Diagnostic value of free triiodothyronine in serum

Serum free T3 concentration has been assessed in various thyroid conditions by a T3 analog method and the results compared with those obtained by equilibrium dialysis in the same individuals. The methodology is easy to perform and reproducible. FT3 determination appears to be especially valuable in detecting borderline thyrotoxicosis as in cases previously cured from thyrotoxicosis but suspected of relapse, or in nontoxic goitrous patients overtreated with T4.