*Geek Box: Measuring Iodine and Defining Deficiency
Biomarkers may be classified according to where they are measured in the body. For example, where levels of a nutrient are measured in adipose tissue or plasma, these are termed ‘concentration biomarkers’ because the concentration of the nutrient in these tissues is being measured. Biomarkers that are measured through excretion are known as ‘recovery biomarkers’, because it is the level of output in, for example, urine, that reflects intake.
Recovery biomarkers are few and far between in nutrition, with sodium and potassium being the two commonly used examples: both are measured through urinary collections. Iodine status may also be measured by urinary sample collections, as the majority of iodine is excreted in urine. Further, iodine may have more reliable immediate measurements because a high proportion of intake appears very rapidly in urine.
Generally, urinary iodine reflects intake in the immediate preceding 2-3 days. For iodine, urinary content practically equals intake, and thus urinary iodine is a highly accurate reflection of short-term intake. However, these spot urinary samples [i.e., where only a single sample is taken during the day, not a full 24hr of all urine collection] are not as accurate as a full 24hr collection. As a result, the ‘next best thing’ is to use the ratio of iodine to creatinine in urine. This is because creatinine excretion is relatively consistent throughout the day, thus the ratio of iodine may be used to estimate 24hr iodine levels from a single spot urinary sample.
Currently, the WHO defines iodine urinary levels of <150μg/g as deficiency, 150 to <50 μg/g as adequate, and ≥500μg/g as excess.