Marker pairs
How to read biomarker pairs together
Some biomarkers are misleading when read alone. Ferritin alone can hide deficiency in inflammation. TSH alone misses central hypothyroidism. Total testosterone alone misses SHBG-driven cases. These guides cover the 5 highest-value pairs to read together.
Iron studies pair
Ferritin and TIBC (total iron binding capacity)
Ferritin and TIBC are interpreted together because ferritin can be misleadingly normal or even raised during inflammation, while TIBC moves in the opposite direction during true iron deficiency.
Thyroid first-line pair
TSH and Free T4
TSH and free T4 are the first-line thyroid axis pair: TSH reflects the pituitary signal and free T4 reflects what the thyroid actually delivers. Together they distinguish primary thyroid disease from secondary pituitary disease.
Calculated free testosterone pair
Total testosterone and SHBG (sex hormone binding globulin)
Total testosterone alone can be misleading because most of it is bound to SHBG and biologically inactive. The pair allows calculated free testosterone (the Vermeulen formula), which is what BSSM 2017 UK guidelines actually use for testosterone deficiency diagnosis.
Methylation B-vitamin pair
Vitamin B12 and Folate
B12 and folate are interpreted together because they share the methylation pathway, present with similar symptoms (macrocytic anaemia, fatigue, glossitis), and supplementing one without the other can mask the deficiency of the other.
Diabetes diagnosis pair
HbA1c and Fasting glucose
HbA1c and fasting glucose are interpreted together because they capture different time windows: HbA1c reflects average glucose over 8-12 weeks, while fasting glucose is a single-moment reading. They agree in most patients but disagree usefully in patients with rapid glucose changes, haemoglobinopathies, or recent illness.