"Minerals reach hair via the blood so it’s going have similar limitations as blood testing since hormones control the levels of many of the minerals giving little indication of the status for the rest of the body. Dr Kellon VMD 'Hair mineral is not an accurate way to determine mineral status and is worthless for formulating the diet and supplements.' NRCPlus

The kidneys will rapidly excrete minerals when they are higher than a tight range, electrolytes like potassium are a good example. The liver can remove minerals before they reach hair. Hair testing can’t tell mineral balance even if the totals were credible as it doesn’t indicate whether it’s due to a deficiency in the first place or competition with another mineral or the body had a high need at that time. Hair can be useful for some heavy metals and selenium but from a nutrition point of view hair testing is not considered overall to be reliable."