Magnesium

The Magnesium in Hair test (hair magnesium; determination of magnesium in hair) is used to assess longer-term magnesium balance as part of a nutritional evaluation. It is employed as an adjunct in the workup of individuals with growth delay, autism spectrum disorder, fibromyalgia, and trichothiodystrophy. Scalp hair composition may indicate chronic alterations in magnesium status even when blood and urine values fall within reference limits. This analysis is not intended for the diagnosis or acute management of magnesium disorders.

Magnesium is a major intracellular divalent cation that serves as a cofactor for more than 200 enzymes, membrane transport systems, and nucleic acid–related processes, supporting ATP-dependent energy metabolism, DNA/RNA synthesis, signal transduction, and other physiologic pathways. In a 70‑kg adult, total body magnesium is approximately 25 g, predominantly stored in bone; only about 1% resides in the extracellular space, so circulating measurements can underrepresent whole-body stores. Homeostasis is governed chiefly by renal tubular reabsorption, with a lesser contribution from intestinal absorption. Magnesium metabolism is interconnected with calcium and phosphorus pathways and is modulated by vitamin D and parathyroid hormone. Deficiency can accompany inadequate intake or increased losses and is seen with alcoholism, vomiting or diarrhea, malabsorption and enteric fistulae, vitamin D deficiency, renal disorders, and exposure to specific medications, including diuretics, cyclosporine, cisplatin, aminoglycosides, and amphotericin B, as well as rare hereditary tubulopathies such as Bartter and Gitelman syndromes. Hair is a keratinized tissue that incorporates minerals during active follicular growth; once formed, the shaft entraps elements and integrates exposure over weeks (approximately 6–8 weeks). Compared with blood or urine, scalp hair may better reflect chronic disturbances of magnesium metabolism, and its painless collection can be advantageous in pediatric patients. Reports describe lower hair magnesium in children with growth retardation and in individuals with autism spectrum disorder, although causality remains unresolved; magnesium deficiency may amplify the neurotoxic effects of aluminum, cadmium, mercury, and lead, to which children show heightened susceptibility. In fibromyalgia, reduced hair magnesium with normal serum concentrations has been observed, potentially indicating intracellular deficiency that could contribute to symptoms and to impaired vitamin D activation with secondary hypocalcemia; for this reason, hair magnesium has been proposed as an adjunct in nutritional assessment in such patients. Within dermatology, hair is an accepted specimen in selected contexts, including trichothiodystrophy. Limitations include notable interindividual variation in hair structure and composition and vulnerability to external contamination (for example, water and cosmetic treatments), both of which can affect reported concentrations. Accordingly, magnesium, hair analysis is complementary to conventional testing and is not used for acute diagnostic decision-making.