Effect of lifelong iodine supplementation on thyroid 131-I uptake: a decrease in uptake in euthyroid but not hyperthyroid individuals compared to observations 50 years ago

Background: In Sweden, iodine has been added to table salt (10 mg/kg) since 1936; this amount was increased in 1966 to 50 mg/kg. Objective: To investigate a euthyroid Swedish population (n=44, 60-65 years) with its entire lifespan with iodine supplementation as for 24-h 131-I uptake (24h IU) and thyroid nodularity (thyroid scintigraphy). To compare the euthyroid 24h IU with uptake of thyrotoxic individuals, and with observations from 1955. Methods: The 24h IU was used in euthyroid individuals after oral administration of 0.1 MBq/2.7 microcurie radioiodine and imaging of the thyroid gland was carried out using 99mTc-pertechnetate. Results: In 1999-2000, the mean 24h IU in the euthyroid individuals was 21% (range 11-33%) and the normal (central 95%) reference interval was 14-30%. Scintigraphy suggested multinodular goitre in three euthyroid individuals. In Graves' patients (n=53, 50-65 years), the mean 24h IU was 61% (range 29-89%). In 1955, the 24h IU in euthyroid individuals was higher (38%, range 10-70%), while hyperthyroid patients had uptake values similar to those recorded in the present investigation (mean 62%, range 40-90%). Conclusions: The population sample studied had to be small for ethical reasons. We conclude that the reference interval for 24h IU is 14-30% in this population that had spent its entire lifespan with iodine supplementation. This is lower than that recorded in a Swedish euthyroid population half a century ago having had low-grade table-salt iodine supplementation for 20 years. Values for hyperthyroid patients, however, do not appear to have been affected likewise.