Laboratory Test Information Guide

Details for Free T4, Plasma/Serum

Test Name: Free T4, Plasma/Serum
Alternate Name(s): Free Thyroxine
Laboratory: Core
Specimen Type: Adult:
4.5 mL Light Green top (Li-Heparin) Vacutainer tube

0-2 years: 0.5 mL Light Green top (Li-Heparin) Microtainer
2-10 years: 3 mL Light Green top (Li-Heparin) Vacutainer tube

Red, Gold, or Lavender (EDTA) top tubes are also acceptable
Collection Information: Minimum volume of plasma or serum required is 700 L for adult samples or 200 L for pediatric samples.
Test Schedule: As required
Turnaround time from when specimen
is received in testing laboratory:
4 hours
Reference Range: 0 - 6 days: 11 - 32 pmol/L
6 days - 3 months: 12 - 28 pmol/L
3 - 12 months: 12 - 26 pmol/L
1 - 6 years: 12 - 23 pmol/L
6 - 11 years: 13 - 22 pmol/L
11 - 20 years: 13 - 21 pmol/L
> 20 years: 12 - 22 pmol/L
First Trimester of Pregnancy: 12 - 20 pmol/L
Second Trimester of Pregnancy: 10 - 17 pmol/L
Third Trimester of Pregnancy: 8 - 16 pmol/L
Effective Date: 2018-03-06
Revised Date: 2019-06-24
Critical Value:  
Interpretive Comments:  
Special Processing:  
Comments: TSH should be the initial test to screen for clinically-suspected hypothyroidism or hyperthyroidism. If TSH is below the lower cut-off, FT4 and FT3 testing will be performed reflexively by the laboratory. If TSH is between the lower and upper cut-offs, no FT4 or FT3 testing will be performed reflexively. If TSH is above the upper cut-off, FT4 testing will be performed reflexively by the laboratory. These cut-offs are the TSH reference intervals in children and the optimal cut-offs to predict abnormal FT4 levels in adults.

The TSH cut-offs are:
2 <6 years: <0.70 or >5.97 mIU/L
6 <11 years: <0.60 or >4.84 mIU/L
11 <18 years: <0.51 or >4.30 mIU/L
≥18 years: <0.58 or >6.30 mIU/L

FT3 and/or FT4 testing should only be ordered directly for patients:
With known or suspected pituitary/hypothalamic disease
Being treated for primary hyperthyroidism, severe hyperthyroidism (thyroid storm), or severe hypothyroidism (myxedema coma)
With resistance to thyroid hormones
Treated with lithium carbonate, an anticonvulsant, or amiodarone
Who are pregnant
Under 2 years of age

Furosemide may cause elevated free T4 results.

Biotin may interfere with this test. Samples should not be taken from patients receiving high biotin doses (i.e. > 5 mg/day) until at least 8 hours after the last biotin administration.
Method of Analysis: Roche Electrochemiluminescence
Critical Information Required:  
Storage & Shipment:  
System Codes: 
Referred To:  

Questions? Comments? Contact Laura Gopaul