thyroxine [T4] (free) in serum

Reference interval

• 0.8-2.3 ng/dL (adult)
• 10.3-29.7 pmol/L

Clinical significance

• free T4 is the active form of T4
• 0.02-0.04% of total plasma T4 exists unbound as free T4, the majority is bound to 1 of 3 plasma proteins, albumin, thyroxine-binding globulin, prealbumin compare with free T3 0.2-0.5%
• serum free thyroxine in the high-normal range associated with a 2.5 fold increased risk of sudden cardiac death
  • absolute 10-year risk increases from 1% to 4%
• higher free T4 levels associated with increased risk for solid cancers (RR=1.8 for lung cancer, highest vs lowest tertile, lower risks for other solid cancers)
• serum free T4 is positively related risk of mortality
  • serum free T3 is inversely related risk of mortality

Increases

• hyperthyroidism
• high doses of biotin may falsely increase serum free thyroxine

Decreases

• hypothyroidism

Methods

• equilibirum dialysis immunoassay

Specimen

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

More general terms

• thyroxine [T4] (free) in body fluid
• thyroid function test

Additional terms

• thyroxine (T4)

Component of

• thyroid panel