This chapter explores the roles, metabolism, deficiencies, and toxicities of four essential trace elements—iodine, cobalt, molybdenum, and chromium—integral to human health. Iodine, stored predominantly in the thyroid gland, is critical for thyroid hormone synthesis, with deficiencies leading to iodine deficiency disorders (IDD), a major cause of intellectual disabilities worldwide. Cobalt, central to vitamin B12, supports red blood cell production and neurological health, with its deficiency resulting in megaloblastic anemia and myelopathy. Molybdenum, a key component of enzymes in detoxification and redox reactions, can cause severe neurological impairments when deficient, though such cases are rare. Conversely, chromium, in its biologically active trivalent form, facilitates insulin function and glucose metabolism. Chromium deficiency, often observed in older adults and patients with reduced absorption, can result in glucose intolerance and neuropathy. Toxicities of these elements, while less common, can disrupt physiological balance, emphasizing the necessity of optimal intake. This chapter underscores the importance of understanding the intricate mechanisms governing these trace elements to manage deficiencies and mitigate risks, highlighting their profound impact on public health and clinical outcomes.