Markers · Thyroid
A free T3 at the low end, beside a TSH that looks fine. Free T3, read against the rest of you.
What free T3 measures, what it does in the body, why it's tested, and the two ranges it's read against. Everything said about the marker here comes from Merlin's own biomarker catalogue.
What free T3 is
Free T3 represents the small fraction of triiodothyronine circulating in the blood that is not bound to carrier proteins and is therefore biologically active. It is the most metabolically potent thyroid hormone, directly influencing cellular energy production, metabolism, heart rate, and body temperature regulation. Low Free T3 levels can indicate hypothyroidism or poor T4-to-T3 conversion, while elevated levels suggest hyperthyroidism or excessive thyroid hormone replacement.
What it does in the body
T3 is the primary active thyroid hormone that binds to nuclear receptors in nearly every cell of the body, regulating basal metabolic rate, thermogenesis, protein synthesis, cardiac output, neurological development, and the metabolism of carbohydrates, lipids, and vitamins. Approximately 20% is secreted directly by the thyroid gland, while 80% is produced through peripheral conversion of T4 to T3 via deiodinase enzymes in tissues such as the liver and kidneys.
Why it's tested
Free T3 testing is used to assess thyroid function, diagnose hyperthyroidism or hypothyroidism, evaluate T4-to-T3 conversion efficiency, monitor thyroid hormone replacement therapy, and investigate symptoms such as fatigue, weight changes, cold intolerance, palpitations, or mood disturbances. It is particularly valuable when TSH and Free T4 alone do not fully explain a patient's symptoms.
The two ranges
The reference range reflects population statistics. The target reflects physiology.
- Lab reference range
- 2.0-4.4 pg/mL (3.1-6.7 pmol/L)
- Functional target
- 3.0-4.5 pg/mL (4.6-6.9 pmol/L)
Those are different numbers, and you deserve to know which one you're looking at. Merlin reads your result against both.
Where this lives
A Field Guide to Thyroid & Adrenal Health
The field guide that covers free T3 in depth, whole and public.
In a real read
The real read on this site is a Cardiovascular one, so there's no thyroid panel in it. What it shows is the shape: one system, thought all the way through, with the score, the narrative and the layers underneath. A thyroid panel gets one of these too, with free T3 read beside TSH and everything else you brought.
A real Cardiovascular surface from a real anonymized panel: the score, the narrative, the layers underneath. Yours will be built the same way, on your own numbers.
Educational context for your conversations with your licensed healthcare provider, not a diagnosis or a treatment plan. Merlin reads results; it doesn't diagnose.
Read next to
No free T3 result yet? You can order it inside Merlin: it's on Thyroid Complete ($99).
Bring the panel this number came from. Merlin reads it against everything else you have, every system and every year, then shows you what it means and what's worth finding out next. Start with your first month.