T4 and T3 determination (in thyrotoxicosis)

The thyroid status of the patient is determined by the concentration of free T3 and T4. Thyroid hormone which is bound to thyroid binding globulin (TBG) is inactive. It is clear that for a given total T3 or T4 level, the TBG concentration determines the free thyroid hormone levels and hence the clinical state.

Raised free T4 and free T3 indicates hyperthyroidism.

• in overt hyperthyroidism - serum free T4,T 3, or both are elevated
• in subclinical hyperthyroidism - normal serum free T4 and normal total T3 or free T3
• T3 toxicosis - serum T4 and free T4 can be normal, only serum T3 may be elevated (1)

Factors which tend to increase the concentration of TBG include (1):

• oestrogen therapy
• pregnancy

Reference:

• (1) Ross DS et al. 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. Thyroid. 2016;26(10):1343-1421
• (2) Prescribers' Journal (1999), 39 (1), 1-8