Free T4

T4 (thyroxine) is the main hormone the thyroid produces. Most of it circulates bound to proteins and is inactive; free T4 measures the small unbound fraction that is actually available to tissues, where it is converted to the more active T3.

Free T4 is what distinguishes overt from subclinical thyroid disease — and that distinction determines whether someone is treated. A raised TSH with a LOW free T4 is overt hypothyroidism, which should be treated. A raised TSH with a NORMAL free T4 is subclinical, where treatment frequently delivers no benefit. Free T4 is the number that decides.

1. 1

   FoundationHormonal HealthHow to read hormones properly — and why both over-treatment and under-treatment cause harm.
2. 2

   ConditionHypothyroidismAn underactive thyroid
3. 3

   Signal RecordLevothyroxineSynthetic T4 replacement for hypothyroidism
4. 4

   Body SystemHormonesEndocrine signaling that regulates the whole body.
5. 5

   BiomarkerThyroid-Stimulating HormoneA pituitary hormone — and the best first test of the thyroid

1. Measure it
2. Understand your result
3. See what raises and lowers it
4. Apply evidence-based interventions
5. Recheck on your review cadence

An orientation to how this topic is typically approached — not medical advice.

Free T4 is the second half of the thyroid picture, and it is what turns a TSH result into a diagnosis. Low free T4 with high TSH confirms overt hypothyroidism. Normal free T4 with high TSH means subclinical disease, where the case for treatment is weak. High free T4 with low TSH indicates thyroid hormone excess — from disease, or from too much levothyroxine. If TSH and free T4 move in the same direction, suspect pituitary disease or assay interference rather than a straightforward thyroid problem.

• Serum free T4
• Always interpreted with TSH
• Free T3 in selected cases

Free T4, read alongside TSH, is the guideline-standard means of distinguishing overt from subclinical thyroid disease — the distinction that determines treatment. Its interpretation is unreliable in acute illness and pituitary disease.

• ATA Guidelines for the Treatment of Hypothyroidism

Sources are named authoritative guidelines and scientific bodies. Biomarker pages present established clinical context; calibrated evidence verdicts on specific interventions live in the linked Signal Records.