Preparation of a monospecific antiserum to thyroxine binding globulin for its quantitation by rocket immunoelectrophoresis

The effects of drugs on tests of thyroid function

Many drugs affect tests of thyroid function through alterations in the synthesis, transport and metabolism of thyroid hormones, as well as via influences on thyrotrophin (TSH) synthesis and secretion. Despite effects on circulating thyroid hormone and TSH levels, few drugs result in important changes in clinical thyroid state, but difficulty in interpretation of thyroid function tests often results. Commonly prescribed drugs including anti-convulsants, non-steroidal anti-inflammatory drugs, beta-adrenoceptor antagonists, steroid hormones and heparin may result in abnormal thyroid function tests in the absence of clinical features of thyroid dysfunction. In contrast, lithium and iodine containing drugs, including radiographic contrast agents and amiodarone, may result rarely in overt thyroid disease.

Time-resolved immunofluorometric assay for thyroxine-binding globulin in serum

We describe a new "sandwich"-type immunofluorometric assay for thyroxine-binding globulin (TBG) in serum. The assay involves a solid-phase monoclonal antibody immobilised in white microtiter wells, and a soluble biotinylated monoclonal antibody that reacts with the captured TBG molecules. Addition of streptavidin labeled with the europium chelator, BCPDA (4,7-bis(chlorosulfophenyl)-1,10 phenanthroline-2,9-dicarboxylic acid), and excess europium results in the formation of a highly fluorescent product. The fluorescence signal of the final complex is quantitated on the dried solid-phase with a pulsed-laser time-resolved fluorometer. The assay requires a 151-fold sample pre-dilution and a total incubation time of 90 minutes. It has a broad dynamic range of 0-100 mg/L and a minimum detection limit of 0.4 mg/L. The coefficients of variation for within-run and between-run assays averaged 4.5% and 5.4%, respectively. The mean analytical recovery of TBG added to serum was 103%. Results obtained by this method correlated well with those determined by a commercial radioimmunoassay (r = 0.96, n = 112) and by an immunoradiometric procedure (r = 0.95, n = 131).

Purification and partial characterization of thyroid hormone binding proteins in canine serum

Thyroxine-binding prealbumin (TBPA) and thyroxine-binding globulin (TBG) were isolated from canine serum and partially characterized. TBPA was isolated by retinol-binding protein (RBP) affinity chromatography and further purified by preparative agarose gel electrophoresis or FPLC ion exchange chromatography. TBG was purified by thyroxine (T4)-Sepharose chromatography followed by gel filtration on Sephacryl S-300 and preparative electrofocusing in a granulated dextran gel. Molecular weights were estimated by SDS-polyacrylamide gradient gel electrophoresis. Canine TBPA had a tetramer molecular weight of 56,000, an extinction coefficient of 12.8 cm2cg.1, an isoelectric point of 5.26-5.70 and a microheterogeneity pattern similar to that of human TBPA. Partial immunochemical identity with human TBPA was also found. Plasma concentrations of TBPA were measured by rocket immunoelectrophoresis in 43 normal and 35 hypothyroid dogs. Reference levels for TBPA ranged between 205 and 474 mg/l. Hypothyroid dogs had a mean TBPA level of 315.0 mg/l (SD: 91.1 mg/l). TBG had a molecular weight of 75,000 and an isoelectric point of 5.0. No immunochemical identity with human TBG was found. Gel filtration of serum on Sephacryl S-200, identification of T4-binding proteins with 125I-T4, and protein- and lipoprotein staining of fractions was performed. Thyroxine-binding was found to TBG in the beta-globulin region, TBPA in the alpha 2-region, albumin, and to the high density lipoprotein (HDL2) in the alpha 1-region and the very low density lipoprotein (VLDL) in the pre-beta region. A corresponding band to the latter protein in serum was masked by TBG and TBPA, and T4-binding in the alpha 1-region was not always seen in serum. Many similarities were found between man and dog regarding TBPA, but not TBG. The differences in structure of TBG may in part be responsible for the low serum T4 levels and rapid T4 metabolism seen in dogs.

Abnormalities of thyroid function tests in hospital inpatients

Results of thyroid function tests were analysed in 199 clinically euthyroid inpatients with normal serum thyroid stimulating hormone values. Serum total triiodothyronine was less than 1.25 nmol/l in 61.8% of samples, free triiodothyronine less than 3.9 pmol/l in 57.8%, total thyroxine less than 63 nmol/l in 21.1% and free thyroxine less than 9.5 pmol/l in 17.6%. In contrast, thyroxine binding globulin ratio was below normal (less than 5) in only 5 samples. A significant positive correlation (P less than 0.001) of serum free thyroxine with total thyroxine, thyroxine/thyroxine binding globulin ratio and free triiodothyronine was present as well as a significant negative correlation (P less than 0.001) with serum thyroid stimulating hormone. There was no correlation of free thyroxine measurements with serum albumin or non-esterified fatty acid concentrations. Although serum free thyroxine is low in a number of patients with non-thyroidal illnesses, this does not appear to be due to a rise in non-esterified fatty acids or a fall in albumin as has been proposed. Serum thyroid stimulating hormone measurements are essential to confirm the diagnosis of hypothyroidism in such subjects.

The influence of age and sex on tests of thyroid function

Serum concentrations of total thyroid hormones, thyroid hormone binding proteins, free thyroid hormones and thyroid stimulating hormone (TSH) were measured in a large group of healthy normal subjects (n = 218) to determine the influence of age and sex on tests of thyroid function. No effect of age on measurements of total thyroxine (T4), total triiodothyronine (T3) or reverse T3 was evident. Concentrations of free T4, free T3 and TSH were similarly unaffected, suggesting that increasing age alone does not significantly influence thyroid hormone measurements. There was, however, a rise in thyroxine-binding globulin (TBG) concentration with increasing age and a fall in thyroxine-binding prealbumin (TBPA). An influence of sex on tests of thyroid function was clearly evident. Levels of total T4, total T3, free T4 and the T4: TBG ratio were lower in women between 16 and 49 years than in men, despite an increase in TBG. Lower levels of free T3 were confined to the group of women aged 16-29 years. TSH values were unaffected by sex. These results suggest that sex hormone status in the pre-menopausal female subject may result in an increase in TBG concentration, and in addition may have an opposing effect on circulating thyroid hormone concentrations.

Thyroid hormone binding in serum of 15 vertebrate species: isolation of thyroxine-binding globulin and prealbumin analogs

The binding of [125I]T4 to serum proteins was studied in human, monkey, cattle, sheep, goat, water buffalo, horse, swine, dog, cat, rabbit, rat, chicken, frog, and salmon. Attempts were made to isolate thyroxine-binding globulin (TBG) and thyroxine-binding prealbumin (TBPA) from serum of all species, utilizing purification methods based on the specific properties of these proteins. TBPA was found to exist in all species examined. The protein was found anodal to albumin only in human, monkey, horse, and chicken. In cattle, swine, dog, cat, rabbit, frog, and salmon, TBPA was found cathodal to albumin, while sheep, goat, water buffalo, and rat had identical mobility of albumin and TBPA. The presence of TBG was demonstrated in larger mammals. In cat, rabbit, rat, chicken, frog, and salmon, TBG could not be demonstrated. The thyroxine-binding capacity of TBPA in serum varied from 1000 to greater than 6000 nmol/l and that of TBG between 150 and 600 nmol/l. TBPA from all species except salmon showed affinity to human retinol-binding protein. The presence of TBPA in all vertebrates suggests prealbumin to be a far more important thyroxine carrier than earlier anticipated.

A comparison of propranolol and nadolol pharmacokinetics and clinical effects in thyrotoxicosis

For a comparison of propranolol and nadolol pharmacokinetics and clinical effects, 20 newly diagnosed patients with thyrotoxicosis were randomly selected to receive as sole therapy, either propranolol, 40 mg every 6 hours for 14 days, or nadolol, 80 mg each morning for 14 days. The study was repeated when the patients were in the euthyroid state. There was a similar and highly significant degree of beta blockade and amelioration of symptoms of thyrotoxicosis at the end of the dosage interval (trough), i.e., 24 hours after nadolol or 6 to 8 hours after propranolol. Trough and peak serum levels of propranolol were significantly lower when the patients were in a thyrotoxic state than when they were in a euthyroid state, whereas the pharmacokinetics of nadolol were not appreciably altered by thyrotoxicosis. In thyrotoxicosis, trough levels of propranolol and nadolol were significantly inversely correlated with derived creatinine clearance values. In the symptomatic treatment of thyrotoxicosis, nadolol, a once-daily nonmetabolized beta blocker, has certain advantages compared with propranolol. It is preferred by patients, is more convenient, and probably aids compliance.

Measurement of free thyroxine and free triiodothyronine in asymptomatic subjects with high concentrations of serum TSH and a history of radioiodine therapy

Total and free thyroid hormones were measured in a group of asymptomatic subjects with high circulating TSH concentrations and a previous history of radioiodine therapy. Subjects in the high TSH group all fell within the normal range for measurements of total T4, total T3 and T4/TBG ratio. Concentrations of free T4 or free T3 were below the normal range in more than half (9 out of 17) of the high TSH group suggesting that these measurements are more sensitive indices of thyroid failure than measurement of total hormones or T4/TBG ratio.

Measurement of free thyroxine and free triiodothyronine in thyrotoxicosis and hypothyroidism

Free T4 and free T3 concentrations in thyrotoxic and hypothyroid patients at presentation differentiated those with thyroid disease from euthyroid control subjects and were closely correlated with T4/TBG ratio. A further group of thyrotoxic patients was followed serially during treatment with carbimazole. Measurements of free T4 and free T3 against closely reflected thyroid status with changes in the hormone measurements being paralleled by changes in T4/TBG ratio.

Thyroid hormones in non-insulin-dependent diabetes before and after dietary treatment

In a group of 8 moderately obese non-insulin-dependent diabetics, both serum total T3 and T4 concentrations and serum free T3 and T4 concentrations were significantly lower than control values. Serum TBG levels did not differ. Five patients demonstrated an apparently successful response to dietary treatment with loss of symptoms and glycosuria and fall in post-prandial blood glucose and HbA1. Serum concentrations of thyroid hormones and TBG, however, were not significantly altered. These abnormalities may be unrelated to hyperglycaemia and may reflect the nutritional status of the patients.

Increased serum concentration of T4-binding globulin in patients with hypogammaglobulinaemia

Patients with hypogammaglobulinaemia have been regularly found to have abnormal conventional thyroid function test results. The abnormality is due to an increased plasma concentration of T4-binding globulin (TBG). As the prevalence of autoimmune thyroid disease is probably increased in hypogammaglobulinaemia this further abnormality in the plasma proteins may lead to diagnostic confusion. Administration of gammaglobulin by infusion causes a rapid but transient fall in plasma concentrations of TBG and T4 which is probably due to a temporary redistribution of the plasma proteins.

An improved method for the estimation of relative binding constants of t4 and its analogues with serum proteins

A new method is reported for the determination of the binding constants of homologous ligands relative to that of the parent ligand. The technique minimizes the effects of random and bias errors inherent in absolute methods and obviates the need to quantitate the radiolabelled tracer concentration. In addition, it is likely to be of general applicability for monovalent proteins. The method has been applied in the determination of the binding constant (relative to that of T4) of the T4-analogue tracer used in the Amerlex free T4 radioimmunoassay, in respect of its binding to both T4 binding globulin and prealbumin. The binding constants of the analogue (relative to T4) were approximately 3% for both proteins. These levels were judged unlikely to interfere significantly with the measurement of free T4 in serum samples from patients with very wide variations in concentration of either T4 binding globulin or prealbumin.

A ligand-binding assay for thyroxine-binding globulin based on polyethylene glycol separation

A simple ligand-binding assay for thyroxine-binding globulin (TBG) is described. The method depends on the partition of labelled thyroxine between bovine serum albumin and TBG. Differentiation between labelled thyroxine bound to TBG and bovine serum albumin is achieved by the addition of excess TBG antiserum and subsequent precipitation with polyethylene glycol. The results are compared with the Corning Immophase kit method for TBG.

Thyroxine binding globulin (TBG) and thyroxine binding prealbumin (TBPA) measurement, compared with the conventional T3 uptake in the diagnosis of thyroid disease

An electroimmunoassay for the determination of thyroxine binding prealbumin is described. The diagnostic efficiency of the assay when used in conjunction with the serum thyroxine as a thyroxine: thyroxine binding prealbumin ratio, is compared with the conventional free thyroxine index and the more recently developed thyroxine: thyroxine binding globulin ratio. The population studied included euthyroid, hypothyroid and hyperthyroid patients and also those who were either pregnant or receiving oral contraceptive therapy. Despite recent evidence establishing the theoretical/practical advantages of using a direct measurement for thyroid binding proteins rather than an indirect method (tri-iodothyronine uptake), results obtained from this study suggest that, for the majority of patients requiring biochemical assessment, the free thyroxine index is still the superior discriminator of thyroid abnormality.

Concentration of serum thyroid hormone binding proteins after 131I treatment of hyperthyroidism

Serum concentrations of the thyroid hormone binding proteins, thyroxine binding globulin, prealbumin, and albumin were determined in 30 thyrotoxic patients before and after 131I treatment. Each patient was placed into one of three groups according to response to treatment. The serum concentration of all three proteins rose significantly in 10 patients who became euthyroid, and a greater increase was seen in 10 patients who developed hypothyroidism. There was no significant change in thyroid hormone binding protein concentrations in 10 subjects who remained hyperthyroid. Changes in the concentration of thyroid hormone binding proteins should be borne in mind when total thyroid hormone concentrations are used to monitor the progress of patients receiving treatment for hyperthyroidism.

Ratio of thyroxine to thyroxine-binding globulin. An assessment of the validity of a single reference range

The thyroxine:thyroxine-binding globulin ratio in serum (T4:TBG) has been proposed as a measure of thyroid status unaffected by altered binding protein levels. Here 320 apparently euthyroid patients are used to derive euthyroid ranges for serum thyroxine concentrations at specific TBG levels. These ranges are compared with those predicted by a T4:TBG reference range derived from the same patient data. The comparison suggests that the ratio would give false positives for hyperthyroidism at low TBG levels and false negatives at high TBG levels. To overcome this the use of graphical reporting of results or the relation of patient results to empirical reference ranges appropriate to the TBG level is suggested.

Determination of thyroxine binding globulin (TBG) in human serum by fluorescence excitation transfer immunoassay

Human thyroxine binding globulin (TBG) was purified to near homogeneity by chromatography on thyroxine-substituted Sepharose 4B, DEAE-Sephadex A-50, Sephadex G-150 and n-decylamine-substituted Sepharose 4B. TBG was quantitated by homogeneous fluorescence excitation transfer immunoassay; the preparation of dye-protein conjugates is described. The fluorescence of fluorescein-TBG is quenched when an immune complex is formed with tetramethylrhodamine-antibody. In the presence of unlabeled TBG, the relative fluorescence intensity is increased and can be related quantitatively to the concentration of competing antigen. The described assay for TBG has a range corresponding to serum concentrations of 5--75 microgram/ml; the correlation with radioimmunoassay is 0.96. The coefficients of variation for replicate determinations of single samples of serum with low, normal and elevated TBG concentrations were 3.5%, 2.7% and 2.6% respectively; between-assay variations were 8.8%, 8.6% and 2.6% respectively.

Radioimmunoassay of thyroxine-binding globulin: evaluation of a kit and diagnostic application

Serum thyroxine-binding globulin (TBG) was measured by using a radioimmunoassay method. The within-batch coefficient of variation (CV) of the assay was 5% or less at different TBG concentrations with a between-batch variation of less than 10%. The mean TBG concentration in hypothyroidism (28.5 +/- 6.7 mg/l) was significantly higher, and that in hyperthyroidism (22.3 +/- 3.5 mg/l) was significantly lower, than the euthyroid mean (25.4 +/- 6.9 mg/l). Acutely ill patients suffering from various non-thyroidal illnesses were found to have significantly low mean TBG (13.8 +/- 4.3 mg/l). A group of women in late pregnancy had a higher mean value, as expected. Serum TBG level, however, was ineffective as a thyroid function test because of overlapping concentrations between groups. The derived index T4:TBG ratio was found to be superior to free thyroxine index (FTI) in many cases of acutely ill patients, pregnancy, and hereditary TBG abnormality. The 95% confidence limit for the ratio in euthyroids was 2.3 to 7.1. Serum TBG level with T4:TBG ratio is recommended as a replacement for the T3-uptake test and FTI.

A method for the preparation of human thyroxine-binding globulin; its importance in the establishment of an accurate and specific radioimmunoassay

Pure thyroxine-binding globulin (TBG) is required in radioimmunoassay to prepare monospecific antisera, [125I]TBG and as primary standard. Homogeneous TBG was prepared by a three-stage affinity chromatography procedure; it could not be dissociated into subunits and its molecular weight by SDS polyacrylamide gel electrophoresis was 59 000. The amino acid composition was in agreement with two earlier reports. The secondary structure determined by circular dichroism in the far U.V. showed it to contain 24% each of alpha-helix and beta-pleated sheet. Serum TBG was determined by a 24-h radioimmunoassay using polyethyleneglycol to separate bound and free TBG. Serum TBG (mg/l, mean +/- S.D.) was: normal men 15.3 +/- 2.11 (n = 34), normal women 18.4 +/- 2.72 (n = 32) (P less than 0.005), in women on oral oestrogens 24.0 +/- 5.0 (n = 23), in normal pregnancy 38.6 +/- 3.0 (n =37), in cord blood 21.7 +/- 3.5 (n = 25) (P less than 0.001) and in euthyroid subjects aged over 60 years 17.8 +/- 4.5 (P n.s.). In women with thyroid disease TBG was reduced in hyperthyroidism: 15.5 +/- 2.5 (n = 28) and elevated in hypothyroidism: 21.0 +/- 4.0 (n = 25). Wider use in TBG-assay of non-denatured TBG of proven composition and structure should decrease the scatter in reference ranges and improve its value as a routine thyroid function test as both a primary measurement and as the T4 : TBG ratio.

Local IgA production in patients with chronic bronchitis: effect of acute respiratory infection

The immunoglobulin A and secretory IgA concentrations were studied in the serum and sputum of patients with chronic bronchitis to determine the effect of active chest infection and inflammation upon the estimation of local IgA production. The sputum/serum albumin ratio was higher during chest infection (5.51; SE+/-1.60x10(-2)) than in the non-infected samples (0.75; SE+/-0.14x10(-2);p less than 0.01) suggesting increased transudation as a result of inflammation. There was a similar increase in sputum/serum IgA during infection from 9.47 (SE+/-1.87)x10(-2) to 41.21 (SE+/-6.92)x10(-2)(p less than 0.01). However, the proportion of IgA locally produced when assessed by conventional techniques was unchanged. There was a significant inverse relationship between the estimated local IgA production of the infected samples and the degree of inflammation (r= -0.972;2p less than 0.001) indicating that inflammation was a major determinant of local IgA production. However, the secretory IgA concentrations of the samples were independent of the degree of inflammation. Furthermore, secretory IgA was found in samples that appeared to have failure of local IgA production using the method of Soutar. Clearer information about local IgA production can be obtained by measuring protein components unique to the bronchial secretions rather than applying correction factors to estimate the contribution of serum components, particularly in the presence of inflammation.

Thyroid function tests in thyroxine-binding globulin deficiency

Results are presented on two patients with complete and two with partial thyroxine-binding globulin (TBG) deficiency. All four subjects had lowered serum thyroxine but were clinically euthyroid. While thyroid hormone uptake tests or TBG assay were effective in the recognition of such individuals, indices based on these tests were misleading in assessing their thyroid status. Results within the reference range were obtained with the Immophase Free Thyroxine assay.

The T3/TBG ratio and the biochemical investigation of thyrotoxicosis

The relationship between serum tri-iodothyronine (T3) and thyroxine-binding globulin (TBG) has been studied in euthyroid subjects. This relationship is similar to that previously described for thyroxine (T4) and TBG. Serum T3, TBG and T4 data are presented for euthyroid subjects of all ages and the T4:T3 and T3:TBG ratios calculated. Similar data has been derived for patients with clinically proven thyrotoxicosis and for patients in which the diagnosis was in doubt. The results suggest the T3:TBG ratio enhances the use of the T3 assay to confirm thyrotoxicosis. The fact that 25% of our patients suffering from thyrotoxicosis had apparently normal serum T4 levels reinforces the contention that a serum TBG assay must be carried out as part of a basic thyroid function screen.

The relationship between the 'idiopathic oedema syndrome' and subclinical hypothyroidism

Thyroid function has been studied in eleven patients with the 'idiopathic oedema syndrome'. Six patients have been shown to have subclinical hypothyroidism associated with the presence of thyroid antibodies. All responded to treatment with sodium thyroxine. Five patients had normal thyroid function, absence of thyroid antibodies and failed to respond to treatment with sodium thyroxine.

An assessment under routine conditions of an immunoelectrophoretic assay for thyroxine-binding globulin

The correct clinical interpretation of serum total thyroxine requires a knowledge of the thyroxine binding globulin (TBG) concentration. An immunoelectrophoretic method for TBG assay has been assessed under routine conditions for precision, sensitivity and recovery. TBG has been compared with T3 Resin Uptake for euthyroid subjects, and patients with myxoedema and thyrotoxicosis. The relationship between TBG and age has also been studied. The method proved to be reliable, sensitive and a technically simple method for the routine determination of TBG and an excellent replacement for the indirect T3 Resin Uptake Test.

Measurement of serum thyroxine-binding capacity

A new method for free thyroxine-binding sites on thyroxine-binding globulin has been described. This method for thyroxine-binding capacity has the virtues of reasonable specificity and convenience for routine use. When used with serum thyroxine levels it should be valuable in the calculation of free thyroxine and total thyroxine-binding globulin. A more directly estimated TBC as reported here should have more value than the various and more empirical T3 uptake methods.

The T4/TBG ratio and the investigation of thyroid function

The relationship between serum thyroxine (T4) and thyroxine-binding globulin (TBG) has been studied in "euthyroid" subjects. The T4/TBG ratios in these subjects has been calculated and compared with those found in patients suffering from myxoedema and thyrotoxicosis. The ratio allows a more precise selection of the borderline cases requiring Thyroid Stimulating Hormone and triiodothyronine assays, particularly when the serum TBG is raised or lowered.

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.

A reassessment of 8-anilino-1-naphthalene sulphonic acid as a thyroxine binding inhibitor in the radioimmunoassay of thyroxine

The effectiveness of 8-anilino-1-naphthalene sulphonic acid (ANS) in the radioimmunoassay (RIA) of thyroxine (T4) as an inhibitor of the binding of T4 to serum T4-binding proteins is assessed. The optimum ANS concentration is dependent upon the antiserum and the method used for separating free and bound T4. If T4 binding to serum proteins is not completely inhibited, resin separation methods may yield low values, while polyethylene glycol and double-antibody methods may produce high values for T4 concentration. Even with optimum ANS concentration gross errors in measurement of T4 may occur in patients with high thyroxine-binding globulin (TBG) concentrations.