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At least 19 recordsLinked to original sources
PubMed1978

Correlation of free thyroxine index and thyroxine: thyroxine-binding globulin ratio with the free thyroxine concentration as measured by the thyroxine and thyroxine-binding globulin radioimmunoassays.

The concentration of thyroxine-binding globulin in the serum can now be measured by a simple and specific radioimmunoassay. Triiodothyronine uptake and measurement of total thyroxine have been combined to yield a free thyroxine index which has been found to correlate with the clinical state of the patients. An estimate of the free thyroxine concentration, as measured by the thyroxine and thyroxine-binding globulin radioimmunoassays, provided a good correlation with the free thyroxine index and the thyroxine: thyroxine-binding globulin ratio. However, the thyroxine: thyroxine-binding globulin ratio is inaccurate when thyroxine-binding globulin concentrations are high or low.

Adult↗
PubMed1978

Thyroxine-protein interactions. Binding constants for interaction of thyroxine analogues with the thyroxine binding site on human thyroxine-binding globulin.

The binding constants for interaction of various thryoxine analogues with the thyroxine binding site on human thyroxine-binding globulin have been determined. Equilibrium dialysis, at pH 7.4 and 37 degrees C, was used to measure the competitive effects of different iodothyronine compounds on the binding of 125I-labeled thyroxine to highly purified thyroxine-binding globulin. Relative to L-thyroxine, K = 6 . 10(9) M-1, the association constants of some important analogues were D-thyroxine, 1.04 . 10(9) M-1, 3,5-diiodo-3'-isopropyl-L-thyronine, 4.9 . 10(8) M-1; L-triiodothyronine, 3.3 . 10(8) M-1, 3,3',5'-DL-triiodothyronine (reverse triiodothyronine), 3.1. 10(8) M-1; tetraiodothyropropionic acid, 2.7 . 10(8) M-1; tetraiodothyroacetic acid, 2.6 . 10(8) M-1; 3', 5'- diiodo-DL-thyronine, 8.3 . 10(7) M-1; and 3,5-diiodo-DL-thyronine, 7.1 . 10(7) M-1. Calculation of the deltaG0 values for binding of the analogues indicates that a major contribution to the free energy favoring binding is made by the alanine side chain of thyroxine. A change in configuration of the alpha-amino group from the L to D form causes an unfavorable change of 1 kcal/mol in the free energy of binding. Removal of the alpha-amino group as in tetraiodothyropropionic acid causes an unfavorable change of 1.9 kcal/mol in the free energy of binding. With regard to ring substituents, the results indicate that the two inner 3,5-iodines make about the same contribution to binding as the two outer 3', 5'-iodines.

Binding Sites↗
PubMed1989

[The role of binding proteins in radioimmunologic determination of the free thyroxine concentration using thyroxine analog methods: thyroxine immunoextraction versus thyroxine analog immunoextraction].

Amerlex FT4-analogue one step RIA was investigated to estimate the 125-Iodine thyroxine immunoextraction in sera with normal and abnormal concentrations of binding proteins. The following results were found: Amerlex analogue FT4 RIA equals an equilibrium-immunoassay. Thyroxin-analogue tracer is strongly bound to an albumin (80%). During the assay procedure, there is no analogue binding to serum proteins. If serum and FT4-antibody diluted in an adequate manner, FT4-analogue values in all serum dilution steps are determined identically. FT4-analogue values are influenced only by thyroxine, bound to albumin and/or prealbumin and not by TBG-bound thyroxine. Serum albumin concentration does not influence FT4-analogue values.

Humans↗
PubMed1971

Effects of norethandrolone on the transport and peripheral metabolism of thyroxine in patients lacking thyroxine-binding globulin. Observations on the physiological role of thyroxine-binding prealbumin.

Studies of the effect of norethandrolone on the transport and peripheral metabolism of thyroxine were carried out in four patients lacking thyroxine-binding globulin. Before norethandrolone administration, values for serum protein-bound iodine (PBI) were decreased (1.8 +/-0.5 mug/100 ml) and the proportion of free thyroxine increased (0.036 +/-0.008%). As a result, values for the absolute concentration of free thyroxine iodine were at the lower end of the normal range (0.63 +/-0.12 mmug/100 ml). During the control thyroxine-turnover study, the thyroxine distribution space was strikingly increased (18.2 +/-7.9 liters) and the fractional rate of thyroxine turnover moderately increased (17.1 +/-11.3%/day), as compared to the expected mean values for normal subjects. Therefore, calculated values for the daily rate of thyroxine clearance were increased even more, ranging between 255 and 500% of normal values. However, owing to the low PBI in these patients, the daily disposal of thyroxine iodine was similar to that expected in normals on the basis of age and weight. During the administration of norethandrolone, the thyroxine-binding capacity of the thyroxine-binding prealbumin increased strikingly in all patients, values averaging 162% of those found during the control period. This increase was associated with a highly significant increase in PBI (133% of control values) and a small but significant decrease in the proportion of free thyroxine, resulting in no significant change in the absolute concentration of free thyroxine iodine. In all four patients, administration of norethandrolone was associated with a pronounced decrease in the thyroxine distribution space to values which averaged 69% of those found during the control period. Values for the fractional rate of thyroxine turnover increased slightly. As a result, thyroxine-clearance rate decreased in all patients. Owing to the reciprocal changes in clearance rate and PBI, no significant change in total daily thyroxine disposal was observed. The present studies reveal that when the thyroxine-binding prealbumin is increased in patients lacking thyroxine-binding globulin, several indices of peripheral thyroxine transport and metabolism are altered. However, these changes were small, even in the absence of thyroxine-binding globulin. It is suggested, therefore, that the effect of changes in thyroxine-binding prealbumin would be even smaller in individuals in whom thyroxine-binding globulin is present.

Adult↗
PubMed1983

Influence of age and sex on the concentration of free thyroxin in serum and on the free thyroxin: total thyroxin ratio.

The influence of age and sex on the concentration of free thyroxin and on the free thyroxin/total thyroxin ratio was studied in 104 men and 89 women, ages 15 to 83 years. Free thyroxin gradually declined with age in men but not in women. In addition, sex- and age-related variations for free thyroxin were similar to those for total thyroxin and the two indirect assessments of free thyroxin: the free thyroxin index and the total thyroxin/thyroxin binding globulin ratio. Further, free thyroxin and the free thyroxin/total thyroxin ratio were higher in men than in women. However, the free thyroxin/total thyroxin ratio showed relatively little change with age in either sex. This study suggests that the free thyroxin/total thyroxin ratio is helpful in minimizing the age-related variations in both free and total thyroxin.

Adolescent↗
PubMed1976

Thyroxine-protein interactions. Interaction of thyroxine and triiodothyronine with human thyroxine-binding globulin.

The effect of temperature on the binding of thyroxine and triiodothyronine to thyroxine-binding globulin has been studied by equilibrium dialysis. Inclusion of ovalbumin in the dialysis mixture stabilized thyroxine-binding globulin against losses in binding activity which had been found to occur during equilibrium dialysis. Ovalbumin by itself bound the thyroid hormones very weakly and its binding could be neglected when analyzing the experimental results. At pH 7.4 and 37 degrees in 0.06 M potassium phosphate/0.7 mM EDTA buffer, thyroxine was bound to thyroxine-binding globulin at a single binding site with apparent association constants: at 5 degrees, K = 4.73 +/- 0.38 X 10(10) M-1; at 25 degrees, K = 1.55 +/- 0.17 X 10(10) M-1; and at 37 degrees, K = 9.08 +/- 0.62 X 10(9) M-1. Scatchard plots of the binding data for triiodothyronine indicated that the binding of this compound to thyroxine-binding globulin was more complex than that found for thyroxine. The data for triiodothyronine binding could be fitted by asuming the existence of two different classes of binding sites. At 5 degrees and pH 7.4 nonlinear regression analysis of the data yielded the values n1 = 1.04 +/- 0.10, K1 = 3.35 +/- 0.63 X 10(9) M-1 and n2 = 1.40 +/- 0.08, K2 = 0.69 +/- 0.20 X 10(8) M-1. At 25 degrees, the values for the binding constants were n1 = 1.04 +/- 0.38, K1 = 6.5 +/- 2.8 X 10(8) M-1 and n2 = 0.77 +/- 0.22, K2 = 0.43 +/- 0.62 X 10(8) M-1. At 37 degrees where less curvature was observed, the estimated binding constants were n1 = 1.02 +/- 0.06, K1 = 4.32 +/- 0.59 X 10(8) M-1 and n2K2 = 0.056 +/- 0.012 X 10(8) M-1. When n1 was fixed at 1, the resulting values obtained for the other three binding constants were at 25 degrees, K1 = 6.12 +/- 0.35 X 10(8) M-1, n2 = 0.72 +/- 0.18, K2 = 0.73 +/- 0.36 X 10(8) M-1; and at 37 degrees K1 = 3.80 +/- 0.22 X 10(8) M-1, n2 = 0.44 +/- 0.22, and K2 = 0.43 +/- 0.38 X 10(8) M-1. The thermodynamic values for thyroxine binding to thyroxine-binding globulin at 37 degrees and pH 7.4 were deltaG0 = -14.1 kcal/mole, deltaH0 = -8.96 kcal/mole, and deltaS0 = +16.7 cal degree-1 mole-1. For triiodothyronine at 37 degrees, the thermodynamic values for binding at the primary binding site were deltaG0 = -12.3 kcal/mole, deltaH0 = -11.9 kcal/mole, and deltaS0 = +1.4 cal degree-1 mole-1. Measurement of the pH dependence of binding indicated that both thyroxine and triiodothyronine were bound maximally in the region of physiological pH, pH 6.8 to 7.7.

Binding Sites↗
PubMedDate not supplied

[Behavior of the levels of free triiodothyronine, triiodothyronine, free thyroxine, thyroxine, thyrotropin and thyroxine-binding globulin in the serum of children with nephrotic syndrome].

Concentrations of free triiodothyronine, triiodothyronine, free thyroxine, thyroxine, thyrotropin, thyroxine-binding globulin, urea, creatinine, cholesterol and total protein were determined in serum of four children (ages from 8 to 16 years) with nephrotic syndrome undergoing therapy. The results showed that at serum protein concentration of less than 4.5 g/dl the concentration of free thyroxine was 3.35 +/- 2.32 pg/ml and that of free triiodothyronine 2.65 +/- 0.96 pg/ml. Elevation of the protein concentration to 4.5-5.7 g/dl lead to an increase in the concentration of free thyroxine to 6.53 +/- 3.69 pg/ml and of free triiodothyronine to 3.32 +/- 1.08 pg/ml. The age-matched reference values for free thyroxine are 15.72 +/- 1.9 pg/ml and for free triiodothyronine 5.10 +/- 1.29 pg/ml. The concentration of thyroxine, triiodothyronine and thyroxine-binding globulin were decreased whereas that of thyrotropin was elevated. Although free triiodothyronine and triiodothyronine were decreased they remained close to the normal range thus preventing apparent hypothyroidism. Improvement in the concentration of serum protein and cholesterol lead to an improvement of serum levels of thyroid hormones and thyroxine-binding globulin; concentrations of thyrotropin remained elevated.

Adolescent↗
PubMed1983

Clinical evaluation of two direct procedures for free thyroxin, and of free thyroxin index determined nonisotopically and by measuring thyroxin-binding globulin.

We have investigated the clinical utility of two direct radioimmunoassays for free thyroxin, an enzyme-inhibition immunoassay, and a direct measurement of thyroxin-binding globulin (TBG) by radioassay. All assay methods correctly identified greater than or equal to 90% of euthyroid, hyperthyroid, and hypothyroid patients who had normal TBG concentrations. In patients with altered TBG concentrations, none of the assays correctly classified all categories of patients. However, the direct assays of free thyroxin concentrations were able to classify correctly more patients with altered TBG concentrations than did the free thyroxin index methods. The free thyroxin index methods evaluated may be acceptable for routine use, if the concentration of thyroxin and the measurement of TBG capacity are reported along with the index value. Patients with altered TBG concentrations included a group of euthyroid pregnant patients. Significant decreases in free thyroxin in the third trimester were detected by all the assays studied. For patients in the first and second trimester, the mean free thyroxin concentration measured varied with the assay method.

Evaluation Studies as Topic↗
PubMed1974

Evaluation of new kits for the assessment in vitro of thyroid function by determination of serum total thyroxine, free TBG capacity, and free thyroxine index Sephadex G-25 and 125I-labelled triiodothyronine and thyroxine.

A trial was carried out on 134 patients of new kits (Ames Co) using columns of Sephadex G-25 for the determination of serum total thyroxine (Tetralute test) and for the indirect estimation of serum free thyroxine-binding globulin capacity (Trilute test). Both new methods were quicker and easier than the reference resin methods and of similar precision. The two measurements when combined to give a free thyroxine index (Trilute-Tetralute-FTI) increased further the diagnostic discrimination and usefulness of the tests. The method for the determination of serum thyroxine can be modified to give a direct estimate of serum free thyroxine, expressed as a free thyroxine index. This new single-column technique, called the ;single-column free thyroxine index', gave a good correlation with clinical thyroid status in a preliminary trial of 45 patients.

Adolescent↗
PubMed1990

Unforeseen effect of thyroxine binding globulin when using the microencapsulated antibody method to determine free thyroxine (FT4): misleading results due to circulating unsaturated thyroxine binding globulin.

The effect of varying concentrations (0-52 mg/l) of purified thyroxine binding globulin (TBG) on the microencapsulated antibody method for free thyroxine was investigated. The results demonstrated that the free thyroxine values were strongly influenced by the concentration of thyroxine binding globulin in the samples. The standard curve could no longer be distinguished at a concentration of purified thyroxine binding globulin of 52 mg/l. In the clinical application, we observed that the values obtained using the microencapsulated antibody method were significantly higher than the expected values in patients receiving triiodothyronine treatment after total thyroidectomy (theoretically nil) and in patients with untreated primary hypothyroidism with negligible thyroxine (less than 12.9 nmol/l). These false positive values are considered to be due to the methodological problem mentioned above, i.e. the microcapsule membrane is not efficient and therefore must be improved. Consequently, any data based on this method should be interpreted with caution.

Adult↗
PubMed1977

[Studies on the binding capacity of thyroxin-binding globulin (TBC), total thyroxin (T4), free thyroxin index (FT4-I) and the ETR-test in gestosis and placental insufficiency].

Total serum thyroxine (T4), thyroxine binding capacity (TBC), free thyroxine index (FT4-I) and effective thyroxine ratio (ETR) were measured in 53 toxemias of pregnancy and in 5 cases with placental insufficiency. Total serum thyroxine, ETR and FT4-I were found in physiological ranges of the normal pregnancy, the TBC-index was decreased. Between the 19. and 34. week of pregnancy, the decrease of the TBC-index was smaller than after the 34. week of pregnancy.

Adult↗
PubMed1975

[Changes in the binding capacity of thyroxine-binding globulin (TBC), of total thyroxine (T4), of the free thyroxine index (FT4-I) and of thyrotropin (TSH) during normal pregnancy and in hydatidiform mole].

TBC-index and total serum thyroxine were measured in 100 healthy nonpregnant and in 163 pregnant women during the 8. and 41. weeks of gestation. The free thyroxine index was calculated. The TBC-index was found to be elevated in pregnant women and rose continously with duration of pregnancy. The amount of total serum thyroxine was greater in pregnant women (p less than 0,01) without difference between early and late pregnancy. The free thyroxine index decreased continously during pregnancy (p less than 0,01). Serum TSH level were elevated during the first two trimesters of pregnancy. At the third trimester the TSH level were found within the normal range. In patients with hydatidiform mole TSH, total thyroxine, FT4-index, ETR-index as well as TSH levels were increased.

Adult↗
PubMed1987

A new radioimmunoassay of free thyroxine using 125I-labelled thyroxine-protein complex uninfluenced by albumin and thyroxine-binding globulin.

We describe a double-antibody solid phase radioimmunoassay for free thyroxine (FT4) in serum with use of 125I-labelled thyroxine-human chorionic gonadotropin conjugate. Since the labelled conjugate does not bind to thyroxine binding globulin (TBG) and albumin because of its large molecular weight, the method is uninfluenced by TBG or albumin. The measurable range of FT4 in serum was 2.0 to 128 ng/l. The mean coefficients of variation within and between assays were 4.6-8.6% and 6.3-11.6%, respectively. The FT4 values determined by the proposed method correlated well with those determined by commercial radioimmunoassay in subjects with normal albumin concentration (r = 0.98). The FT4 concentrations in serum as determined by this method were 9 to 17 ng/l for healthy adult subjects; high for patients with hyperthyroidism; low for patients with hypothyroidism; and within normal limits for pregnant women, and patients with high or low concentrations of thyroxine-binding globulin.

Albumins↗
PubMed1982

Photoaffinity labeling of human thyroxine-binding prealbumin with thyroxine and N-(ethyl-2-diazomalonyl)thyroxine.

To facilitate studies of thyroid hormone-binding proteins, we have synthesized and tested the photoaffinity analogues N-(ethyl-2-diazomalonyl)-3,5,3'-triiodothyronine (EDM-T3) and N-(ethyl-2-diazomalonyl)thyroxine (EDM-T4). The binding affinities of L-EDM-T4 and D-EDM-T4 to human thyroxine-binding prealbumin were 4% and 13.2%, respectively, that of L-thyroxine (L-T4). For comparison the affinities of L-EDM-T3 and D-EDM-T3 to crude rate liver nuclear receptor preparation were 0.1% and 0.7%, respectively, that of L-triiodothyronine (L-T3). Photolysis of prealbumin-[125I]-L-EDM-T4 complexes at 254 nm resulted in covalent linkage of [125I]-L-EDM-T4 to prealbumin as judged by sodium dodecyl sulfate gel electrophoresis. Virtually no labeling was observed in the absence of photolysis. Photolabeling of prealbumin was specific for the high-affinity hormone binding site since it was (a) completely blocked during photolysis in the presence of excess 3,5,3',5'-tetraiodothyroacetic acid, (b) saturated at high [125I]-L-EDM-T4 concentrations, (c) prevented when the hormone binding site had been previously blocked by dansylation of prealbumin, and (d) blocked competitively by T3 and T4 with inhibition constants (K1) similar to the dissociation constants (Kd) for these ligands. Analysis of prealbumin photolabeling by direct attachment of [125I]-L-EDM-T4 or attachment of unlabeled L-EDM-T4 followed by titration of the remaining sites with [125I]-L-T4 indicated a photolabeling efficiency of 54-61% at 63-67% site occupancy. After destruction of the diazo group by preirradiation, L-EDM-T4 was found to label prealbumin after further irradiation; 19-26% photolabeling efficiency could be achieved by using preirradiated reagent at 87-91% site occupancy. This carbene-independent photoattachment was also specific for a high-affinity hormone binding site. The mechanism of the carbene-independent process may involve attachment via radical formation following photoinduced loss of the thyronine ring iodine. Accordingly, specific covalent cross-linking of L-T4 to prealbumin was demonstrated; however, the photolabeling efficiency was much lower than that with preirradiated EDM-T4. Both EDM-T4 and T4 were employed to photolabel prealbumin, thyroxine binding globulin, and albumin in unfractionated human serum.

Affinity Labels↗