Thyroxine, Free is a topic covered in the Davis's Lab & Diagnostic Tests.

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Free T4, Free4.

Common Use:
A reflex test for thyroid function to assist in diagnosing hyperthyroidism and hypothyroidism in the presence of an abnormal TSH level.

Serum collected in a gold-, red-, or red/gray-top tube. Plasma collected in a green-top (heparin) tube is also acceptable.

Normal Findings:
(Method: Immunoassay)

AgeConventional UnitsSI Units (Conventional Units × 12.9)
Newborn0.8–2.8 ng/dL10–36 pmol/L
1–12 mo0.8–2 ng/dL10–26 pmol/L
1–18 yr0.8–1.7 ng/dL10–22 pmol/L
Adult–older adult0.8–1.5 ng/dL10–19 pmol/L
Pregnancy0.7–1.4 ng/dL9–18 pmol/L

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