Triiodothyronine (T3), Free, Serum

Triiodothyronine(T3), Free, Serum

CPT Test code: 84481

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Related Information:
Specimen: Serum
Volume: 0.8 mL
Minimum Volume: 0.3 mL (Note: This volume does not allow for repeat testing.)
Container: Red-top tube or gel-barrier tube
Collection: If a red-top tube is used, transfer separated serum to a plastic transport tube.
Storage Instructions: Refrigerate
Causes for Rejection: Citrate plasma specimen; improper labeling
Reference Interval: See table.1

Age (pg/mL)
0-3 d 2.0-7.9
4-30 d 2.0-5.2
31 d to 12 mo 1.6-6.4
13 mo to 5 y 2.0-6.0
6-10 y 2.7-5.2
11-19 y 2.3-5.0
>19 y 2.0-4.4
Use: Evaluate thyroid function and assess abnormal binding protein disorders
Limitations: Circulating antibodies to thyroid hormones have also been reported to interfere with free hormone immunoassays.
Additional Information: Triiodothyronine (T3) normally represents only approximately 5% of the thyroid hormone and like thyroxine is almost entirely bound to the carrier proteins, with only 0.25% of the total being in the free state. Measurement of free triiodothyronine is of value in confirming the diagnosis of hyperthyroidism, when an elevated free or total thyroxine level is found. Abnormal total and free triiodothyronine concentrations may appear in T3 toxicosis, in the presence of normal thyroxine levels. Free T3 levels are unaffected by carrier protein variation.
Footnotes: 1. “Reference Intervals for Children and Adults,” Elecsys Thyroid Test, Roche Diagnostics, May 2005.PubMed 8595709
References: Albertini A, Ekins RP, eds, Free Hormones in Blood, Amsterdam: Elsevier Biomedical Press, 1982.

Parslow ME, Oddie TH, Fisher DA, et al, “Evaluation of Serum Tri-iodothyronine and Adjusted Tri-iodothyronine (Free Tri-iodothyronine Index) in Pregnancy,” Clin Chem, 1977, 23(3):490-2.PubMed 402243