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Thyroid
Free triiodothyronine (free T3) (FT3)
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The unbound fraction of the more potent thyroid hormone, mostly produced by peripheral deiodination of T4. It is mainly useful to confirm hyperthyroidism, particularly T3-toxicosis, when TSH is suppressed.
Reference interval
| Conventional (pg/mL) | SI (pmol/L) | |
|---|---|---|
| Adults | 2.3–4.2 pg/mL | 3.5–6.5 pmol/L |
Specimen: Serum · Conventional × 1.536 = SI
Causes of high values
Hyperthyroidism (Graves disease, toxic nodular goitre), T3-toxicosis, thyroiditis in the early phase, excess liothyronine, thyroid hormone resistance.
Causes of low values
Severe non-thyroidal illness (low T3 syndrome), fasting and malnutrition, hypothyroidism (a late and insensitive sign), drugs that block deiodination (for example amiodarone, propranolol, glucocorticoids).
Pre-analytical and method notes
Free T3 is not recommended for routine diagnosis of hypothyroidism because it stays normal until late. Interpretation depends on the assay, and values fall during acute illness without true thyroid disease. Reference intervals vary by laboratory, method and population; use the interval printed on the report.
References
- Williams Textbook of Endocrinology. Elsevier, 14th edition (2019) — Tier 3 (textbook)
- Rifai N, Horvath AR, Wittwer CT. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Elsevier, 6th edition (2018) — Tier 3 (textbook)
- McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. Elsevier — Tier 3 (textbook)
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