Decreased radioiodine uptake values without increase in urinary iodine excretion

A total of 12842 radioiodine uptake measurements (RIU), performed for diagnostic purposes at Malmö General Hospital 1955-75, have been reviewed. In agreement with the general trend in Europe and the USA, there was a significant decrease in uptake during this period. The 24-hour RIU (mean +/- S.D.) was 51 +/- 18% in 1955, 41 +/- 17% in 1965 and 37 +/- 18% in 1975. The 3- and 48-hour uptake values, as well as the protein-bound radioiodine, fell in a parallel fashion. The fall can not be explained by an increased dietary supply of iodine, because the mean urinary excretion did not show any consistent change from 1965 to 1975, apart from a falling trend from 1967 to 1971. The cause of the decrease in thyroid iodine uptake is not elucidated by this study.