Triiodothyronine (T3), Free
Code:9004|CPT:84481|LOINC:3051-0
| Includes | Triiodothyronine (T3), Free |
|---|
Analysis details
Methodology
- Electrochemiluminescence immunoassay (ECLIA)
Expected Turnaround Time
1 day
Special Instructions
- Report any use of biotin (vitamin B7/B8, vitamin H, coenzyme R) and stop biotin for at least 72 hours before the blood draw.
- Infants younger than 1 year: hold feeds for 30–40 minutes before specimen collection.
- Do not eat for 2–3 hours before collection; drinking water is allowed.
- With clinician guidance, discontinue thyroid hormones and iodine-containing products for 72 hours prior to testing.
- With clinician guidance, avoid steroid hormones for 48 hours before testing.
- Minimize vigorous physical activity and emotional stress for 24 hours prior to collection.
- Do not smoke during the 3 hours before the sample is obtained.
How to use
Triiodothyronine (T3), Free (Free T3; free triiodothyronine) is used to assess thyroid function and to interpret suspected thyroid disorders alongside thyroid-stimulating hormone (TSH) and other thyroid tests. Measuring the free, unbound hormone is particularly helpful when changes in binding proteins alter total T3 concentrations. This test supports evaluation during routine thyroid assessment, tracking therapy for thyroid disease in conjunction with TSH, and investigating thyroid status in pregnancy and in neonates born to mothers with thyroid disease.
Limitations
Free triiodothyronine is the small, unbound portion of T3 that is not carried by thyroid-binding globulin, transthyretin, or albumin. Because only the free fraction can enter cells and activate thyroid hormone receptors, Free T3 better reflects biologic activity than total T3 when binding protein concentrations are altered. Physiologic and clinical states that change binding protein levels, such as pregnancy, can raise or lower total T3 without parallel shifts in the free hormone. Interpreting Triiodothyronine (T3), Free alongside TSH and other thyroid tests improves assessment of thyroid status in these settings and during management of established thyroid disease. Immunoassay methods that use streptavidin–biotin capture can be affected by recent high-dose biotin ingestion; adherence to pretest biotin restrictions reduces the risk of spurious results.
| Unit | pg/mL | ||||
|---|---|---|---|---|---|
| Reference interval |
| ||||
| Indications | Assessment of possible thyrotoxicosis in patients with symptoms such as tachycardia, weight loss, insomnia, tremor, irritability, diarrhea, or ophthalmopathy., Evaluation of possible hypothyroidism in those with weight gain, xerosis, constipation, cold intolerance, edema, alopecia, or menstrual irregularity; in children, assessment of developmental delay., Thyroid function testing as part of periodic health evaluation in combination with TSH and related laboratory studies., Monitoring response to treatment for thyroid disease together with TSH., Pregnancy with known or suspected thyroid disease to identify abnormal thyroid hormone secretion., Early evaluation of thyroid status in neonates born to mothers with thyroid disease. |
Specimen Requirements
| Specimen | Serum |
|---|---|
| Container | Gold/Tiger Top (SST, Gel Separator) |
| Volume | 1 mL (min 0.7 mL) |
| Storage Instructions | Room temperature, Refrigerated, Frozen |
References
Albertini A, Ekins RP, eds. Free Hormones in Blood. Amsterdam, Holland: 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 Mar;23(3):490-492. PubMed 402243