Interlaboratory/intermethod differences in functional sensitivity of immunometric assays of thyrotropin (TSH) and impact on reliability of measurement of subnormal concentrations of TSH

Clinically relevant interassay precision profiles for thyrotropin (thyroid-stimulating hormone; TSH) were constructed with human serum pools measured over 4-8 weeks by six immunometric assays, in at least two different reagent lots. Functional sensitivities (the concentration at which the interassay CV is < or = 20%) were determined in four to eight clinical laboratories plus the respective manufacturer's laboratory. These studies revealed that the manufacturer's stated functional sensitivity limit is rarely duplicated in clinical practice. Loss of specificity (indicated by artifactually high values) was seen with some methods when used to measure certain unrefrigerated low-TSH sera. Measurement of TSH in four human serum pools (TSH < 0.05-0.25 mIU/L) by 16 different methods (each in at least eight UK or US laboratories) showed that some methods could not reliably distinguish subnormal from normal TSH values. Better pool rankings and fewer misclassifications of low-TSH sera as "normal" were seen with use of assays capable of "third-generation" functional sensitivity (0.01-0.02 mIU/L) than with assays with "second-generation" functional sensitivity (0.1-0.2 mIU/L). Because inter- and intramethod differences in functional sensitivity negatively impact the diagnostic accuracy and cost-effectiveness of a TSH-centered thyroid-testing strategy, laboratories should independently establish an assay's functional sensitivity by a clinically relevant protocol. Moreover, manufacturers should assess functional sensitivity more realistically and improve the robustness of assays to ensure that their performance potential is consistently met in clinical practice.

Effect of selenium depletion on thyroidal type-I iodothyronine deiodinase activity in isolated human thyrocytes and rat thyroid and liver

The effects of dietary selenium deficiency on hepatic and thyroidal type I iodothyronine deiodinase (ID-I) and selenium-dependent glutathione peroxidase (GPx) activities have been studied in weanling rats. In selenium-deficient animals hepatic ID-I activity was reduced to 11% of the activity found in the selenium-replete groups, whilst thyroidal ID-I activity increased by 42%. Hepatic and thyroidal GPx activities were also reduced by selenium deficiency to approximately 0.6 and 70%, respectively, of the values found in the selenium-replete animals. We have also studied the effects of thyrotropin (TSH), and selenium supply on the activity of IDI and GPx in human thyrocytes grown in primary culture. When thyrocytes were grown in selenium-deficient (< 1 nmol l-1 Se) medium in the absence of TSH, addition of sodium selenite up to 1000 nmol l-1 had little or no effect on ID-I activity. In the absence of added selenite, TSH addition produced a significant increase in ID-I activity and this stimulation was increased further when selenite was added at concentrations of 50-1000 nmol l-1 with an optimal effect on ID-I activity being observed at a 500 nmol l-1. Selenium content and GPx activity in human thyrocytes grown in selenium-free media (selenium content < 1 nmol l-1) were not significantly lower than the corresponding measurements made in cells grown in media containing selenium at a concentration of 5.4 nmol l-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of fetal plasma levels of glutathione S-transferase B1 as an indicator of damage to the liver caused by hypoxia in utero

Glutathione S-transferase B1 (GST B1) concentration in blood and amniotic fluid from fetuses investigated for a variety of conditions including rhesus (Rh) allo-immunisation was assessed for its usefulness as a measure of liver damage caused by hypoxia in utero. The concentration in blood from the intrahepatic vein (IHV) was 10-fold higher than that from the placental cord insertion suggesting that parenchymal liver cells are damaged during blood sampling from the IHV. As a measure of hepatocellular impairment caused by intra-uterine hypoxia, levels were higher in frankly acidotic fetuses than in normally managed Rh fetuses. The degree of hypoxia required to trigger the release of GST B1 into the plasma remains unclear.

Hepatotoxicity of sodium stibogluconate therapy for American cutaneous leishmaniasis

Sodium stibogluconate is the mainstay of treatment for all forms of leishmaniasis. Therapy is associated with an increase in serum aminotransferases. In this study liver damage was assessed during treatment of American cutaneous leishmaniasis with sodium stibogluconate and also in a control group given aminosidine. In addition to standard liver function tests, acute hepatocellular damage was assessed by measuring plasma glutathione S-transferase B1 (GST), and hepatic metabolic capacity was assessed by a caffeine clearance (CCL) test, before, during and after treatment. Thirteen patients were treated; 5 received sodium stibogluconate, 6 received aminosidine and a further 2 patients received aminosidine followed by sodium stibogluconate. Treatment with sodium stibogluconate was associated with an increase in both alanine aminotransferase (ALT) and GST and a fall in the CCL, indicating both hepatocellular damage and functional impairment. Six weeks after treatment had stopped ALT and GST had returned to pre-treatment levels and the CCL remained depressed in only one patient. Patients given aminosidine did not show any evidence of liver damage. Sodium stibogluconate is associated with significant hepatocellular damage and hepatic functional impairment. However, this is rapidly reversible on drug withdrawal. We suggest that liver function is monitored throughout treatment and that patients with pre-existing liver disease receive alternative treatment.

Hormone-nuclear receptor interactions in health and disease. The iodothyronine deiodinases and 5'-deiodination

Two types of iodothyronine deiodinase (ID-I and ID-II) catalyse the 5'-deiodination of thyroxine (T4) to produce the biologically active triiodothyronine (T3). Under normal circumstances ID-I in liver and kidney provides the main source of T3 to the circulation, whilst ID-II is largely responsible for local T3 production in the CNS, brown adipose tissue and pituitary. In some circumstances ID-II in brown adipose tissue and ID-I in the thyroid may provide a significant source of plasma T3, and ID-I in the pituitary may be important for local T3 production in this gland. The IDs thus play a pivotal role in controlling the supply of T3 to the nuclear receptors. ID-I is a selenoenzyme and, although ID-II activity is reduced in selenium deficiency, this is a consequence of increased plasma T4 concentration, rather than ID-II activity being directly dependent on selenium. Changes in 5'-deiodination occur in a number of situations such as poor nutrition, illness, iodine and selenium deficiency, and drug therapy. In iodine deficiency these changes appear to have evolved to ensure that the plasma T3 level is maintained and also to provide the brain with a degree of protection from hypothyroxinaemia. Relatively little is known about the importance of selenium deficiency on thyroid function in humans but, in combination with iodine deficiency, selenium deficiency may prove to be a contributing factor in the pathogenesis of myxodematous cretinism. The changes that occur in ID-I and ID-II in illness produce abnormalities in thyroid function tests which, although of no direct clinical significance, may lead to interpretative problems.

Chemiluminescent third-generation assay (Amerlite TSH-30) of thyroid-stimulating hormone in serum or plasma assessed

We assessed the laboratory performance and clinical utility of a new commercial third-generation assay of thyroid-stimulating hormone (TSH), Amerlite TSH-30. The interassay CV was 6% at TSH concentrations of approximately 0.08 mIU/L, and the analytical and functional detection limits of the assay were 0.005 and 0.0125 mIU/L, respectively. Although the assay recovered approximately 96% of TSH International Reference Preparation (TSH-IRP) 80/558 added to serum samples, the endogenous TSH concentrations in basal samples were significantly lower than those found by using two other TSH assays; bias data obtained from thyroliberin stimulation tests suggested that the negative bias found with TSH-30 may be due to the heterogeneity of TSH in basal samples. TSH-30 completely discriminated hyperthyroid and hypothyroid patients from euthyroid ambulatory patients but also detected TSH (> 0.0125 mIU/L) in 3 of 46 untreated hyperthyroid patients. Compared with two second-generation assays, TSH-30 better discriminated between patients with subnormal TSH due to hyperthyroidism, thyroxine overreplacement, and nonthyroidal illness but there was still significant overlap between results for these groups.

Chemiluminescent third-generation assay (Amerlite TSH-30) of thyroid-stimulating hormone in serum or plasma assessed

We assessed the laboratory performance and clinical utility of a new commercial third-generation assay of thyroid-stimulating hormone (TSH), Amerlite TSH-30. The interassay CV was 6% at TSH concentrations of approximately 0.08 mIU/L, and the analytical and functional detection limits of the assay were 0.005 and 0.0125 mIU/L, respectively. Although the assay recovered approximately 96% of TSH International Reference Preparation (TSH-IRP) 80/558 added to serum samples, the endogenous TSH concentrations in basal samples were significantly lower than those found by using two other TSH assays; bias data obtained from thyroliberin stimulation tests suggested that the negative bias found with TSH-30 may be due to the heterogeneity of TSH in basal samples. TSH-30 completely discriminated hyperthyroid and hypothyroid patients from euthyroid ambulatory patients but also detected TSH (&gt; 0.0125 mIU/L) in 3 of 46 untreated hyperthyroid patients. Compared with two second-generation assays, TSH-30 better discriminated between patients with subnormal TSH due to hyperthyroidism, thyroxine overreplacement, and nonthyroidal illness but there was still significant overlap between results for these groups.

The role of thyroidal type-I iodothyronine deiodinase in tri-iodothyronine production by human and sheep thyrocytes in primary culture

We have studied the origin of tri-iodothyronine (T3) secreted by human and sheep thyrocytes in primary culture and also the expression of type-I thyroidal iodothyronine deiodinase (ID-I) in the thyroid and liver of man and various other animals. Inhibitors of ID-I reduced T3 secretion from human but not sheep thyrocytes. In contrast, inhibitors of de-novo thyroid hormone synthesis reduced both thyroxine (T4) and T3 production in sheep thyrocytes, but had no effect on the T3 secreted by human thyrocytes. Human thyrocytes did not produce T4 under the culture conditions used, although some endogenous T4 was present in the cells following their isolation. Although thyrotrophin (TSH) stimulated T3 production in both human and sheep thyrocytes, iodine in the form of potassium iodide was only essential for T3 and T4 production by the sheep cells. Although 125I from Na125I was incorporated into T3 and T4 in TSH-stimulated sheep thyrocytes, no 125I incorporation into T3 or T4 was detected in TSH-stimulated human thyrocytes. Using activity measurements and affinity labelling, ID-I was present in the livers of all species studied, but ID-I could not be detected in thyroid tissue from cattle, pigs, sheep, goats, rabbits, deer or llamas. In contrast, thyroid tissue from man, mice, guinea-pigs and rats had significant ID-I activity and expressed an affinity-labelled protein with a molecular mass of approximately 28.1 kDa on SDS-PAGE. These data show that under the culture conditions used, sheep thyrocytes produced T3 by de-novo synthesis, whilst human thyrocytes produced T3 by deiodination of endogenous T4. We conclude that thyroidal ID-I shows marked species difference in its expression and that, in those species which express the enzyme (man, mice, guinea-pigs and rats, in this study), it appears that it may make an important contribution to thyroidal T3 production.

Effects of combined iodine and selenium deficiency on thyroid hormone metabolism in rats

This paper compares the effects of combined iodine and selenium deficiency, of single deficiencies of these trace elements, and of no deficiency on thyroid hormone metabolism in rats. In rats deficient in both trace elements, thyroidal triiodothyronine (T3), thyroidal thyroxin (T4), thyroidal total iodine, hepatic T4, and plasma T4 were significantly lower, and plasma thyroid-stimulating hormone (TSH) and thyroid weight were significantly higher than in rats deficient in iodine alone. Plasma and hepatic T3 concentrations were similar in the dietary groups. Hepatic type I iodothyronine deiodinase (ID-I) activity was inhibited by selenium deficiency irrespective of the iodine status. Type II deiodinase (ID-II) activity in the brain was significantly higher and in pituitary, significantly lower in combined deficiency than in iodine deficiency alone. These data show that selenium can play an important role in determining the severity of the hypothyroidism associated with iodine deficiency.

Selenium deficiency, thyroid hormone metabolism, and thyroid hormone deiodinases

Much research into the functions of selenium in the cell has concentrated on its role in selenium-containing glutathione peroxidases. However, selenium was recently shown to be an essential component of type I iodothyronine 5'-deiodinase in rats, which converts thyroxin to the more biologically active hormone 3,5,3'-triiodothyronine. Thus, selenium-deficient rats have low tissue deiodinase activities and abnormal thyroid hormone metabolism. The discovery of this function for selenium in thyroid hormone metabolism has important implications for the interpretation of the effects of selenium deficiency, especially in individuals with an adequate vitamin E status.

The role of selenium in thyroid hormone metabolism and effects of selenium deficiency on thyroid hormone and iodine metabolism

Selenium deficiency impairs thyroid hormone metabolism by inhibiting the synthesis and activity of the iodothyronine deiodinases, which convert thyroxine (T4) to the more metabolically active 3,3'-5 triiodothyronine (T3). Hepatic type I iodothyronine deiodinase, identified in partially purified cell fractions using affinity labeling with [125I]N-bromoacetyl reverse triiodothyronine, is also labeled with 75Se by in vivo treatment of rats with 75Se-Na2SeO3. Thus, the type I iodothyronine 5'-deiodinase is a selenoenzyme. In rats, concurrent selenium and iodine deficiency produces greater increases in thyroid weight and plasma thyrotrophin than iodine deficiency alone. These results indicate that a concurrent selenium deficiency could be a major determinant of the severity of iodine deficiency.

Selenium deficiency and type II 5'-deiodinase regulation in the euthyroid and hypothyroid rat: evidence of a direct effect of thyroxine

Selenium deficiency in rats is characterized by elevated serum T4 and decreased serum T3 concentrations, and low liver type I (5'D-I) and brain type II (5'D-II) iodothyronine 5'-deiodinase activities. These findings are partially explained by the demonstration that type I 5'D is a selenoprotein; however, 5'D-II does not contain selenium. Since 5'D-II varies inversely with serum T4 concentrations, and serum T4 is elevated in selenium deficiency, the decreased cerebrocortical 5'D-II activity may be secondary to the increased serum T4 levels. To determine the mechanism(s) by which selenium influences 5'D-II activity, we examined the effects of altered selenium intake on brain 5'D-II levels and enzyme turnover in euthyroid and thyroidectomized rats. Rats were fed a selenium-supplemented or selenium-deficient diet for 5 weeks from weaning; half of the animals were also thyroidectomized 3 weeks before death. Selenium deficiency was confirmed by decreased liver and brain glutathione peroxidase activities. In euthyroid rats, selenium deficiency caused a 38% increase in serum T4, and 91% and 39% decreases in 5'D-I and 5'D-II, respectively, compared to those in selenium-supplemented rats. In the thyroidectomized hypothyroid rats, selenium deficiency caused a 60% decrease in 5'D-I, but had no effect on 5'D-II activity, fractional turnover of the enzyme, or the calculated enzyme synthesis rate. The lack of effect of selenium deficiency on 5'D-II levels in hypothyroid rats is consistent with the finding that 5'D-II is not a seleno-enzyme. Thus, the decrease in brain and pituitary 5'D-II activity in selenium-deficient euthyroid rats is due to the T4-dependent increase in the turnover of the enzyme polypeptide.

Aspartate Aminotransferase, Alanine Aminotransferase, and Glutathione Transferase in Plasma During and after Sedation by Low-Dose Isoflurane or Midazolam

To assess the effect of prolonged administration of midazolam or isoflurane on hepatocellular integrity, we measured the concentrations of glutathione transferase (EC 2.5.1.18) B1 subunit and the activities of alanine aminotransferase (ALT; EC 2.6.1.2) and aspartate aminotransferase (AST; EC 2.6.1.1) in 40 patients who required long-term sedation with low-dose midazolam or isoflurane. Blood samples were collected before and 24 h after the start of the sedation and 0, 24, 72, 120, and 172 h after the last dose. ALT and AST activities did not change appreciably, but the glutathione transferase B1 concentration decreased significantly (P less than 0.03) at all times studied. The patients who received isoflurane and those who received midazolam showed no significant differences in any of the enzyme tests. We conclude that long-term sedation with midazolam or isoflurane is unlikely to affect hepatocellular integrity.

Aspartate aminotransferase, alanine aminotransferase, and glutathione transferase in plasma during and after sedation by low-dose isoflurane or midazolam

To assess the effect of prolonged administration of midazolam or isoflurane on hepatocellular integrity, we measured the concentrations of glutathione transferase (EC 2.5.1.18) B1 subunit and the activities of alanine aminotransferase (ALT; EC 2.6.1.2) and aspartate aminotransferase (AST; EC 2.6.1.1) in 40 patients who required long-term sedation with low-dose midazolam or isoflurane. Blood samples were collected before and 24 h after the start of the sedation and 0, 24, 72, 120, and 172 h after the last dose. ALT and AST activities did not change appreciably, but the glutathione transferase B1 concentration decreased significantly (P less than 0.03) at all times studied. The patients who received isoflurane and those who received midazolam showed no significant differences in any of the enzyme tests. We conclude that long-term sedation with midazolam or isoflurane is unlikely to affect hepatocellular integrity.

The role of selenium in thyroid hormone metabolism and effects of selenium deficiency on thyroid hormone and iodine metabolism

Selenium deficiency impairs thyroid hormone metabolism by inhibiting the synthesis and activity of the iodothyronine deiodinases, which convert thyroxine (T4) to the more metabolically active 3,3'-5 triiodothyronine (T3). Hepatic type I iodothyronine deiodinase, identified in partially purified cell fractions using affinity labeling with [125I]N-bromoacetyl reverse triiodothyronine, is also labeled with 75Se by in vivo treatment of rats with 75Se-Na2SeO3. Thus, the type I iodothyronine 5'-deiodinase is a selenoenzyme. In rats, concurrent selenium and iodine deficiency produces greater increases in thyroid weight and plasma thyrotrophin than iodine deficiency alone. These results indicate that a concurrent selenium deficiency could be a major determinant of the severity of iodine deficiency.

Effect of selenium deficiency on hepatic type I 5-iodothyronine deiodinase activity and hepatic thyroid hormone levels in the rat

Selenium deficiency in rats for a period of up to 6 weeks inhibited both the production of 3,3',5-tri-iodothyronine (T3) from thyroxine (T4) (5'-deiodination) and also the catabolism of T3 to 3,3'-di-iodothyronine (5-deiodination) in liver homogenates. The hepatic stores of T3 were decreased by only 8% in selenium deficiency, despite the T3 production rate from T4 being only 7% of the rate found in selenium-supplemented rats. Hepatic glutathione S-transferase (GST) activity was increased in both hypothyroidism and selenium deficiency, but apparently by different mechanisms, since mRNA expression for this family of enzymes was lowered by hypothyroidism and increased in selenium deficiency. It is concluded that, since both T3 production and catabolism are inhibited by selenium deficiency, there is little change in hepatic T3 stores, and therefore the changes in the activity of certain hepatic enzymes, such as GST, that are found in selenium deficiency are not the result of tissue hypothyroidism.

Inter-relationships between selenium and thyroid hormone metabolism in the rat and man

Labelling of rat kidney microsomes in vitro with [125I]-bromoacetyl T4 produced two bands on SDS/PAGE with Mr of 55 kDa and 27.5 kDa representing protein disulphide isomerase and type I iodothyronine deiodinase (ID-I) respectively. The amount of the 55 kDa band was unchanged by selenium (Se) deficiency but the 27.5 kDa protein was markedly decreased in kidney microsomal fraction obtained from Se-deficient rats. Concurrent Se and iodine deficiency produced a significant increase in thyroid weight, plasma thyrotrophin (TSH) and a decrease in thyroidal iodine when compared with either single Se or iodine deficiency. These results suggest that ID-I is a selenoprotein and that Se deficiency can exacerbate the hypothyroidism observed in iodine deficiency. In man, blood glutathione peroxidase and blood Se levels were decreased in hyperthyroidism due to Graves' disease whilst normal levels of these analytes were found in patients treated for Graves' disease. These results suggest that thyroid status can affect Se balance rather than Se deficiency predisposes to Graves' disease.

Glutathione S-transferase mu locus: use of genotyping and phenotyping assays to assess association with lung cancer susceptibility

In mammals, the cytosolic glutathione S-transferases (GSTs; EC 2.5.1.18) are a supergene family comprised of four multigene families, named alpha, mu, pi and theta. In man, within the mu class gene family there is a gene (the GSTmu 1 locus) that is polymorphic and is only expressed in 50-55% of individuals. It has previously been reported, using trans-stilbene oxide (tSBO) as a specific substrate for the expressed phenotype, that smokers with the null phenotype had a greater susceptibility to lung cancer. In a subsequent study, it was shown that on Southern blot analyses of human DNAs using a GSTmu 1 cDNA probe a DNA fragment was absent in certain individuals. The absence of this band correlated with the tSBO null phenotype. In the present work, DNA clones derived from GST mu class genomic sequences were used as probes in Southern blot analyses and confirmed the correlation between the lack of a DNA fragment and the null phenotype; moreover in this case, using radioimmunoassay for the GST mu protein, these probes were then used in a genotyping assay to investigate further the association of GSTmu 1 polymorphism with susceptibility to lung cancer. It was found that in a control group of 225 individuals, of unknown smoking history, 42% lacked the restriction fragment and were homozygous null, and therefore 58% were either heterozygous or were homozygous normal. Among 228 lung cancer patients, which included all tumour types, a similar distribution occurred, namely 43% were homozygous and 57% were heterozygous or homozygous normal. If, however, the tumours were analysed by tumour type a small but significant positive correlation with the homozygous null genotype was seen in squamous carcinoma of the lung, and an apparently negative correlation with adenocarcinoma of the lung.

The clinical utility of a non-isotopic two-step assay (DELFIA) and an analogue radioimmunoassay (SimulTRAC) for free thyroxine compared

The analytical and diagnostic performance of a new non-isotopic, two-step immunoassay (DELFIA) for the measurement of free thyroxine (free T4) in plasma or serum has been compared with an established second generation analogue radioimmunoassay (SimulTRAC). Both methods had a good diagnostic specificity in pregnancy, thyroid clinic patients, and patients taking anticonvulsant drugs. In patients presenting to a general medical ward the diagnostic specificity of both methods was poor. Two samples appeared to contain substances which produced assay interference by DELFIA but not by SimulTRAC assays. When free T4 was measured by equilibrium dialysis a clear association between sample dilution and free T4 concentration was demonstrated in sick euthyroid patients. In contrast, using samples obtained from patients with known thyroid disease, free T4 was little influenced by sample dilution. The effects of sample dilution on free T4 measured by DELFIA were similar to those found using equilibrium dialysis. It would appear that free T4 measurements have a relatively poor diagnostic specificity in non-thyroidal illness irrespective of the method used.

Impairment of iodothyronine 5'-deiodinase activity in brown adipose tissue and its acute stimulation by cold in selenium deficiency

The activity of the type II iodothyronine 5'-deiodinase enzyme in brown adipose tissue has been examined in rats-fed a selenium-deficient diet. Iodothyronine 5'-deiodinase activity was threefold lower in brown adipose tissue of deficient rats than in control animals. The activity of glutathione peroxidase, a biochemical index of selenium deficiency, was also greatly decreased in deficient animals. Cytochrome oxidase activity in brown fat was, however, unaltered by selenium deficiency. Acute exposure to cold (4 degrees C for 18 h) resulted in a substantial increase in iodothyronine 5'-deiodinase activity in brown adipose tissue of control rats, but the stimulatory effect of cold was attenuated in selenium-deficient animals. These results support the concept that the iodothyronine 5'-deiodinases are selenium-dependent enzymes, and indicate that the thermogenic response to cold may be impaired in selenium deficiency.

Human glutathione S-transferases: radioimmunoassay studies on the expression of alpha-, mu- and pi-class isoenzymes in developing lung and kidney

The developmental expression of the alpha-, mu- and pi-class glutathione S-transferases has been defined in human lung and kidney using radioimmunoassay, immunohistochemistry and column chromatography. Expression of alpha-class enzymes increased significantly after about 40 weeks gestation in kidney but not lung, while expression of mu isoenzymes was continuous throughout development in both tissues. Expression of the pi isoenzyme fell during in utero ontogeny in lung, the pattern of down-regulation being similar to that previously observed in liver. There was no change in the expression of this isoenzyme in kidney. Comparison of the expression of the glutathione S-transferases in developing lung, kidney and liver shows some common patterns of expression suggesting these genes are under similar regulatory control.