triiodothyronine [T3] (free) in serum

Indications

• suspected T3 thyrotoxicosis

* rarely necessary

Reference interval

• 2.6-4.8 ng/L (child & adult)
• 3.6-5.6 ng/L

Clinical significance

• free T3 is the active form of plasma T3
• 0.2-0.5% of total plasma T3 exists unbound as free T3,
  • compare with free T4 0.02-0.04%
  • normally, T3 total in serum represents ~5% of thyroid hormone in serum
• T3 is more active than T4, estimates range from 3-100 fold more active
• serum free T3 is inversely related risk of mortality
  • serum free T4 is positively related risk of mortality

* assays are highly variable

Methods

• equilibirum dialysis immunoassay

Specimen

• serum, refrigerate immediately, stable for 7 days at room temp, but storage at 4 C preferred

More general terms

• general chemistry test
• thyroid function test

Additional terms

• T3 total in serum/plasma
• triiodothyronine (T3, liothyronine, Cytomel)