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Test Code T4FREE T4, Free, LAB127

Performing Laboratory

St. Luke's Hospital Cedar Rapids

Specimen Requirements


Preferred:  Green-top (lithium heparin) tube

Acceptable:  Gold-top serum gel tube, plain, red-top tube or lavender-top (EDTA) tube

Specimen: 0.1 mL of plasma or serum

Stability:  Separated specimens stored at 2 – 8 °C are stable for 14 days

Collection Instructions:

Note: 1. Human anti-mouse antibodies or other heterophile antibodies may be present in patient specimens. This assay has been specially formulated to minimize the effects of these antibodies, however results from patients known to have these antibodies should be carefully evaluated.

2. Non-thyrometabolic disorders may cause abnormal free T4 levels. Anti-convulsant drugs (especially phenytoin), anti-inflammatory drugs (ie, salicylate), and heparin therapy may interfere with this assay.


Reference Values


Age Group



1 – 23 months

0.9 – 1.5 ng/dL


2 years – 12 years

0.8 – 1.4 ng/dL


13 years – 20 years

0.8 – 1.3 ng/dL


≥21 years

0.8 – 1.5 ng/dL


Useful For

Thyroid Function

Day(s) Test Set Up

Monday through Sunday



Test Classification and CPT Coding