What does the Free Triiodothyronine test measure?

Why is Free Triiodothyronine measured?

Free T3 is not recommended as a standalone first-line thyroid screening marker by mainstream endocrine guidelines — its value lies in comprehensive thyroid panels where it identifies conditions not detectable by TSH and Free T4 alone. Most critically, impaired T4-to-T3 peripheral conversion — producing normal TSH and normal Free T4 with low-normal Free T3 — represents a clinically significant thyroid hormone insufficiency at the receptor level that is missed entirely by standard two-marker TSH/Free T4 testing. This pattern is common in chronic illness, caloric restriction, selenium deficiency, high-stress states, and is the primary reason comprehensive thyroid panels extend beyond the two-marker standard. Free T3 is also elevated in T3 thyrotoxicosis and T3-predominant Graves' disease, and provides the therapeutic monitoring marker in patients on T3-containing thyroid hormone replacement (desiccated thyroid extract or liothyronine combinations).

Reference ranges

Standard reference

2.0–4.4 pg/mL (3.1–6.8 pmol/L) # REVIEWER FLAG: Confirm Access Medical Laboratories Free T3 reference range — assay platform variation is substantial for this analyte and direct immunoassays for Free T3 are subject to greater pre-analytical interference than Free T4 assays.

Functional medicine optimal

3.2–4.2 pg/mL (functional medicine optimal — the upper half of the reference range, reflecting adequate T3 for receptor saturation and metabolic activity). # REVIEWER FLAG: Not standardised in mainstream endocrinology — reviewer to confirm framing.

Units

pg/mL, pmol/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.