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Thyroid
Free thyroxine (free T4) (FT4)
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The unbound, biologically active fraction of the main hormone secreted by the thyroid, which is less affected by binding-protein changes than total T4. It is interpreted together with TSH.
Reference interval
| Conventional (ng/dL) | SI (pmol/L) | |
|---|---|---|
| Adults | 0.8–1.8 ng/dL | 10.3–23.2 pmol/L |
Specimen: Serum · Conventional × 12.87 = SI
Causes of high values
Hyperthyroidism (Graves disease, toxic adenoma, toxic multinodular goitre), thyroiditis in the thyrotoxic phase, excess levothyroxine, factitious thyrotoxicosis, heparin effect in vitro, TSH-secreting adenoma.
Causes of low values
Primary hypothyroidism, central (pituitary or hypothalamic) hypothyroidism, severe non-thyroidal illness, iodine deficiency, antithyroid drug overtreatment.
Pre-analytical and method notes
Free T4 immunoassays can be affected by binding-protein abnormalities, biotin and heparin, and equilibrium dialysis or mass spectrometry is more reliable in difficult cases. Units differ: ng/dL multiplied by 12.87 gives pmol/L. 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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