Euthyroid Hyperthyroxinemia

Excerpt

To the editor: The article by Borst and coworkers (1) on euthyroid states accompanied by elevated serum thyroxine (T4) levels lists several conditions where the elevated total T4 is due to abnormal binding of T4 to serum proteins. In these conditions, free T4 is normal. The classic example for this type of discrepancy is an altered thyroxine-binding globulin (TBG) concentration. Increased T4 binding to albumin or thyroxine-binding prealbumin (TBPA) as a cause for elevated total T4 has only recently been identified.

The free T4 index, based on total T4 and resin triiodothyronine (T3) uptake, is a widely used laboratory test