Free Triiodothyronine

Also written Free T3

ThyroidMeasured in pg/mL

What it measures

The unbound, biologically active fraction of the more potent thyroid hormone.

Why it matters

It is useful in diagnosing and characterizing hyperthyroidism, and in specific clinical situations where TSH is unreliable.

Where the claimed ranges actually sit

Measured in pg/mL. Each row is one claim about where your result should fall.

What a laboratory reportsWhat a seller markets as “optimal”

Laboratory reference range2.3–4.2 pg/mL

Laboratory reference intervals

Upper third of the reference rangeD3.2–4.2 pg/mL

Hormone optimization clinics and thyroid protocols

›View as table

Claim	Range	Source
Laboratory reference range	2.3–4.2 pg/mL	Laboratory reference intervals
Upper third of the reference range	3.2–4.2 pg/mL	Hormone optimization clinics and thyroid protocols

Where the sources disagree

Mainstream endocrinology does not use free T3 to diagnose hypothyroidism, while hormone clinics frequently treat a low-normal value as the explanation for fatigue. Low free T3 with normal TSH most often reflects non-thyroidal illness syndrome, the body deliberately down-regulating metabolism in response to stress, illness, or under-eating. Prescribing T3 in that setting treats a thermostat reading rather than the room.

The marketed ranges, examined

Each claim gets its strongest possible case before it gets its criticism. Several of these are substantially correct.

Upper third of the reference range

Promoted by hormone optimization clinics and thyroid protocols

DUnsupported

The strongest case for it

Some people with normal TSH and low-normal free T3 report symptoms, and the concept of impaired peripheral conversion is biologically plausible.

What it leaves out

No outcome evidence supports treating a low-normal free T3 in someone with a normal TSH. Free T3 falls as a normal adaptive response to illness, caloric restriction, and overtraining; in those settings it is a signal to address the underlying cause, not a deficiency to medicate.

Before you act on your result

Falls adaptively in illness, caloric restriction, and overtraining.
Not recommended as a screening test for hypothyroidism.
Assay variation between laboratories is substantial.

Last reviewed July 28, 2026How we grade evidence and attribute claims →

More in Thyroid

Thyroid Stimulating Hormone

TSH

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