Detailed definition
Triiodothyronine (T3) is one of the two main thyroid hormones. It contains three iodine atoms (versus four in T4) and is several times more biologically active at the nuclear thyroid receptor. The thyroid produces a small fraction of circulating T3 directly; the majority is generated by peripheral conversion of T4 to T3 by deiodinase enzymes (DIO1 and DIO2) in the liver, kidney, brain, and other tissues. Free T3 — the unbound, active fraction — is the most physiologically relevant measure but circulates at very low concentrations. Total and free T3 are usually measured as second-line tests after TSH and free T4 when there is suspicion of hyperthyroidism, T3 toxicosis, or a conversion problem. T3 supplementation (liothyronine, Cytomel) is sometimes added to T4 (levothyroxine) in patients who feel suboptimal on T4 monotherapy, although the evidence for routine combination therapy is mixed.
Why it matters in menopause
In menopause workup, T3 is rarely the first lab ordered — TSH is. But women on existing thyroid replacement who feel unwell despite a normal TSH sometimes have low free T3 from poor T4-to-T3 conversion, and addressing this can reduce overlap with menopausal fatigue and brain fog. T3 testing is more specialized and is best interpreted alongside TSH and free T4.
Related terms
Sources
External references: Wikipedia.