A simplified method has been described for the measurement of triiodothyronine (T3) in human serum. The sensitivity was sufficient for determinations on hypothyroid as well as normal and thyrotoxic sera. The values obtained have been in reasonable agreement with a double isotope derivative assay.

The normal T3 concentration in human serum approximates 0.2 μg/100 ml; the mean ±SD of 31 normal sera was 220 ±27 ng/100 ml. Elevations were observed in sera from 40 patients with thyrotoxicosis (752 ±282 ng/100 ml), and diminutions were found in sera from 10 hypothyroid patients (98±48 ng/100 ml).

In rare instances thyrotoxicosis may be due to elevated serum T3 with normal thyroxine (T4) concentration. The incidence of this condition remains to be determined.

In approximately half the cases with low serum T4 after ¹³¹I therapy, the eumetabolic state may be maintained by normal or elevated T3 concentration.

From these data and kinetic studies indicating a rapid turnover it may be inferred that T3 rather than T4 may be the more important hormone in health and in disease.