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J E Midgley

Publications and source records attributed to J E Midgley.

At least 19 recordsLinked to original sources
PubMed1992

One-step, labeled-antibody assay for measuring free thyroxin. I. Assay development and validation.

We describe a one-step, labeled-antibody radioassay for measuring free thyroxin (FT4) in serum or plasma, based on a novel principle. FT4 in the sample competes with a gross molar excess (over antibody) of a cross-reactant (L-triiodothyronine, T3), chemically coupled to magnetizable polymer particles, for binding to avid 125l-labeled monoclonal anti-thyroxin antibodies. As in conventional immunoassays, 125I counts bound to the solid phase (T3-magnetizable particles) are inversely proportional to sample FT4 concentration. We demonstrate here the development and technical validity of this new method.

Antibodies, Monoclonal↗
PubMed1990

Concentrations of free thyroxin and albumin in serum in severe nonthyroidal illness: assay artefacts and physiological influences.

Free thyroxin (FT4) estimates by two immunoassays were compared with the concentrations of albumin in serum of apparently euthyroid subjects who either were (n = 99) or were not (n = 327) suffering from severe nonthyroidal illness (sNTI). In neither group was FT4 significantly correlated with albumin (P greater than 0.05), according to a "labeled antibody" radioassay (Amerlex-MAB). On amalgamating both groups, correlation with albumin was positive and significant (P less than 0.001). In the group with sNTI, both FT4 and albumin concentrations were decreased (mean FT4 to 77% and mean albumin to 61% of the respective reference means). For an analog radioimmunoassay (Amerlex-M), FT4 in all groups was significantly (P less than 0.001) correlated with albumin. Correlation coefficients were greater than with Amerlex-MAB for both sNTI and euthyroid groups, as well as for the joint panel. Mean FT4 in sNTI was only 44% of the reference mean. Lower radio-tracer "analog" values in sNTI are exaggerated by additional technical artefacts resulting from tracer binding to albumin.

Adolescent↗
PubMed1989

Low-normal concentrations of free thyroxin in serum in late pregnancy: physiological fact, not technical artefact.

Free thyroxin (FT4) concentrations, total thyroxin/thyroxin-binding globulin (T4/TBG) ratios, and thyrotropin (TSH) and albumin concentrations were measured in serum in a longitudinal study in each of the three trimesters of 25 normal pregnancies. In late pregnancy, FT4 estimates by assays reputedly either affected or unaffected by albumin were in the lower half of the reference range for nonpregnant subjects. T4/TBG ratios and albumin concentrations were similarly lower. FT4 overall was significantly (P less than 0.001) correlated with these latter two values. Serum TSH concentrations increased as FT4 declined in late pregnancy. Nonesterified fatty acid (NEFA) concentrations were too low to displace T4 from its binding proteins and were not correlated with other measurements. Within any one of the trimesters, FT4 and T4/TBG were independent of variations in TBG or albumin concentrations. This implies that lower FT4 concentrations in late pregnancy are real, merely coinciding with parallel decreases in albumin. They are not artefacts of albumin-affected assays.

Adult↗
PubMed1987

Relationship between effects of added albumin, initial free thyroxine value and endogenous serum-binding protein concentrations on Amerlex free thyroxine estimations.

We studied the effect of adding purified human albumin to sera on free thyroxine (FT4) values obtained with Amerlex radioimmunoassays. Apparent FT4 values increased with progressive addition of albumin in vitro. The effect was smallest with low and greatest with high initial FT4 concentrations, which were also linearly correlated with the incremental increase in FT4 values per g/l albumin added. Wide variations in either endogenous thyroxine binding globulin (TBG) or albumin concentrations in patient serum had little effect on the rate of increase in FT4 values when albumin was added in vitro. From Mass Action theory, calculations of the binding affinity of the endogenous albumin for the analog (2.1 X 10(5) l/mol) gave values nearly half that of the added albumin (3.94 X 10(5) l/mol). Distortions in Amerlex FT4 values caused by adding albumin in vitro may exaggerate its importance as a tracer binder and such results may be unrepresentative of patient samples.

Albumins↗
PubMed1986

Assay performance and tracer properties for two analog-based assays of free triiodothyronine.

We determined binding characteristics of the triiodothyronine (T3) analog tracer used in the Amerlex and Amerlex-M FT3 radioimmunoassay for the three endogenous binding proteins in serum: thyroxin-binding globulin (TBG), thyroxin binding prealbumin (PA), and albumin. Both T3 and its analog bind to the same sites on TBG and PA. However, the analog has significantly lower association constants (1.0% and 3.8%, respectively, of T3 binding affinity) and it binds to different sites on albumin. Analog binding is characterized by two (weak) specific binding sites [K = 0.46 (SD 0.03) X 10(5) L/mol]; T3 is bound at about 28 very weak, nonspecific sites [K = 0.41 (SD 0.03) X 10(4) L/mol]. Sera from healthy subjects with a wide range of concentrations of binding proteins showed no interference from analog binding in the FT3 assay. In contrast, in vitro studies of albumin binding revealed a weak dependence of both assays on albumin concentration (0.05 pmol of FT3 per gram of albumin per liter), an interference probably unimportant for most laboratory samples. Nonesterified fatty acids (NEFA) and the T3 analog apparently bind to different sites on albumin; thus the Amerlex FT3 assay is insensitive to moderately increased concentrations of NEFA in serum.

Binding Sites↗
PubMed1985

Comprehensive study of a thyroxin-analog-based assay for free thyroxin ("Amerlex FT4").

The basic theory of thyroxin-analog-based radioimmunoassays for free thyroxin has been extended to evaluate definitively the effects arising from residual binding of the tracer analog to serum proteins. Using experimentally determined binding constants and computer simulation techniques, we studied the effects of thyroxin-analog binding to serum proteins on results of Amerlex FT4 radioimmunoassay, using this improved mathematical model. Results from computer-simulation studies were compared both directly with in vitro experimental results and indirectly with clinical studies. Agreement was good among all three approaches. The relatively weak binding of analog to thyroxin-binding globulin and prealbumin does not significantly perturb Amerlex FT4 assay results. Binding of the analog by albumin has a small but quantifiable effect on assay results, amounting to an intrinsic bias of 0.08 pmol of free thyroxin per liter per gram of albumin per liter for euthyroid serum samples. This bias is unlikely to be important for most clinical laboratory samples, but it may be significant when one is interpreting results for those rare patients with genetic albumin abnormalities such as analbuminemia or familial dysalbuminemic hyperthyroxinemia. Massively increased concentrations of nonesterified fatty acids (e.g., after treatment with heparin) will lead to a spurious increase in free thyroxin in this and most other techniques, including equilibrium dialysis.

Computers↗
PubMed1985

Effects of age and health on the euthyroid reference ranges for serum free thyroxine and free triiodothyronine.

Age-related trends in serum free thyroxine (FT4) and free triiodothyronine (FT3) concentrations were measured in 7248 euthyroid subjects (age-range 3 months to 106 years). 5700 were patients referred to hospitals for investigation of suspected thyroid dysfunction, but who were diagnosed euthyroid. 1548 were healthy blood donors (age-range 18-63 years) with no indication of thyroid dysfunction. FT4 concentrations were little affected by the age, the sex or the state of health of the subjects in either group. Serum FT3 concentrations were significantly affected by both age and health factors. The upper limit of the euthyroid reference range for young subjects up to 15 years was about 20% higher (10.4 pmol/l) than for adult subjects older than 25 years (8.8 pmol/l). The change in the upper limits typical of young subjects to that typical of adults occurred steadily over the decade 15-25 years. After this age, little further change occurred, especially in healthy subjects. Additionally, the lower limit of the euthyroid range for FT3 was extended by the inclusion in the reference group of patients referred to hospitals. Compared with the lower limit of the FT3 range for healthy subjects (5 pmol/l), the corresponding limit for referred subjects (young or adult) was 3.5-3.8 pmol/l. Broadening of the FT3 reference range was probably brought about by a significant number of patients in the hospital-referred group with the "low-T3 syndrome" of mild non-thyroidal illness. Accordingly, FT3 was inferior to FT4 in the discrimination of hypothyroidism, as FT4 was unaffected by this phenomenon.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1982

A comparison of free T4 and the ratio of total T4 to T4-binding globulin in serum through pregnancy.

Serum free T4 concentrations have been variously reported as either constant or falling in pregnancy. In this study, 122 serum samples from apparently normal pregnancies were used to derive euthyroid ranges throughout pregnancy for T4 and free T4 (measured by the Amerlex kits) and for T4-binding globulin (TBG). The mean free T4 concentration fell with increasing gestational age, the range of values in the third trimester being narrower than the non-pregnant normal range. Evidence is reviewed which shows that the Amerlex free T4 assay is unaffected by the elevation of TBG in non-pregnant subjects. The decline in mean free T4 as pregnancy proceeds is in good agreement with the change in free T4 predicted by the falling T4/TBG ratio.

Alpha-Globulins↗