Diagnostic Use
Increased levels of free T3 are found in T3 toxicosis and in thyrotoxicosis. Free T3 may be elevated earlier than free T4 in thyrotoxicosis.
Free T3 is increased in hyperthyroidism, but usually only measured when increased T3 is thought to be the cause.
It is decreased in hypothyroidism, but not used for diagnosing or monitoring this condition.
Reference Intervals
| >=4 days – 7 years |
3.0 – 10.0 pmol/L |
| >7 years – 19 years |
3.5 – 8.0 pmol/L |
| >19 years – Adult |
3.9 – 6.8 pmol/L |
The reference interval is highly variable during the first three days of life.
Please contact a Paediatric Endocrinologist or Paediatrician if interpretation of these results is required.
Test Method
Principle: Electrochemiluminescence - Competition principle
Analyser: Roche Diagnostics Cobas e801
Reagents: Elecsys FT3 III
Limitations / Interference
Free T4 and Free T3 assays attempt to measure a very small fraction of free hormone against a much larger background of bound hormone. Consequently, these assays are prone to give incorrect estimates of free hormone when the concentration or affinity of binding proteins (TBG and albumin) are abnormal, or when drugs displace the hormone from the binding proteins. These effects vary with the assay used.
Uncertainty of Measurement
10%