Understanding the marker
Free T4 measures the unbound part of thyroxine, the main hormone made by the thyroid. The body converts some T4 into the more active hormone T3.
Free T4 is usually read with TSH, the signal that helps control thyroid hormone production. The combination helps assess thyroid function or treatment. Illness, medicines and interference with the laboratory method can affect the result.
What does high Free T4 mean?
High free T4 may reflect increased thyroid hormone production or a treatment effect. TSH, symptoms and sampling or medicine information are needed before drawing a conclusion.
What does low Free T4 mean?
Low free T4 may suggest reduced thyroid hormone availability. TSH, symptoms and any treatment show whether the overall pattern fits an underactive thyroid or needs another explanation.
Panels that measure Free T4
Thyroid Care
Thyroid Care Plus
Midlife Health
Energy & Fatigue Plus
Northstar
Vitals Pro
Menstrual Health Advanced
Fertility Advanced
Vitals Max
Founders Panel Plus
Reading
- Tired all the time? The blood values that can explain why
- PMOS (previously called PCOS) and blood tests: the values that bring clarity
- Normal TSH but still tired: what could be behind it?
- Menopause or your thyroid: the symptoms that overlap and the blood tests that tell them apart
- Can a blood test explain your sleep problems?
- Which blood tests are relevant for hair loss?
- Can a blood test explain weight gain?
More markers in the same area
This is general information about the marker, not an assessment of your values and not medical advice. Last updated: October 2026.