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Paediatric Laboratory Medicine

T4 - Free, Blood

Clinical Significance

The free fraction of the circulating thyroxine (T4) is considered to exert the main influence on metabolic control. Consequently, the FT4 concentration is believed to be the most direct indicator of an individual’s thyroid status. FT4 concentrations are generally depressed in hypothyroidism and raised in hyperthyroidism. Measurement of FT4 thus provides an aid to the differential diagnosis of thyroid disease. FT4 concentrations are independent of the concentration of thyroid hormone binding proteins  and may therefore be measured in patients with elevated or reduced binding protein concentrations without the need for additional tests of binding capacity. In borderline cases of suspected thyroid malfunction, additional tests such as free T3 or TSH may be necessary

Test Name

T4 - Free, Blood

Test Code



Biochemistry - Biochemistry


Abbott Ci4100

External Proficiency Testing


Turn Around Time

Monday to Friday

Specimen Type

Serum, Sodium/Lithium Heparin / K EDTA plasma

Minimum Specimen Requirements

150 uL



Approval is not required

CPT Codes


Shipping and Contact Information

The Hospital for Sick Children
Rapid Response Laboratory
170 Elizabeth Street, Room 3642
Toronto, ON
M5G 2G3
Phone: 416-813-7200
Phone: 1-855-381-3212

Reference Range

<3d : 12.0-40.7 pmol/L
3 days - < 1 month : 11.0-40.7 pmol/L
1 - < 6 months : 10.0-23.0 pmol/L
6 months - <1 year : 10.0-20.4 pmol/L
1-<19y : 10.0-17.6 pmol/L