The normal range for thyroid radioactive iodine (RAI) uptake values in the Minneapolis area has shifted twice in the past 20 years. The mean 24hour thyroid RAI uptake value in 29 normal subjects was 20.5%±6.1% (SD) in 1975, whereas only four years previously it was found to be 11.5%±4.4% (SD) in 51 normal subjects. In a previously published study, the mean thyroid RAI uptake value in normal subjects in this area was 25%±8% (SD) in 1957. The iodine content of commercially baked white bread currently available in this area was found to average only 26 μg per single slice of bread; the change in the mean 24-hour thyroid RAI uptake value that we observed between 1971 and 1975 is most likely due to a marked reduction in this source of dietary iodine.
(JAMA 238:1741-1743, 1977)
Wong ET, Schultz AL. Changing Values for the Normal Thyroid Radioactive Iodine Uptake Test. JAMA. 1977;238(16):1741–1743. doi:10.1001/jama.1977.03280170035022