Biomarker 101: T3 (triiodothyronine)

T3 is the thyroid hormone that does the work — and most of it is not made by the thyroid at all.

What T3 is

Triiodothyronine, or T3, is the biologically active thyroid hormone. It sets metabolic rate, and influences body temperature, heart rate, growth and brain function.

The counterintuitive part is where it comes from. The thyroid secretes far more T4 than T3, and roughly 80 per cent of your circulating T3 is produced by converting T4 into T3 in tissues such as the liver and kidneys, through a process called deiodination.

So thyroid function is not only a question of how much hormone the gland produces. It also depends on how efficiently the body converts it.

Free T3 versus total T3

As with T4, most T3 in the blood is bound to carrier proteins and unavailable to tissue. Free T3 is the unbound, active portion, and it is what laboratories generally report.

Total T3 measures both bound and free together. It has occasional uses, particularly where binding protein levels are abnormal, but free T3 more closely reflects what is actually available to cells.

Reverse T3

T4 can also be converted into reverse T3, an inactive form that is chemically similar to T3 but cannot activate cells. It appears to act as a natural brake on thyroid hormone activity.

Reverse T3 is not routinely measured. There is limited evidence on how to interpret a raised result, which is why it tends to be confined to specialist settings rather than general panels.

Why it's measured

T3 is not always included in a basic thyroid panel. It earns its place when TSH and T4 do not adequately explain what someone is experiencing — because conversion from T4 to T3 varies, and a picture that looks unremarkable on TSH and T4 alone can look different with T3 added.

What the numbers mean

A typical adult reference range for free T3 is around 3.1 to 6.8 pmol/L, varying by laboratory and assay.

Lower T3 is associated with an underactive thyroid, and also with iodine deficiency, some medications, and chronic liver or kidney conditions that reduce conversion.

Higher T3 is associated with an overactive thyroid, and can also rise in pregnancy as demand for thyroid hormone increases.

What T3 won't tell you

• It isn't a standalone test. T3 is interpreted alongside TSH and free T4, never on its own.
• A borderline result may mean nothing. Where TSH and T4 are normal and free T3 is marginally off, this often reflects temporary variation.
• It doesn't identify the cause. Determining why levels have shifted requires further testing.
• Reverse T3 is not a shortcut. Despite the attention it receives online, the evidence for interpreting it in isolation is thin.

The bottom line

T3 is the active hormone, and most of it comes from converting T4 rather than directly from the thyroid. That makes it useful when the standard tests do not fit the picture — and unremarkable when they do.