This is to assess thyroid function, eg detect hyperthyroidism.


T3 is from 80% by deiodination of T4.

Some free T3 is present in the normal state in the blood: the majority is a TBG (thyroxine binding globulin).


Sampling 10 mL EDTA blood.


Normal values:

0.3 to 0.4 mU / L.

Pathological changes:

– In hypothyroidism:

  – The first sign is the elevation of TSH greater than 20 mU / L, whereas free T4 is itself, diminished,

  – The therapeutic goal is to obtain TSH levels less than 1 mU / L;

– In hyperthyroidism:

  – The ultrasensitive TSH is low, less than 0.1 or 0.2 mU / L,

  – Q3 rate is increased, that of T4 less.



Practical advice:

Medications containing thyroid hormones may distort the results.

It is the same for those that contain iodine causing a diversion from the conversion of T4 to T3.


Editor-in-chief of the Medical Actu website; general practitioner graduated from the Faculty of Medicine of Algiers in 2005 currently practicing as a liberal.

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