Urinary iodine, thyroid function, and thyroglobulin as biomarkers of iodine status

Abstract

The accurate assessment of population iodine status is necessary to inform public health policies and clinical research on iodine nutrition, particularly the role of iodine adequacy in normal neurodevelopment. Urinary iodine concentration (UIC) directly reflects dietary iodine intake and is the most common indicator used worldwide to assess population iodine status. The CDC established the Ensuring the Quality of Iodine Procedures program in 2001 to provide laboratories that measure urinary iodine with an independent assessment of their analytic performance; this program fosters improvement in the assessment of UIC. Clinical laboratory tests of thyroid function (including serum concentrations of the pituitary hormone thyrotropin and the thyroid hormones thyroxine and triiodothyronine) are sometimes used as indicators of iodine status, although such use is often problematic. Even in severely iodine-deficient regions, there is a great deal of intraindividual variation in the ability of the thyroid to adapt. In most settings and in most population subgroups other than newborns, thyroid function tests are not considered sensitive indicators of population iodine status. However, the thyroid-derived protein thyroglobulin is increasingly being used for this purpose. Thyroglobulin can be measured in either serum or dried blood spot (DBS) samples. The use of DBS samples is advantageous in resource-poor regions. Improved methodologies for ascertaining maternal iodine status are needed to facilitate research on developmental correlates of iodine status. Thyroglobulin may prove to be a useful biomarker for both maternal and neonatal iodine status, but validated assay-specific reference ranges are needed for the determination of iodine sufficiency in both pregnant women and neonates, and trimester-specific ranges are possibly needed for pregnant women. UIC is currently a well-validated population biomarker, but individual biomarkers that could be used for research, patient care, and public health are lacking.

Iodine status of euthyroid adults: A cross‐sectional, multicenter study

“…In this study, the subgroups with a higher UIC were associated with a higher median serum TSH, but not with statistical significance, and a relationship between UIC and FT3 or FT4 levels was not evident. Due to the large interindividual variation in the ability of the thyroid to adapt, thyroid hormones, including FT3, FT4, and TSH, are not considered sensitive indicators of the population iodine status . Evidence suggests that levels of thyroid hormones will remain within normal range in mild iodine deficiency, while the hormone levels will fall outside the normal ranges only in cases of severe iodine deficiency .…”

“…Iodine status has been historically assessed by goiter rates, although the definition of goiter is subjective. Currently, the iodine status is usually evaluated using the urine iodine concentration (UIC), which, based on population studies, is a recommended and reliable method to assess iodine status . Spot urine is a cost‐efficient, easily obtainable, and well‐acceptable method, since >90% of dietary iodine appears in urine within 24‐48 hours…”

Suboptimal Maternal Iodine Intake Is Associated with Impaired Child Neurodevelopment at 3 Years of Age in the Norwegian Mother and Child Cohort Study

“…To our knowledge, there are no valid biomarkers for assessing iodine status at an individual level (51). UIC is useful as an indicator of ID at the population level but not at the individual level because of large day-to-day variation.…”

The joint role of thyroid function and iodine concentration on gestational diabetes risk in a population‐based study

“…Cases and controls were selected at random from national administrative records covering the entire population of pregnant Finnish women, thus reducing risk of selection bias. While most studies of iodine status have used urinary iodine concentrations—which are susceptible to high day‐to‐day variability from recent dietary intake and urine dilution—our study used a novel measurement of iodide concentrations in serum, reducing the shorter‐term variability common in most urinary biomarkers . Diagnoses of GDM were made by physicians according to standardized guidelines.…”