Concentration of Metals and Trace Elements in the Normal Human and Rat Thyroid: Comparison with Muscle and Adipose Tissue and Volcanic Versus Control Areas

Background: The concentration of trace elements and metals in the thyroid is the result of exposure, uptake, retention, and clearance. The specificity and selectivity of thyroid capacity to concentrate these elements relative to other tissues are not known. To obtain this information, we measured the tissue concentration of 26 elements in the thyroid, muscle, and fat of euthyroid human subjects and also in normal rats. Methods: At programmed surgery, small (<1 g) tissue fragments were collected in 77 euthyroid subjects. Macroscopically normal thyroid tissue, sternothyroid muscle, and neck subcutaneous fat samples were excised, and thyroid tissue was confirmed to be morphologically normal through microscopy. Tissue specimens (thyroid, hindlimb muscle, and abdominal fat) were also obtained from normal rats. Measurements of trace elements were performed on tissues using inductively coupled plasma mass spectrometry (DRC-ICP-MS). Results: Only 19 of the 26 investigated elements were measurable as 7 elements were below the limit of detection. The ranking concentration in human thyroid tissue, not considering iodide, indicated that Zn, Br, Cu, Cr, Se, and Mn represented over 95% of the measured elements. A similar ranking was observed in the rat thyroid. A comparison with other tissues indicated that in addition to I, also Br, Mn, Se, and Sn were significantly more concentrated in the thyroid, and this was also the case for the recognized carcinogens As, Cd, and Hg. As and Hg, but not Cd (which was not detectable in any of the rat tissues), were also more concentrated in the rat thyroid. Since human thyroid specimens were also obtained from residents of a volcanic area, where environmental pollution may cause human biocontamination, we compared the trace element concentration in specimens from the volcanic area with controls. Many trace elements were slightly, but not significantly, increased in the volcanic area specimens. Conclusions: In the normal human thyroid, many trace elements, including Br, Mn, Se, and Sn, and the recognized carcinogens, As, Cd, and Hg, are significantly more concentrated than in muscle and fat of the same individual. Similar data were observed in rats. The reason for the differential element accumulation in the thyroid is unclear; a better understanding may be useful to further clarify thyroid biology.