FT3

FT3, or free triiodothyronine, is the unbound fraction of the thyroid hormone triiodothyronine circulating in the bloodstream. It represents the biologically active portion that is available to tissues and is largely derived from peripheral conversion of thyroxine (T4) rather than direct secretion by the thyroid gland. FT3 works together with free thyroxine (FT4) and thyroid-stimulating hormone (TSH) to regulate metabolism and energy expenditure.

FT3 is measured in blood using immunoassays or more direct methods such as equilibrium dialysis or ultrafiltration. Immunoassays are common but can be affected by assay variability and interfering substances; dialysis-based methods are more accurate but less widely available. Results are reported in pmol/L or pg/mL with reference ranges varying by assay and laboratory. A typical adult reference range is around 3.1–6.8 pmol/L, but clinicians should consult the specific lab’s report.

Clinical use of FT3 is generally focused on cases where thyroid function is unclear or where T3-specific disorders are suspected. In overt hyperthyroidism, FT3 is often elevated (especially in T3 toxicosis, where FT4 may be normal). In classic hypothyroidism, FT3 can be normal or low, but TSH and FT4 are usually more informative. FT3 can be low in non-thyroidal illness (euthyroid sick syndrome) and in some pituitary disorders. Pregnancy and changes in binding proteins can influence FT3 measurements, though the free fraction is typically maintained within the reference range.

Limitations include assay variability, interference from biotin or heterophile antibodies, and pre-analytical factors. FT3 is most informative when interpreted alongside FT4 and TSH, as part of a complete thyroid function assessment.