What causes yellowing of the hair?

Hair yellowing can result from several physiological, environmental, and pathological factors. In many cases, it is a benign and reversible change. Common causes include sun exposure—particularly ultraviolet (UV) radiation—which oxidizes melanin pigment in the hair shaft, leading to photobleaching and a lighter, often yellowish or brassy appearance, especially in lightened or previously colored hair. Chemical treatments such as bleaching, perming, or frequent use of harsh shampoos strip natural lipids and disrupt the cuticle, increasing porosity and allowing environmental pollutants or metals (e.g., copper or iron from hard water or swimming pools) to bind to keratin, resulting in yellowish discoloration. Nutritional deficiencies—especially of protein, iron, zinc, biotin, or vitamin B12—can impair melanin synthesis or keratin production, contributing to dullness and abnormal pigmentation. Certain systemic conditions, including liver disease (e.g., jaundice-related hyperbilirubinemia), thyroid dysfunction (particularly hypothyroidism), or rare genetic disorders like kwashiorkor or Menkes disease, may also manifest with hair color changes, though these are typically accompanied by other clinical signs such as fatigue, skin changes, or growth delays. Additionally, aging naturally reduces melanocyte activity in hair follicles, leading to gradual graying that may appear yellowish before turning fully white. A thorough evaluation—including patient history, physical examination, and, when indicated, laboratory testing (e.g., ferritin, TSH, liver function tests)—is essential to distinguish benign causes from underlying pathology.