Drugs may also bind with thyroid hormone binding proteins and may interfere with the estimation of thyroid function. The causes of increased and decreased TBG capacity are summarised in Fig. 15.16. It should be noted that, although alteration in the concentration of serum proteins or in their residual binding capacity will affect total thyroid hormone levels (e.g. by competitive binding from drugs such as phenytoin), free hormone levels are not usually affected. Increased TBG capacity will produce ΓÇÿfalselyΓÇÖ elevated total T4 and levels in euthyroid patients, while decreased TBG capacity will produce low T4 and T3 levels in euthyroid patients. These may be corrected by using the T4:TBG ratio or by deriving the Free Thyroxine Index. Direct measurement of TBG also enables altered TBG states to be identified. Anomalous total T4 levels may be ΓÇÿcorrectedΓÇÖ by deriving T4:TBG ratio.
