Today, human iodine deficiency is, after iron, the most common nutritional deficiency in developed European and underdeveloped third world countries. A current biological indicator of iodine status is urinary iodine, which reflects very recent iodine exposure; a long-term indicator of iodine status remains to be identified.We analyzed hair iodine in a prospective, observational, cross-sectional, and exploratory study involving 870 apparently healthy Croatians (270 men and 600 women). Hair iodine was analyzed with inductively coupled plasma mass spectrometry.The hair iodine median was 0.499 μg/g, and was 0.482 and 0.508 μg/g for men and women respectively, suggesting no sex-related difference. We studied hair iodine uptake by analyzing the logistic sigmoid saturation curve of the median derivatives to assess iodine deficiency, adequacy, and excess. We estimated overt iodine deficiency to occur when hair iodine concentration was below 0.1-0.15 μg/g. Then there was a saturation range interval of about 0.1-2.0 μg/g where the deposition of iodine in the hair was linearly increasing (R(2)=0.994). Eventually, the sigmoid curve became saturated at about 2.0 μg/g and upward, suggesting excessive iodine exposure.Hair appears to be a valuable and robust biological indicator tissue for assessing long-term iodine status. We propose that an adequate iodine status corresponds with hair iodine uptake saturation of 0.565-0.739 μg/g (55-65%).