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

Abstract

Objective: The iodine intake level in a population is determined in cross-sectional studies. A fraction of samples with iodine content below a certain level, e.g. 25 μg/l, may suggest iodine deficiency in part of the population. However, urinary iodine varies considerably from day to day and the fraction 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 serum TSH, total thyroxine (T4), free T4 index and total tri-iodothyronine (T3) in samples collected monthly for 1 year. Results: Average urinary iodine excretion was 57.0 μg/l (49.1 μg/24 h (corrected for creatinine excretion) and varied from 29 to 81 μg/l (28 to 81 μg/24 h) between participants. Individual samples varied between 10 and 260 μg/l, and the variation around the mean was 2.4 times larger when calculated for the 180 individual samples compared with the 15 average annual values (1.7 times larger for estimated 24 h iodine excretion values). The fraction of individual samples below 25 μg/l was 6.7% (7.2% <25 μg/24 h), whereas none of the participants had average iodine excretion below 25 μg/l or 25 μg/24 h. Participants with average annual iodine excretion below 50 μg/24 h had a negative correlation between iodine excretion and TSH, whereas a positive correlation was observed when average annual iodine excretion was above this level. Conclusions: Seven per cent of individual urine samples indicated severe iodine deficiency without this being present in the group studied. Dispersion was reduced by 24% when using estimated 24 h urinary iodine excretion rather than urinary iodine concentration. Participants with moderate iodine deficiency (average annual urinary iodine excretion 25-50 μg/24 h) showed clear signs of substrate deficiency for thyroid hormone synthesis while participants with mild iodine deficiency (50-100 μg/24 h) did not.