Determination of serum estriol levels using a125I-radioimmunoassay

A new method of radioimmunoassay (RIA) for serum estriol (E3) which utilizes125I-estriol as a tracer has been developed. This steroid was radioiodinated with chloramine T and purified by Sephadex LH-20 column chromatography. Two hundred and sixty-two samples from 130 normal pregnant patients at different stages of gestation were analysed. From 5 women, blood samples were drawn at 15 min intervals for 1 h. An increase in the circulating levels of E3 was observed throughout normal pregnancy and the results were widely distributed. The episodic fluctuations of E3 showed a variation of 13–20 ng/ml during the same 1-h sampling period. The quality control results showed the viability of this RIA in the determination of serum E3 utilizing the iodinated steroid as a tracer.