Production and evaluation of high-quality thyroxine antisera for use in radioimmunoassay

Fractionation of polyclonal antibody by isoelectric focusing and chromatofocusing: separation of high-affinity rabbit clonotype anti-thyroxine antibody

Immunoglobulin G fractions prepared from conventional rabbit anti-thyroxine (T4) antisera were fractionated by agarose gel isoelectric focusing (IEF) in the range of pH 3 to 10, and by chromatofocusing using a Fast Protein Liquid Chromatography (FPLC) system. The clonotype antibodies were recovered from the fractions and subjected to Scatchard plot analysis. The highest affinity constants of the initial antibody (shown in parentheses) and those of the antibodies recovered were IEF, 1.8 X 10(9) to 8.3 X 10(9) M-1 (2.2 X 10(9) M-1); FPLC, 2.4 X 10(9) to 6.0 X 10(9) M-1 (2.5 X 10(9) M-1). A sensitive radioimmunoassay of T4 was achieved with the isolated high-affinity anti-T4 antibody. The minimum detectable concentration of T4 was 6.3 X 10(-15) to 1.5 X 10(-14) mol/tube, which was three to five times lower than detectable with the initial antibodies.

Thyroid function tests in thyroid and nonthyroid disease

Modern day evaluation of thyroid disorders requires a combination of accurate clinical judgement and reliable, sensitive, and specific thyroid functions tests. Principle among the latter are thyroxine (T4) 3, 5, 3'-triiodothyronine (T3), and thyroid-stimulating hormone (TSH). Also playing an important role in special situations are free thyroxine, an assessment of bound and unbound thyroid-binding globulin, TRH stimulation, long-acting thyroid stimulator (LATS), antibodies to thyroid hormone and to thyroid receptors. Basic to interpretation of these tests in the clinical setting is a comprehension of the relationship of the hypothalamus, the pituitary, and the thyroid gland as well as a knowledge of the peripheral metabolism of thyroxine and triiodothyronine. The role of each of these laboratory tests in the evaluation of hyper- and hypometabolic states, their alteration in nonthyroid and other endocrine disorders, and the effects of environmental and physiological factors on these tests are reviewed.

Nation-wide TSH-screening with low recall rate. Laboratory results of a two-year study

The laboratory results from the Danish neonatal TSH screening programme for congenital hypothyroidism are reported for a period of 2 years. The screening was based on a single determination of TSH in blood samples taken on the fifth day of life (PKU-filterpaper). The TSH assay (double antibody radioimmunoassay) has a sensitivity of 10 mU TSH/1 blood and a precision of 20-30%. 127,111 samples have been tested. In 3% TSH was greater than 10mU/1 and in 0.64% greater than 25 mU/1. The cut-off point was fixed at 25 mU TSH/1. Values above this level were corroborated by a T-4 radioimmunoassay on the original blood specimen. TSH greater than 25 mU/1 together with T-4 less than 45 nmol/1 blood was found in 37 samples. In 18 samples with TSH greater than 25 mU/1 there was no material left for the T-4 determination. These 55 samples (37 + 18) resulted in a request for a venous sample (recall rate 0.043%). An elevated TSH was verified in 32 of the 55 samples. The repeat samples were collected on average on the 25th day after birth. Three of the 32 babies were clinically diagnosed before the screening result was available.

Neonatal screening for hypothyroidism in Scotland: results of a pilot study

We have assessed the feasibility of screening for neonatal hypothyroidism based on thyroxine (T4) or thyrotrophin (TSH) assays on dried blood spots collected for the phenylketonuria (PKU) screening programme in Scotland. The TSH assay was both more sensitive and specific than T4 in detecting primary hypothyroidism and was sufficiently rugged and rapid for routine application. Using a labelled antibody assay for TSH, 99.7 per cent of the first 30,000 babies tested had values less than 25 mU/l, and 0.24 per cent had values between 25 and 50 mU/l. Of the 11 suspected positives (TSH greater than 50 mU/l), nine were confirmed on serum specimens as primary hypothyroidism (prevalence 1:3,300) and the two false positives (0.007% of total) were explicable by inappropriate time of sampling. No false negatives have so far been identified. The median time to detection by blood spot assay was 16 days (range 6 to 22 days). Five of the nine cases were not suspected clinically at the time of screening. It is concluded that TSH fulfils most of the criteria for an ideal screening test for primary hypothyroidism and a screening programme based on blood spot TSH will allow therapy to be commenced within one month of birth.

Removal of endogenous ligands from a high-affinity antiserum for radioimmunoassay

A method for removal of endogenous ligands from high-affinity antisera (stripping) is described. A thyroxine antiserum of very high affinity was used to develop the method. The antiserum was incubated at 50 degrees C in a glutamate buffer at pH 4.4 together with some ethanol and methyl cellulose and a large amount of activated charcoal. After incubation for up to 2 days, the stripped antibodies were separated from the ligand adsorbed to the charcoal by centrifugation. The estimated titre increased several fold by the stripping, although the stripping method caused a loss of about 13% of the total number of binding sites over a period of 2 days. When stripped antiserum was used instead of unstripped antiserum in an assay system, the sensitivity was up to 3 times better, and the concentration, which could be measured with the best relative precision, was 3 times lower. The stripped antiserum showed a poorer specificity than the unstripped antiserum when a short incubation period was used. However, when a long incubation period was used, the specificity was nearly the same.

Dissociation rate constants and fractional binding of tracer estimated for three antibody populations in unstripped and stripped antiserum

The dissociation rate constants of a high-affinity thyroxine antiserum were estimated on the basis of curves showing the dissociation of labelled thyroxine-antibody complexes as a function of time. The dissociation curves were fitted by computer to an equation with a sum of three exponentials. The analysis of the curves gave estimates of the dissociation rate constants for fast, medium and slowly dissociating antibody populations, and also estimates of the size of the fraction of tracer bound to each of the three populations of antibodies. The coefficients of variation for the estimates were of the order of 10%, and the estimates were reproducible. The dissociation rate constants for the thyroxine antibodies were 0.55, 0.045 and 0.0058 h-1 at 23 degrees C. When labelled thyroxine and antiserum were incubated for increasing periods of time, there was an increase in the fractional binding of tracer to the slow-dissociating antibodies. The use of stripped antiserum as against unstripped antiserum also improved the fractional binding of tracer to the slow-dissociating antibodies. The stripping effect was slight for three other antisera which contained relatively fast dissociating antibodies.

Unconjugated thyroxine and triiodothyronine in urine: influence of age, sex, drugs and thyroid function

The 24 h urinary excretion (dU) of T4 and T3, determined by radioimmunoassays using extraction and separation on Sephadex columns, is higher in men than in women (mean dU-T4 and dU-T3 +/- 2 SD: 2.6 +/- 0.9 vs. 2.2 +/- 1.0 and 2.5 +/- 1.0 vs. 2.0 +/- 0.8 nmol, respectively). The excretion of T3 decreases with age, but expressed relatively to dU-creatinine there is no effect of age and sex. Children have increased T4 and T3 to creatinine excretion ratios and neonates excrete predominantly T4. Oestrogens, acute salicylate loading and diurnal variation yielded results, which were at variance with the hypothesis that dU-T4 and dU-T3 mirror corresponding free hormone concentrations in serum, and there was only a weal positive correlation to total serum hormones in thyrotoxicosis as well. General clinical use of urine T4 and T3 is obviated by poor diagnostic discrimination and inherent analytical and interpretative disadvantages.

Radioimmunoassay of thyroxine and triiodothyronine in urine using extraction and separation of Sephadex columns

Radioimmunological methods for the determination of thyroxine (T4) and triiodothyronine (T3) in urine have been developed. The methods are based on trapping of T4 and T3 from unextracted urine, followed by separation between free and antibody bound hormone on the same Sephadex column. The T4 method has been compared to a competitive protein binding (CPB) assay using ethyl acetate extraction. The methods are evaluated in sixty-seven euthyroid controls, twenty-four hyperthyroid and seven hypothyroid patients. In the T4 RIA detection limit was 3.9 pg, intra-assay coefficient of variation (cv) was 5.2% and inter-assay cv was 6.9%. In the T3 RIA detection limit was 7.2 ng, intra-assay cv 3.9% and inter-assay cv 10.8%. Recovery of added amounts of hormones and serial dilutions gave satisfactory results. The CPB assay was found unreliable with unspecific and false high values. In euthyroid controls 24 h urinary T4 excretion as measured by RIA was 1.8 +/- 0.5 nmol, and urinary T3 excretion was 0.7 +/- 0.3 nmol. T4 and T3 excretion was greatly elevated in hyperthyroid patients and decreased in hypothyroidism.

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.