What is Free T4 (only if TSH is abnormal)?
Free T4 helps clarify an abnormal TSH and supports distinguishing different thyroid function patterns. In many protocols, Free T4 is only measured when TSH is abnormal. The combined interpretation of TSH and Free T4 is typically most informative.
Why is Free T4 (only if TSH is abnormal) relevant?
TSH reflects what the brain thinks the thyroid should do; Free T4 reflects what the thyroid is actually producing. Together they untangle where a problem sits: elevated TSH with low T4 classically signals primary hypothyroidism, whereas elevated TSH with normal T4 is 'subclinical' — a grey zone that warrants attention but is not always treated right away.
How to read Free T4 (only if TSH is abnormal) in context
Free T4 is almost always interpreted alongside TSH; on its own it carries little meaning. Both markers are ideally drawn in the morning because they follow a daily rhythm. In edge cases or for patients on thyroid medication, Free T3 can add value, since some conversion issues only show up in T3 levels. For follow-up after a dose change, allow 6–8 weeks for the system to stabilize before re-testing.