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TEST ID T3 T3 (Triiodothyronine), Total, Serum

Reporting Name

T3 (Triiodothyronine), Total, S

Specimen Type

Serum


Specimen Required


Collection Container/Tube:

Preferred: Serum gel

Acceptable: Red top

Submission Container/Tube: Plastic vial

Specimen Volume: 1 mL

Collection Instructions: Centrifuge and aliquot serum into plastic vial.


Specimen Minimum Volume

0.75 mL

Specimen Stability Information

Specimen Type Temperature Time
Serum Refrigerated (preferred) 7 days
  Frozen  30 days

Testing Algorithm

For information see Thyroid Function Ordering Algorithm.

Method Name

Electrochemiluminescence Immunoassay

Reject Due To

Gross hemolysis Reject
Gross lipemia OK
Gross icterus OK

Reference Values

Pediatric

0-5 days: 73-288 ng/dL

6 days-2 months: 80-275 ng/dL

3-11 months: 86-265 ng/dL

1-5 years: 92-248 ng/dL

6-10 years: 93-231 ng/dL

11-19 years: 91-218 ng/dL

 

Adult (≥20 years): 80-200 ng/dL

 

For International System of Units (SI) conversion for Reference Values, see www.mayocliniclabs.com/order-tests/si-unit-conversion.html

Day(s) Performed

Monday through Saturday

Report Available

1 to 3 days

Specimen Retention Time

2 weeks

Performing Laboratory

Mayo Clinic Laboratories in Rochester

CPT Code Information

84480

Forms

If not ordering electronically, complete, print, and send a Renal Diagnostics Test Request (T830) with the specimen.

Useful For

Second-order testing for hyperthyroidism in patients with low thyroid-stimulating hormone values and normal thyroxine levels

 

Diagnosing triiodothyronine (T3) toxicosis

 

This test is not useful for general screening of the population without clinical suspicion of hyperthyroidism.

Clinical Information

Thyroid hormones regulate numerous developmental, metabolic, and neural activities throughout the body. The thyroid gland synthesizes 2 hormones. The 2 main hormones secreted by the thyroid gland are thyroxine (T4), which contains 4 atoms of iodine, and triiodothyronine (T3). T3 production in the thyroid gland constitutes approximately 20% of the total T3; the rest is generated by the conversion (deiodination) of T4 to T3. T3 is also produced by conversion (deiodination) of T4 in peripheral tissues. Circulating levels of T4 are much greater than T3 levels, but T3 is biologically the most metabolically active hormone (3-4 times more potent than T4), although its effect is briefer due to its shorter half-life compared to T4.

 

Thyroid hormones circulate primarily bound to carrier proteins (eg, thyroid-binding globulin [TBG], prealbumin, and albumin), whereas only a small fraction circulates unbound (free). Only the free forms are metabolically active. While both T3 and T4 are bound to TBG, T3 is bound less firmly than T4. Total T3 consists of both the bound and unbound fractions.

 

In hyperthyroidism, both T4 and T3 levels are usually elevated, but in a small subset of hyperthyroid patients, only T3 is elevated (T3 toxicosis).

 

In hypothyroidism, T4 and T3 levels are decreased. T3 levels are frequently low in sick or hospitalized euthyroid patients.