Zinc, Selenium, and Iodine: The Trace Minerals Your Body Needs in Small Doses

What Are Trace Minerals and Why Do They Matter?

Trace minerals — also called trace elements — are dietary minerals the body requires in amounts typically under 100 milligrams per day. Though tiny, their physiological roles are anything but minor. Zinc, selenium, and iodine each sit at the intersection of critical body systems: immune defence, thyroid regulation, cellular repair, and antioxidant protection.

Unlike macrominerals such as calcium or magnesium, trace minerals are needed in micrograms or low milligrams. Yet falling short of even these small amounts can have measurable health consequences. For a broader view of how minerals and vitamins interact, see our micronutrient reference guide.

Zinc: Immune Function, Wound Healing, and Enzyme Activity

Zinc is involved in the activity of more than 300 enzymes and plays a structural role in proteins called zinc-finger proteins, which help regulate gene expression. Its most widely recognised roles include supporting immune cell development, enabling wound healing, and sustaining normal growth during childhood and pregnancy.

The body does not store zinc in a dedicated reserve, which means regular dietary intake matters. Good food sources include shellfish (particularly oysters), red meat, poultry, legumes, nuts, seeds, and whole grains. Plant-based sources contain phytates that reduce zinc absorption — a consideration worth discussing with a registered dietitian if your diet is primarily plant-based.

Zinc deficiency can impair immune function, delay wound healing, reduce appetite, and affect taste and smell. Excess supplemental zinc can interfere with copper absorption, which underscores why supplementing beyond established tolerable upper intake levels carries risk. For context on how nutrient needs change at different life stages, see how mineral requirements shift across life stages.

Selenium: Antioxidant Defence and Thyroid Support

Selenium functions primarily as a component of selenoproteins — a family of proteins that includes glutathione peroxidases, which neutralise harmful reactive oxygen species (free radicals) that can damage cells. It also supports the enzymes responsible for converting thyroid hormones into their active forms.

Brazil nuts are one of the most concentrated food sources of selenium; just one or two per day can meet daily needs, though intake should remain consistent rather than excessive. Other sources include seafood, organ meats, poultry, eggs, and whole grains — with content varying by soil selenium levels in the region where food is grown.

Selenium deficiency is associated with thyroid dysfunction, weakened immune response, and in severe cases, a heart condition known as Keshan disease, historically observed in regions with very low soil selenium. Selenium toxicity (selenosis) from excessive supplementation can cause brittle nails, hair loss, and neurological symptoms — reinforcing that more is not always better with trace minerals.

Iodine: The Thyroid Hormone Building Block

Iodine is indispensable for the synthesis of thyroid hormones — thyroxine (T4) and triiodothyronine (T3) — which regulate metabolism, growth, and neurological development. Adequate iodine intake is especially critical during pregnancy and early childhood, when thyroid hormone availability directly affects fetal and infant brain development.

The primary dietary sources of iodine include seafood, seaweed, dairy products, and eggs. Iodised table salt has been a cornerstone public health measure for preventing iodine deficiency globally since the early 20th century. In the United States, iodine deficiency is uncommon in the general population, though certain groups — including those avoiding dairy or iodised salt — may have lower intakes.

Iodine deficiency can lead to goitre (enlargement of the thyroid gland) and, in pregnancy, is a leading preventable cause of intellectual disability worldwide. Excessive iodine intake can paradoxically suppress thyroid function in susceptible individuals. For a complementary look at how vitamins support body processes, see what vitamins actually do in the body.

This article is for general informational purposes only and does not constitute medical advice. If you have concerns about your nutrient intake, thyroid health, or immune function, consult a qualified healthcare professional before making changes to your diet or supplement routine.