Reliability of studies of iodine intake and recommendations for number of samples in groups and in individuals

Abstract

The iodine intake level in a population is determined in cross-sectional studies. Urinary iodine varies considerably and the reliability of studies of iodine nutrition and the number of samples needed is unsettled. We performed a longitudinal study of sixteen healthy men living in an area of mild to moderate iodine deficiency. Iodine and creatinine concentrations were measured in spot urine samples collected monthly for 13 months. From these data we calculated the number of urine samples needed to determine the iodine excretion level for crude urinary iodine and for 24 h iodine excretion estimated from age-and gender-specific creatinine excretions. We found that mean urinary iodine excretion varied from 30 to 87 mg/l (31 to 91 mg/24 h). Sample iodine varied from 10 to 260 mg/l (20 to 161 mg/24 h). Crude urinary iodine varied more than estimated 24 h iodine excretion (population standard deviation 32 v. 26; individual standard deviation 29 v. 21; Bartlett's test, P,0·01 for both). The number of spot urine samples needed to estimate the iodine level in a population with 95 % confidence within a precision range of^10 % was about 125 (100 when using estimated 24 h iodine excretions), and within a precision range of^5 % was about 500 (400). A precision range of 20 % in an individual required twelve urine samples or more (seven when using estimated 24 h iodine excretions). In conclusion, estimating population iodine excretion requires 100 -500 spot urine samples for each group or subgroup. Less than ten urine samples in an individual may be misleading.

Intake of Iodine and Perchlorate and Excretion in Human Milk

“…For studies that relied on one-time spot samples (all but this study and ref 27), there is also the problem of large temporal variability for the same individual (27). Recently Andersen et al (47) concluded that estimating population iodine excretion requires 100-500 urine samples and for an individual, g12 urine samples are needed to obtain ( 20% accuracy. If this applied to milk, only the present data and the data set in ref 28 will qualify.…”

Longitudinal trends in thyroid function in relation to iodine intake: ongoing changes of thyroid function despite adequate current iodine status

“…In 1052 participants of this subgroup, the iodine concentration was measured in fasting morning urine samples in 2006. Serum TSH and FT 4 were measured twice with an interval of 4 years (2002 and 2006). The mean interval between the measurements was 4.1 years (range 2.8-5.4 years).…”

“…To assess the iodine status of a population, the median iodine concentration of series of single urine samples is the most widely used measurement (1,3). In large population studies, there is a leveling out of the dayto-day variation, and the median value of the urinary iodine concentration in samples can be used to assess whether a population is iodine sufficient (4). According to the World Health Organization (WHO) criteria, a population has an optimal iodine intake if its median urine iodine concentration is between 100 and 199 mg/l and no more than 20% of the population has an urinary iodine concentration of 50 mg/l or less (1,3).…”

Iodine supplementation during pregnancy of hypothyroid women treated with L-thyroxine neither influences neonatal TSH nor prevents decrease in maternal free thyroid hormone concentrations in second and third trimesters

“…Iodine supplementation resulted in an expected increase in median morning ioduria, as well as an increase in the percentage of patients with desirable urinary iodine concentration above 150 μg/L to 61.1%. We realise that the number of participants in our study is not adequate to assess iodine sufficiency in the population [11]; however, the increase in iodine urinary excretion confirms additional iodine intake in the "thyroxine + iodine" group. In healthy pregnant women fT4 concentrations decrease at the second and third trimester [12].…”