Immune and non-immune T4 radioassays utilizing albumin magnetic microparticles.

Albumin magnetic microparticles reversibly adsorb thyroxine. They quickly establish equilibrium allowing time and temperature independent measurements in T4 radioassays. We used these particles to compare the efficiency of NaOH, HCl, pepsin, sodium trichloroacetate, and 8-anilino-1-naphthalene sulfonic acid to release 125I-T4 from serum, T4-free serum and human serum albumin. We found that the efficiency of the reagents to extract 125I-T4 depended on the concentration and type of proteins to which the labelled hormone was bound. Pepsin was the most effective reagent and we utilized it for a T4 radioimmunoassay, in which albumin magnetic microparticles were used to separate free from bound hormone. We also utilized the particles in a T4 non-immune radioassay. Both assays accurately measured total serum T4, however the radioimmunoassay was simpler, less dependent on protein content of serum, required a smaller serum sample and provided slightly higher T4 values. We describe a magnetic rack which allows simultaneous handling of fifty individual tubes with an intra-assay C.V. of 2.1% for the radioimmunoassay and 2.3% for the non-immune assay and an inter-assay C.V. of 3.1%, respectively.