Blood test biomarkers,
explained.

Ferritin is the protein that stores iron in your body. It is the single best indicator of whether your iron reserves are full or running out.

TSH is the pituitary's instruction to the thyroid. It moves in the opposite direction to thyroid output, which is why a high TSH suggests an underactive thyroid.

25-hydroxyvitamin D is the storage form of vitamin D and the value labs report. It reflects your supply over the past weeks rather than today.

Vitamin B12 is needed for red blood cell formation and nerve function. Serum B12 is the usual first test, but it is a less reliable marker than its widespread use suggests.

LDL carries cholesterol from the liver to the tissues. It is the lipid value most closely tied to cardiovascular risk, and the target depends on your overall risk rather than a single cut-off.

HbA1c shows what proportion of your haemoglobin has sugar attached to it, which reflects average blood glucose over roughly the previous three months.

CRP is made by the liver during inflammation. It rises and falls quickly, which makes it useful for detecting an active process and for judging whether other markers can be trusted.