Semi-automated competitive protein binding analysis of serum thyroxine on reusable Sephadex columns and its advantages over radioimmunoassay

Myeloma immunoglobulin interferes with serum thyroxine analysis by homogeneous enzyme immunoassay

Myeloma immunoglobulin paraproteins interfere with a homogeneous enzyme immunoassay (EMIT) for serum thyroxine. The EMIT assay failed to detect any hormone in three hyperproteinemic sera from multiple myeloma patients, although thyroxine in these sera was accurately measured by our competitive protein-binding radio-assay on small, re-usable Sephadex columns. The interference was due to turbidity of the paraproteins in the EMIT reaction mixture, resulting in an increased absorbance and a marked underestimation of hormone concentration. Thyroxine was detected (82 to 107% recovery) by the EMIT assay in ethanol extracts of myeloma sera. With 82 other sera there was an excellent correlation (r = 0.985, slope = 0.912, Y intercept (EMIT) = 6.8) of the EMIT assay with our competitive radioassay. Thus, although the EMIT thyroxine assay possesses many desirable features and it is an attractive alternative method to competitive radioassays, its susceptibility to interferences by immunoglobulin paraproteins is a troublesome liability.

Procedure for a normalised thyroxine determination in small sephadex columns

A rapid simple test for measuring normalised serum thyroxine is described in detail, using small Sephadex columns. A small portion of test serum is reintroduced into the test system at the competitive binding stage, so that corrected T4 values are obtained when the concentrations of thyroxine binding globulin are variable. Only 125 microliter serum is required for the whole test. Values obtained by the present method correlated well with the product of thyroxine concentration and radioactive T3 uptake (T4--RT3U index) and with a commercially available method for normalized thyroxine (T4N). Day-to-day variation was CV=6%.

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.

A rapid and precise sequential saturation radioimmunoassay for thyroxine

A radioimmunoassay for the measurement of total thyroxine in unextracted serum or plasma is described. The assay is performed according to the sequential saturation principle in less than 4 h. The entire assay is carried out at room temperature. An antiserum of high affinity was used as binder, ammonium 8-anilino-1-naphthalene-sulphonate as displacing agent, and activated charcoal as adsorbent in the separation step. The standard curve was linear in the range 0-136 nmol/l serum. The precision in terms of SD was nearly the same through-out this range. Estimates of within-assay SD varied from 1.3 to 4.1 and between-assay SD from 0 to 3.3 nmol/l. The recovery of thyroxine added to normal plasma was 100% and the sensitivity was 6 nmol/l. Mean thyroxine concentration in plasma from young bulls was 92 nmol/l. The assay appears to be very well suited for experimental purposes when high precision is essential.

Minimal effects of free fatty acids on the competitive protein-binding assay of serum thyroxine on reusable Sephadex columns

In rats; following oral administration of cream and intravenous heparin, plasma free fatty acids and triiodothyronine Sephadex uptake were elevated. Simultaneously apparent serum thyroxine, measured by the Tetrasorb-E kit, was increased, but not when measured using reusable Sephadex columns. Addition of oleate to a human serum albumin solution or to human serum produced similar effects; appreciable rises in apparent thyroxine were noted in the kit assay but only minor changes occurred when Sephadex columns were used. In the latter procedure almost all [14C]oleic acid added to serum was removed in the first barbital buffer wash, before application of the thyroxine-binding globulin solution. In the serum thyroxine assay on reusable Sephadex columns, there is minimal interference by free fatty acids because they are removed by the first barbital buffer wash.