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Test Code LAB127 T4 (Thyroxine), Free, Plasma or Serum

Reporting Title

Free T4

Methodology

Roche Diagnostics, Electrochemiluminescence Immunoassay at RMH, BH, GMC, GMH, MGH, MH, OBH, SH

Used as an aid in clinical assessment of thyroid status. Free T4 correlates with thyroid status better than total T4 because it is not subject to variation of carrier proteins.

Performing Laboratory

OhioHealth Laboratory Services - RMH, BH, GMC, GMH, MGH, MH, OBH, SH Core Laboratories

Specimen Requirements

Collection Instructions:

1. Serum and Plasma gel tubes should be centrifuged within 2 hours of collection.

2. Red-top tubes should be centrifuged and aliquoted within 2 hours of collection.

 

Submit only 1 of the following specimens:

 

Preferred:

Serum

Draw blood in a gold-top serum gel tube(s). Spin down and send 1 mL of serum refrigerated.
Note:  1. Specimen should not be drawn during or soon after administration of heparin.

2. Plain, red-top tube(s) is also acceptable.

3. Indicate serum on request form.

4. Label specimen appropriately (serum).

 

Alternate: 

Plasma

Draw blood in a mint green-top (heparin) gel tube(s). Spin down and send 1 mL of heparinized plasma refrigerated.

Note:  1. Specimen should not be drawn during or soon after administration of heparin.

2. Indicate plasma on request form.

3. Label specimen appropriately (plasma).

 

Specimen Stability Information

Specimen Type Temperature Time
Serum or plasma Refrigerated 7 days
  Ambient  5 days
  Frozen  30 days

 

Specimen Transport Temperature

Refrigerate/Frozen OK/Ambient OK

Reference Values

0.7-1.7 ng/dL

Possible interferences:

1. Drugs which may affect binding of T4 to thyroid hormone carrier proteins or which interfere with it’s metabolism to triiodothyronine.

2. Severe non-thyroidal illness. Thyroid-stimulating hormone can be helpful in this context.

3. Rare conditions such as familial dysalbuminemic hyperthyroxinemia.

4. Autoantibodies to T4 and hormone-binding inhibitors, 5-heterophile antibodies.

Day(s) Test Set Up

Monday through Sunday; Continuously

Test Classification and CPT Coding

84439