Determination of free thyroid hormone fractions using a simple T3-uptake test.

Results from a simple, fast and inexpensive Sephadex T3-uptake test (T3U) have been compared to free fractions of serum thyroxine (T4) and triiodothyronine (T3) determined by a dialysis procedure in forty-six sera from hypothyroids, 154 sera from euthyroids and ninety-one sera from hyperthyroids. Sera with high TBG levels were included in these groups. A second order correlation (r = 0.90) was found between T3U and free T4 and T3 fractions. The T3U had a low diagnostic value for all three groups. Free hormone indices calculated as the product between the T4 or T3 and T3U were linearly correlated to total free T4 (r = 0.94) and total free T3 (r = 0.96) if these are less than 2.5 times the upper nu described.