101 British men, of whom 48 were vegans and 53 were omnivores. The geometric mean thyroid stimulating hormone concentration, adjusted for age and body mass index, was 47% higher in the vegans than in the omnivores (P= 0.001). Five vegans but none of the omnivores had a thyroid stimulating hormone concentration above the reference upper limit of 5 mU/1 (P=0.022). High concentrations of thyroid stimulating hormone are usually indicative of marginal iodine status, but can also result from excessive iodine intake. The three vegans with the highest values of thyroid stimulating hormone reported regular use of kelp tablets or kelp powder. None of the other vegans, and none of the omnivores, reported taking kelp. After exclusion of the three vegans who took kelp, the remaining vegans had an adjusted geometric mean thyroid stimulating hormone concentration 29% higher than that of the omnivores (P=0.012). It was concluded that use of kelp can be associated with raised thyroid stimulating hormone, and that the iodine status of vegans who do not use kelp requires further investigation.