Radioiodine kinetics and thyroid function following the universal salt iodization policy.

BACKGROUND

Universal salt iodization was introduced in Delhi in 1989. The present study quantifies the change in iodine kinetics as a result of this. The previous values were reported 10-30 years earlier, when Delhi was iodine deficient.

METHODS

Thirty subjects (18 men and 12 women, 17-48 years of age) who were residents of Delhi and had no thyroid disorder, were recruited from our outpatient clinic in 1999. The 24-hour urinary excretion of iodine and the iodine content of salt consumed at home by these subjects were estimated. Kinetic studies of iodine using radiotracer 131I were done to determine thyroid iodine clearance, renal iodine clearance, percentage uptake and absolute iodine uptake by the thyroid gland, and plasma inorganic iodine.

RESULTS

The median 24-hour urinary iodine excretion was 341.3 micrograms. The mean (SD) thyroid uptake of radioactive iodine was 4.9 (2.3)% at 2 hours and 19.1 (8.0)% at 24 hours. The median calculated plasma inorganic iodine was 1.36 micrograms/dl, absolute iodine intake 6.5 micrograms/hour and thyroid iodine clearance was 4.8 ml/minute (geometric means 1.68 micrograms/dl, 8.5 micrograms/hour and 8.1 ml/minute, respectively). The serum thyroid hormones and thyroid stimulating hormone were within normal limits.

CONCLUSION

Compared to the values reported 10-30 years ago when the population was iodine deficient, the present urinary iodine excretion, plasma inorganic iodine and absolute iodine intake have increased, while the percentage thyroid uptake of iodine ingested and thyroid clearance have decreased. The lack of change in the serum thyroid hormone levels after 10 years of universal salt iodization indicates that iodine consumption has had no adverse effect on thyroid function in these normal individuals. These changes are consistent with the increase in iodine consumption. Since the iodine ingestion in a community may change with time, assessment of iodine kinetics should be done periodically in different regions of the country.