Variations in urinary iodine excretion and thyroid function. A 1-year study in healthy men

Objective: The iodine intake level in a population is determined in cross-s ectional studies. A fraction of samples with iodine content below a certain level, e.g. 25 mug/l, may suggest iodine deficiency in part of the populat ion. However, urinary iodine varies considerably from day to day and the fr action of low samples caused by dispersion remains unsettled. Design: A longitudinal study of 16 healthy men living in an area of mild to moderate iodine deficiency. Methods: We measured urinary iodine and creatinine concentrations, and seru m TSH, total thyroxine (T-4) free T-4 index and total tri-iodothyronine (T- 3) in samples collected monthly for 1 year. Results: Average urinary iodine excretion was 57.0 mug/l (49.1 mug/24 h (co rrected for creatinine excretion)) and varied from 29 to 81 mug/l (28 to 81 mug/24 h) between participants. Individual samples varied between 10 and 2 60 mug/l, and the Variation around the mean was 2.4 times larger when calcu lated for the 180 individual samples compared with the 15 average annual va lues (1.7 times larger for estimated 24 h iodine excretion values). The fra ction of individual samples below 25 mug/l was 6.7% (7.2% < 25 mug/24 h), w hereas none of the participants had average iodine excretion below 25 mug/l or 25 mug/24 h. Participants with average annual iodine excretion below 50 mug/24 h had a negative correlation between iodine excretion and TSH, wher eas a positive correlation was observed when average annual iodine excretio n was above this level. Conclusions: Seven per cent of individual urine samples indicated severe io dine deficiency without this being present in the group studied. Dispersion was reduced by 24% when using estimated 24 h urinary iodine excretion rath er than urinary iodine concentration. Participants with moderate iodine def iciency (average annual urinary iodine excretion 25-50 mug/24 h) showed cle ar signs of substrate deficiency for thyroid hormone synthesis while partic ipants with mild iodine deficiency (50-100 mug/24 h) did not.