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PubMed1987

Specific labeling of the thyroxine binding site in thyroxine-binding globulin: determination of the amino acid composition of a labeled peptide fragment isolated from a proteolytic digest of the derivatized protein.

[125I] Thyroxine has been covalently bound to the thyroxine binding site in thyroxine-binding globulin by reaction with the bifunctional reagent, 1,5-difluoro-2,4-dinitrobenzene. An average of 0.47 mol of [125I] thyroxine was incorporated per mol protein; nonspecific binding amounted to 8%. A labeled peptide fragment was isolated from a proteolytic digest of the derivatized protein by HPLC and its amino acid composition was determined. Comparison with the amino acid sequence of thyroxine-binding globulin indicated partial correspondence of the labeled peptide with two possible regions in the protein. These regions also coincide with part of the barrel structure present in the closely homologous protein, alpha 1-antitrypsin.

Amino Acids↗

PubMed1985

An evaluation of a fluorescence polarization immunoassay of thyroxin and thyroxin-uptake.

We evaluated the fluorescence polarization immunoassay for total thyroxin (T4) and thyroxin-uptake (T-U) in the Abbott "TDx" Analyzer. Between-assay precision was good when we did once-fortnightly calibration and assayed samples in singleton. Measured T4 concentration was decreased in hemolyzed samples with obvious red coloration and undetectable in severely hemolyzed samples. The T-U assay was unaffected by hemolysis. Unlike the triiodothyronine-uptake methods, the T-U assay utilizes labeled T4 and measures a variable related to serum thyroxin-binding capacity rather than the concentration of unoccupied binding sites in serum. The T4 and T-U values of 422 samples correlated highly with a T4 (in-house) radioimmunoassay and a commercial assay for thyroxin-binding globulin, respectively. The free thyroxin ratio (ratio of T4 to T-U, FTI) correlated highly with free T4 concentration as measured by T4-analog-tracer radioimmunoassay (fT4 analog RIA). FTI and fT4 values were discordant in late pregnancy (normal FTI and low fT4) and euthyroid sick patients (above-normal or normal FTI and low fT4), suggesting that the FTI gives fewer misleading results in these patients.

Adult↗

PubMed1985

Conflict between classifications based on free thyroxine and thyroid stimulating hormone concentrations in thyroxine-treated patients.

The use of free thyroxine assay as the basis for monitoring patients on thyroxine replacement therapy was assessed. Patients with normal free thyroxine levels were divided into sub-groups on the basis of serum TSH levels. Of these, patients with normal TSH levels had higher serum free thyroxine and free 3,5,3'-triiodothyronine concentrations than patients with elevated TSH levels. No significant differences were seen in the level of duration of replacement therapy. Patients were almost equally divided between the high and normal TSH sub-groups. It was concluded that free thyroxine assay had little role to play in the monitoring of these patients.

Humans↗

PubMed1987

Value of free thyroxine (FT4), free triiodothyronine (FT3), and sensitive thyrotropin (TSH) assay in the assessment of optimal thyroxine therapy.

We describe a sensitive thyrotropin (TSH) assay in the evaluation of thyroxine replacement therapy. Patients taking varying amounts of L-thyroxine replacement doses were studied using a thyrotropin-releasing hormone (TRH) test and a sensitive TSH assay as the indices of optimal replacement therapy. There were no differences in the mean thyroxine doses of those patients who had a normal TRH response and those who had a flat response. Similarly there were no significant differences in the serum free thyroxine (FT4), free triiodothyronine (FT3), or total triiodothyronine (TT3) levels between these two groups. The patients in both groups had normal basal serum TSH values as measured by a standard, HTSH RIABEAD (Abbott) method. However, serum TSH values, as measured by a sensitive TSH3 MAIACLONE (Serono) immunoradiometric method, were subnormal in all patients with flat TRH responses. The TSH (Serono) assay provided the best single blood test of optimal thyroxine replacement.

Adult↗

PubMed1981

Free thyroxine and free thyroxine index in women taking oral contraceptives.

This study compares the accuracy of the free thyroxine index and free serum thyroxine radioassay in evaluating the metabolic status of 36 euthyroid women, 17 of whom were taking oral contraceptive drugs. Results indicate that the free thyroxine index does not uniformly compensate for estrogen-induced elevations in total serum thyroxine concentrations and that the free serum thyroxine is the test of choice in assessing the functional thyroid status of women taking oral contraceptive drugs.

Adult↗

PubMed2001

Structure of rat transthyretin (rTTR) complex with thyroxine at 2.5 A resolution: first non-biased insight into thyroxine binding reveals different hormone orientation in two binding sites.

The first observation of the unique environment for thyroxine (T(4)) binding in tetrameric rat transthyretin (rTTR) is reported as determined by X-ray diffraction. These data revealed different modes of hormone binding in the two unique hormone-binding sites in the rat TTR tetramer channel. Differences in the orientation of thyroxine and the position of water molecules in the two binding sites further suggest a mechanism for the docking pathway of the hormone into the channel of TTR. Crystals of the rat transthyretin-thyroxine complex are isomorphous with those reported for apo rTTR and crystallized in the tetragonal space group P4(3)2(1)2 with four independent TTR monomeric subunits in the asymmetric part of the crystal lattice. Data were collected to 2.5 A resolution and the structure was refined to R = 20.9% for 15 384 data in the resolution range 12-2.5 A. Similar to human TTR, the rat protein is also a 54 000 Da tetramer with four identical polypeptide chains of 127 amino-acid residues. Of the 22 amino-acid residues which differ between the human and rat sequences, none are in the thyroxine-binding domains. Analysis of these structural data reveals that the tertiary structure is similar to that of hTTR, with only small differences in the flexible loop regions on the surface of the structure. Conformational changes of the amino acids in the channel result in a hydrogen-bonded network that connects the two binding domains, in contrast to the hydrogen bonds formed along the tetramer interface in the apo transthyretin structure. These changes suggest a mechanism for the signal transmission between thyroxine-binding domains.

Amino Acid Sequence↗

PubMed1988

Comparative evaluation of serum thyroxine, free thyroxine and thyrotropin determinations in screening of thyroid function.

We assessed a highly sensitive immunoradiometric thyrotropin (TSH) assay in screening thyroid dysfunction in 130 consecutive outpatients from a department of medicine and 224 patients from a municipal health centre. In addition to clinical examination, three routine tests were done: a thyroxine radioimmunoassay, an analogue-based free thyroxine assay and an immunoradiometric TSH assay. Triiodothyronine and the TRH test were done, if the findings were discrepant. Discrepancy existed in 24% of cases. The TSH assay had no false negative results (sensitivity 100%). Therefore TSH could screen all patients with thyroid dysfunction. Free thyroxine was the most specific assay (specificity 96%), but many subclinically or overtly hypothyroid patients would have been missed, if that assay had been used alone. We conclude that TSH(IRMA) is the best first-line measurement for thyroid dysfunction testing among outpatients. An abnormal TSH result alone is not diagnostic, but should be followed by the measurement of thyroxine or free thyroxine.

Adolescent↗

PubMed1979

[Effect of diet and L-thyroxine on deiodination of L-thyroxine in rat liver].

Male Wistar rats on 3 or 50% fat diet for 14 weeks were treated during 6 weeks with L-thyroxine (5 or 25 or 50 microgram 100 g body weight/2 times weekly, sc.) and the activity of L-thyroxine-deiodinase was determined in supernatans of liver homogenates. With increasing thyroxine lading the deiodinating activity increases statistically significantly within each diet group. The liver of animals on 50% fat diet deiodinates increasing thyroxine doses to a lesser extent than livers of animals on 3% fat diet. It is discussed that high fat diet influences thyroid function and that thyroxine shares effectiveness and undergoes deiodination also in other tissues, probably in fat tissue, to a higher extent.

Animals↗

PubMed1977

A radioimmunoassay for thyroxine on Sephadex columns. Investigations on the influence of some buffers on the binding of thyroxine to serum proteins.

A thyroxine radioimmunoassay procedure (T4RIA) based on incubation and separation on Sephadex columns is presented. The assay is rapid and easily to perform; if necessary columns may be regenerated. The raising of antibodies against thyroxine in goats is described in detail. The specificity of the antiserum towards the coupling of several compounds related to thyroxine has been tested. Influence of some buffers on the binding of thyroxine to serum proteins has been investigated. The results of T4RIA in patient sera were in agreement with those obtained by a competitive binding method. The within-day variation was approximately 4% (coefficient of variation, C.V.); day-to-day variation was 7% C.V.

Antibody Formation↗

PubMed1988

A comparison of commercially available luminescence enhanced enzyme immunoassays with in-house non-radioisotopic assays for thyroxine binding globulin and total thyroxine.

Commercial luminescence enhanced enzyme immunoassays (Amersham-Amerlite) for thyroxine binding globulin (TBG) and total thyroxine (TT4) were compared with the in-house methods (TT4--Abbott TDx, TBG--Immunoluminometric Assay (ILMA)). The experimental groups consisted of 108 healthy euthyroid blood donors, 165 non-selected thyroid outpatients, 44 tumour bearers and 84 haemodialysis patients. Total thyroxine/thyroxine binding globulin quotients were constructed as an index of thyroid function. The luminescence enhanced enzyme immunoassays were precise (interassay coefficients of variation less than 10% in the range 5-45 mg/l thyroxine binding globulin and 20-100 micrograms/l for total thyroxine) performed similarly to the in-house methods in the differentiation of eu-, hypo- and hyperthyroidism on the basis of total thyroxine/thyroxine binding globulin quotients. Although statistically significant differences often occurred in comparisons of the in-house method with the luminescence enhanced enzyme immunoassays, these only gave rise to thyroid status differences in two cases out of 273, where the in-house method gave a hyperthyroid, the luminescence enhanced enzyme immunoassay a euthyroid answer, when taken from the total thyroxine/thyroxine binding globulin quotients. The luminescence enhanced enzyme immunoassays performed as well as the in-house methods, and quality assessment data were comparable with their radioimmunological counterparts.

Humans↗

PubMed1984

Lowering effect of diphenylhydantoin on serum free thyroxine and thyroxine binding globulin (TBG).

To evaluate the effect of anticonvulsants on serum levels of free thyroxine (FT4) and thyroxine-binding globulin (TBG), 32 patients with epilepsy receiving long-term anticonvulsant treatment (mean duration 81.8 +/- 8.9 months) were employed in the present study. The serum levels of total thyroxine (TT4), total triiodothyronine (TT3), T3 resin uptake (T3U) and thyrotrophin (TSH) were also measured. Free thyroxine index (FT4I) was calculated from TT4 and T3U. Mean levels of these thyroid function tests were as follows: 5.52 +/- 1.51 micrograms/dl (TT4), 1.16 +/- 0.32 ng/dl (FT4), 101.5 +/- 16.2 ng/dl (TT3), 27.5 +/- 2.6% (T3U), 19.64 +/- 3.97 micrograms/ml (TBG), 1.50 +/- 0.36 (FT4I) and 2.21 +/- 0.18 microU/ml (TSH). These values except for T3U and TSH were significantly lower than normal values (P less than 0.01). Using correlation analysis, a significant dose-dependency (daily doses) was found between diphenylhydantoin (DPH) and TT4, FT4 and TBG. Other anticonvulsants (barbiturates, carbamazepine, primidone) or duration of treatment were not significantly correlated with thyroid function tests. Therefore serum levels of DPH were also measured in 19 out of 32 patients receiving anticonvulsant treatment. Significant negative correlation was found between serum levels of DPH and FT4 (r = -0.65) and DPH and TBG (r = -0.61).

Adult↗

PubMed1988

The transport of plasma thyroxine in white storks (Ciconia ciconia) and the association of high levels of plasma transthyretin (thyroxine-binding prealbumin) with moult.

Antisera raised against domestic fowl transthyretin (thyroxine-binding prealbumin) and quail albumin were used to identify thyroxine-binding proteins in the plasma of White storks (Ciconia ciconia) and to measure seasonal changes in these proteins. Three plasma proteins were shown to bind thyroxine (T4). They were albumin, transthyretin and a protein with an electrophoretic mobility on non-denaturing polyacrylamide gel electrophoresis intermediate between albumin and transthyretin. There was no thyroxine-binding globulin. The plasma concentrations of transthyretin and albumin were measured in male and female storks exposed to the photoperiodic and climatic conditions experienced during an annual cycle at 46 degrees N, 11 degrees E. The storks were in four age groups, fledglings and 1-, 2- and 3-year-old birds. A strong correlation (P less than 0.001) was found between daylength and the concentration of transthyretin during the period from February to August in the 1-, 2- and 3-year-old male and female storks, with peak values of 150-210 mg/l in May, June and July coinciding with the annual moult. Between September and January the concentration of plasma transthyretin was low, ranging between 80 and 100 mg/l. The concentration of plasma albumin was not correlated with seasonal changes in daylength and did not change during moult. It is suggested that seasonally high concentrations of plasma transthyretin may have a physiological role in moult.

Animals↗

PubMed1977

[The estimation of thyroxine, triiodothyronine, thyro-binding-index, and free-thyroxine-index in the newborn (author's transl)].

Because of the importance of early diagnosis of hypothyroidism normal values of thyroxine (T4), thyro-binding-index (TBI), free thyroxine-index (FTI) and triiodothyronine (T3) in the serum of newborns were established. In extremely premature babies as in early fetal life the total thyroxine is relatively low. The lowest serum thyroxine in newborns without hypothyroidism was found in a premature infant with a birth weight of 750 g, i.e. 4.8 mug/100 ml on the 4th day of life. Otherwise the T4 values were quite high during the neonatal period, without signs of hyperthyroidism. Our hypothyroid patients scarcely had higher T4 values (0.8-5.1 mug/100 ml) within the first month of life than older hypothyroid patients. Healthy newborns had a mean T4 value of about 15 mug/100 ml during the first week of life (two standard deviations 6.4-23.6 mug/100 ml). Afterwards the T4 values slowly came down to a mean of about 12 mug/100 ml in the 4th week of life. During the first month of life the FTI of hypothyroid patients was below two standard deviations of normal newborn values, whereas TBI-values showed an overlap. During the first 3 days of life newborns with goiter had T4-values in the lower normal or hypothyroid range without obvious signs of hypothyroidism. FTI was quite low, too, and TBI relatively high (as in hypothyroidism). Within the second week of life all these values normalized mostly after treatment with KI ointment, occasionally without this treatment. Where these values did not normalize, hypohtyroidism was assumed. Triiodothyronine in cord blood was very low (0.47 ng/1.6 ng/ml) during the neonatal period (one to two days after birth almost 2.0 mg/ml). Values in our hypothyroid patients fluctuated widely (zero to slightly elevated values with a mean of 0.78 ng/ml). Therefore, T4 seemed more reliable than T3 for the diagnosis of hypothyroidism (the opposite was true for hyperthyroidism.

Fetal Blood↗

PubMed1978

[Study of the binding of thyroxine by thyroxine-binding prealbumin by a dialysis method using a fixed free concentration of ligand (author's transl)].

In order to study ligand-protein binding in solution, a dialysis method was used in which the free concentration of ligand can be controlled. The method has certain advantages and was applied to the binding of thyroxine by thyroxine-binding prealbumin, a system about which the results found in the literature are not in good agreement. From the isotherm drawn according to the Scatchard plot, it was found that thyroxine-binding prealbumin only presents a single binding site for thyroxine per molecule, the association constant being 1.7 . 10(8) M-1.

Dialysis↗

PubMed1984

Estimation of free thyroxin with a new thyroxin analog and a porous-glass solid phase.

We evaluated a new thyroxin analog-based assay for free thyroxin (FT4) (Corning Medical), finding it technically simple and precise (between-assay CVs of 3.3 and 4.2% for FT4 concentrations of 12 and 25 ng/L, respectively). We measured FT4 in a total of 325 serum samples from euthyroid patients; patients receiving replacement thyroxin; patients receiving estrogens or who were pregnant; hyperthyroid, hypothyroid, and non- thyroidally ill patients; and patients receiving salicylates, phenytoin, or heparin. This assay clearly identified hyper- and hypothyroid patients, and produced similar results in euthyroid patients with above-normal, normal, or low concentration of thyroxin-binding globulin. Results in some non-thyroid-illness patients and patients receiving salicylates or phenytoin were low compared with euthyroid patients receiving no medications, but the diagnostic accuracy of the Corning FT4 assay exceeded that of another analog-based assay (Amersham) in these particular groups. We believe the new Corning analog FT4 assay offers an attractive alternative to other commercial FT4 systems.

Female↗

PubMed1980

Decrease in serum free triiodothyronine, thyroxine-binding globulin and thyroxine-binding prealbumin whilst taking a very-low-calorie diet.

Serum free triiodothyronine decreased significantly (P < 0.02) by 18 per cent (range from -8 to +47 per cent) in 13 grossly obese patients after 11 days (median) treatment (range 8-18 days) with a very-low-calorie diet consisting of 43.0 g protein, 30.4 g carbohydrate and 1.1 g lipid. Also a significant decrease was found in serum total triiodothyronine (P < 0.007), thyroxine-binding globulin (P < 0.008) and thyroxine-binding prealbumin (P < 0.005). No significant changes in serum free thyroxine, total thyroxine and TSH could be demonstrated.

Adult↗

PubMed1981

Stereospecific determination of D-thyroxin and L-thyroxin in serum.

A new method for separate determination of D-thyroxin and L-thyroxin in the serum was applied to study the response of serum levels of these isomers and of radioimmunologically determined total T3, total T4, free T4 and TSH upon administration of 2, 4 and 6 mg of a highly purified D-thyroxin preparation in six male patients with diffuse nontoxic goiter. D- and L-thyroxin are determined separately following extraction of the hormone from the serum and formation of diastereomeric dipeptides. The separation and final determination are accomplished by means of ion-pair chromatography on reversed-phase columns using an iodine-selective catalytic detector. A significant decrease in TSH takes place during the 3-days observation period. The values of D-T4, total T3 and free T4 are highest 4 hours after administration of the tablets and get to be to initial values after 24 hours. L-T4 shows no significant change. A direct suppressive effect of D-T4 on the pituitary gland may therefore be assumed as the cause of the suppression of TSH secretion.

Adult↗

PubMed1978

Bovine thyroxine-binding globulin. Purification and comparison of molecular weight and amino acid composition with human thyroxine-binding globulin.

Bovine and human thyroxine-binding globulin were purified from serum by a three-step purification procedure which comprised affinity chromatography consecutively on thyroxine- and Concanavalin A--Sepharose and finally preparative polyacrylamide gel electrophoresis. The molecular weights of the two proteins were similar (54 000) as well as their carbohydrate contents while some differences in amino acid composition were found. Rabbit antiserum against bovine thyroxine-binding globulin reacted with human thyroxine-binding globulin with no sign of spur formation.

Amino Acids↗