Serum Iron

Serum iron measures the amount of iron currently circulating in your bloodstream, bound to a transport protein called transferrin. Think of it as a snapshot of iron in transit: ferritin tells you how much iron is in storage, while serum iron tells you how much is on the move at any given moment. Iron is essential for making haemoglobin, the protein in red blood cells that carries oxygen to your muscles, brain, and every other tissue. It also supports energy production, immune function, and cognitive performance.

Without enough circulating iron, your body cannot efficiently produce new red blood cells, even if your iron stores are not yet depleted. One important thing to know is that serum iron fluctuates throughout the day and can be affected by recent meals, so it is best measured in the morning, ideally between 6am and 10am, and at least 24 hours after any iron or vitamin supplement. A single reading should always be interpreted alongside your ferritin, TIBC, and transferrin saturation to understand your iron status. Low serum iron is common and can leave you feeling tired, short of breath, or unable to concentrate. It is especially prevalent in women with heavy periods, vegetarians, vegans, and endurance athletes.

High serum iron is less common but can occur with excessive supplementation or conditions that cause iron to build up in the body. If your serum iron falls outside the expected range, your GP can help determine whether further investigation or dietary adjustments are needed.