Functional indicators of iodine and selenium status

The micronutrient I, which has only one known biological function as a component of thyroid hormones, contrasts with Se, which is essential for many biochemical pathways through a range of Se-containing proteins. Thus, for I it is less complicated to propose and validate biochemical or functional markers which represent the adequacy or otherwise of dietary intake than it is to perform the same task for Se. Plasma thyroid-stimulating hormone provides a good indicator of functional I status, since levels of the hormone increase to compensate for any inadequacy in available dietary I. This increase occurs even when problems are caused by goitrogens restricting utilization of dietary I that otherwise would be adequate. In contrast, there are many potential indicators of Se status, especially since the many Se-containing proteins relate to different biochemical functions in the body. However, determination of the different GSH peroxidase (EC 1.11.1.9) activities in blood or tissue may give a useful indication of 'functional Se status', since these selenoproteins are often the most sensitive indicators of dietary Se intake. This approach has been successful in many experiments where animals of a similar genetic background have consumed diets which differ only in their Se content. Unfortunately, use of indicators of Se status in human populations may be complicated by differences (possibly genetic) between individuals that increase variability of selenoenzyme activities. The most appropriate indicator of functional Se status may also vary with other dietary conditions such as vitamin E or I deficiencies which may interact with Se deficiency. However, taking particular circumstances into account, determination of blood or tissue selenoprotein activity can provide a useful marker of adequacy of dietary Se supply for maintaining optimal health.