Thyroxine Binding Globulin (Sendout)

General Information

Lab Name

Thyroxine Binding Globulin

Lab Code

RTBG

Epic Name

Thyroxine Binding Globulin

External Test Id

TBGI

Description

Useful For: Cases in which total thyroid hormone levels do not correlate with the thyrometabolic status, most commonly with pregnancy or the use of contraceptive steroids

Thyroxine binding globulin (TBG) is the high-affinity serum binding protein for thyroxine and triiodothyronine. Normally, the thyroid adjusts to changing concentrations of TBG by producing more, or less, thyroid hormone to maintain a constant level of metabolically important free hormone.

Elevated TBG levels are associated with influences such as pregnancy, genetic predisposition, oral contraceptives, and estrogen therapy. TBG levels can decrease with androgenic or anabolic steroids, large doses of glucocorticoids, hypoproteinemic states, liver disease, nephrotic syndrome, and congenital TBG variants.

Patient Preparation: For 12 hours before specimen collection, do not take multivitamins or dietary supplements containing biotin (vitamin B7), which is commonly found in hair, skin, and nail supplements and multivitamins.

References

Mayo Clinical & Interpretive Information: Thyroxine-Binding Globulin (TBG), Serum

Synonyms

TBG, TBG by Immunoassay, TBGI, Thyroid Binding Globulin, Thyroxine-Binding Globulin

Components

Interpretation

Method

Solid-Phase Chemiluminescent Assay

Reference Range

Units: mcg/mL

Female		Male
Age	Range	Age	Range
0-	11-27	0-	12-26

Effective date: 02/13/2004

Ref. Range Notes

Interpretation:

A change in thyroxine-binding globulin (TBG) concentration may be of hereditary, pathophysiologic, or pharmacologic origin.

The TBG concentration indicates whether an abnormally high or low total thyroid hormone concentration is offset by a parallel increase or decrease in TBG concentration.

In TBG deficiency, one may find euthyroid patients with extremely low total thyroxine (T4) values. Conversely, patients with high TBG levels may be clinically euthyroid with high serum total T4 values.

Twenty-four specimens obtained during various stages of pregnancy yielded results ranging from 27 to 66 mcg/mL with a median of 43 mcg/mL. The literature suggests 47 to 59 mcg/mL as the range of TBG values expected during the third trimester of pregnancy.

Interferences and Limitations

Cautions: Females using estrogen-based contraception may exceed the reference range.

References

Mayo Clinical & Interpretive Information: Thyroxine-Binding Globulin (TBG), Serum

Ordering & Collection

Specimen Type: Blood
Collection: 3 mL blood in RED TOP tube

Also Acceptable: GOLD SST
Handling Instructions: Outside Laboratories: Centrifuge sample and transfer serum to a separate plastic vial. Refrigerate serum while awaiting shipment. Transport with a cold pack.

Stability: Refrigerated (preferred): 7 days; Frozen: 30 days: Ambient: Unacceptable.

Reject Due To: Gross hemolysis.
Quantity: Requested: 0.5 mL serum
Minimum: 0.4 mL serum

Processing

Receiving Instructions

Centrifuge sample and transfer serum to a separate plastic vial. Refrigerate serum.

Sendouts:

Order Mayo Test: TBGI
Interfaced: Yes [Interface: 601; Worksheet: MARF]

Stability: Refrigerated (preferred): 7 days; Frozen: 30 days: Ambient: Unacceptable.

Reject Due To: Gross hemolysis.

Misc Sendout

Performance

Lab Department

Sendouts Mayo Lab (RF)(MARF)

Frequency

Performed: Monday, Wednesday, Friday. Report Available: 1-3 days from sample receipt at Mayo Clinic Labs.

Available STAT?

Performing Location(s)

Sendout	Mayo Clinic Laboratories (Superior Drive) 800-533-1710 3050 Superior Drive NW Rochester, MN 55901

Billing & Coding

CPT Codes: 84442
LOINC: 3021-3
Interfaced Order Code: UOW3903
Interfaced Result Code: UOW3903