Free T3
The active, unbound form of triiodothyronine, the more potent thyroid hormone. Helps clarify thyroid status alongside TSH and Free T4.
Typical reference range
| Range | Unit |
|---|---|
| 3.1–6.8 | pmol/l |
What Free T3 measures
Free T3 (free triiodothyronine) is the unbound, most metabolically active thyroid hormone circulating in the blood. Only a small fraction of total T3 is free rather than bound to carrier proteins, and it is this free portion that reaches tissues and drives the pace of metabolism, so the free measurement reflects hormone that is actually available to cells rather than hormone held in reserve.
Most T3 is not released directly by the thyroid gland but is produced when the body converts T4 into T3 within the tissues themselves. Because of this, Free T3 is usually interpreted alongside TSH and Free T4, and sometimes thyroid antibodies, rather than read on its own. Together these markers give a clinician a fuller picture of thyroid function than any single value can.
What a high Free T3 can mean
A high Free T3 result can suggest an overactive thyroid, or hyperthyroidism, in which the body is exposed to more active thyroid hormone than it needs. Conditions commonly associated with this pattern include Graves disease, some forms of thyroiditis, and overactive thyroid nodules. It can also appear when thyroid hormone replacement is set higher than the body requires.
In some cases the pattern is described as T3-toxicosis, where Free T3 rises before Free T4 moves, so T3 may be the earlier signal. People with an overactive thyroid sometimes notice a sped-up feeling: a fast or pounding heartbeat, unexpected weight change, warmth or sweating, tremor, restlessness, or trouble sleeping.
Because the pituitary responds to abundant hormone, a raised Free T3 is often seen together with a low TSH. Any elevated result is best reviewed in context, so discuss it with a clinician.
What a low Free T3 can mean
A low Free T3 result can suggest an underactive thyroid, or hypothyroidism, where too little active hormone is available. Importantly, though, Free T3 also tends to fall during serious non-thyroidal illness, a pattern often called low T3 or euthyroid sick syndrome, in which the thyroid gland itself is working normally but the body dials down T3 as part of its response to being unwell.
This is one reason Free T3 is a poor test to rely on alone. A low value may reflect reduced conversion of T4 into T3 in the tissues rather than a problem with the thyroid, so the number cannot be read in isolation.
Sorting out these possibilities usually means looking at Free T3 together with TSH and Free T4, along with the wider clinical picture. Any low or unexpected result is best discussed with a clinician.
Track your Free T3 over time in Quanome — it reads your lab reports (PDF or photo) on your device, charts each marker against its range, and an AI coach explains what your trends mean alongside your Apple Health and DNA.