Thyroxine (T4), Thyroxine-binding Globulin (TBG), and T4−TBG Index

CPT: 84436; 84442
Print Share


  • TBG, Thyroxine (T4), and T4−TBG Index

Special Instructions

This test may exhibit interference when sample is collected from a person who is consuming a supplement with a high dose of biotin (also termed as vitamin B7 or B8, vitamin H, or coenzyme R). It is recommended to ask all patients who may be indicated for this test about biotin supplementation. Patients should be cautioned to stop biotin consumption at least 72 hours prior to the collection of a sample.

Expected Turnaround Time

2 - 3 days

Related Documents

Specimen Requirements




2 mL

Minimum Volume

0.5 mL (Note: This volume does not allow for repeat testing.)


Red-top tube or gel-barrier tube


If a red-top tube is used, transfer separated serum to a plastic transport tube.

Storage Instructions


Stability Requirements



Room temperature

7 days


14 days


14 days

Freeze/thaw cycles

Stable x3

Causes for Rejection

Plasma specimen


Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
004457 T4, TBG and T4-TBG Index 001735 Thyroxine Binding Globulin ug/mL 3021-3
004457 T4, TBG and T4-TBG Index 001149 Thyroxine (T4) ug/dL 3026-2
004457 T4, TBG and T4-TBG Index 011735 T4-TBG Index index 3027-0

For Providers

Please login to order a test

Order a Test

© 2021 Laboratory Corporation of America® Holdings and Lexi-Comp Inc. All Rights Reserved.

CPT Statement/Profile Statement

The LOINC® codes are copyright © 1994-2021, Regenstrief Institute, Inc. and the Logical Observation Identifiers Names and Codes (LOINC) Committee. Permission is granted in perpetuity, without payment of license fees or royalties, to use, copy, or distribute the LOINC® codes for any commercial or non-commercial purpose, subject to the terms under the license agreement found at Additional information regarding LOINC® codes can be found at, including the LOINC Manual, which can be downloaded at