Serum Thyroid Hormone Binding Proteins Research Articles

Clinical recognition and evaluation of patients with inherited serum thyroid hormone-binding protein mutations.

There are three important thyroid hormone-binding proteins in human serum, thyroxine-binding globulin, transthyretin, and albumin. Genetic variation in these proteins can lead to altered thyroid hormone binding and abnormalities in serum tests of thyroid hormone. Importantly, patients harboring these mutations are euthyroid; thus, the recognition of these conditions is crucial to prevent unnecessary repeated testing and treatment. This article provides an updated overview of serum thyroid hormone transport biology and reviews the underlying genetic alterations, clinical presentation, and appropriate evaluation of patients with suspected mutations in serum thyroid hormone-binding proteins.

Transport of thyroid hormone in brain.

Thyroid hormone (TH) transport into the brain is not only pivotal for development and differentiation, but also for maintenance and regulation of adult central nervous system (CNS) function. In this review, we highlight some key factors and structures regulating TH uptake and distribution. Serum TH binding proteins play a major role for the availability of TH since only free hormone concentrations may dictate cellular uptake. One of these proteins, transthyretin is also present in the cerebrospinal fluid (CSF) after being secreted by the choroid plexus. Entry routes into the brain like the blood–brain-barrier (BBB) and the blood–CSF-barrier will be explicated regarding fetal and adult status. Recently identified TH transmembrane transporters (THTT) like monocarboxylate transporter 8 (Mct8) play a major role in uptake of TH across the BBB but as well in transport between cells like astrocytes and neurons within the brain. Species differences in transporter expression will be presented and interference of TH transport by endogenous and exogenous compounds including endocrine disruptors and drugs will be discussed.

Species-dependent effects of the phenolic herbicide ioxynil with potential thyroid hormone disrupting activity: modulation of its cellular uptake and activity by interaction with serum thyroid hormone-binding proteins

Ioxynil, a phenolic herbicide, is known to exert thyroid hormone (TH) disrupting activity by interfering with TH-binding to plasma proteins and a step of the cellular TH-signaling pathway in restricted animal species. However, comparative studies are still lacking on the TH disruption. We investigated the interaction of [125I]ioxynil with serum proteins from rainbow trout, bullfrog, chicken, pig, rat, and mouse, using native polyacrylamide gel electrophoresis. Candidate ioxynil-binding proteins, which included lipoproteins, albumin and transthyretin (TTR), differed among the vertebrates tested. Rainbow trout and bullfrog tadpole serum had the lowest binding activity for ioxynil, whereas the eutherian serum had the highest binding activity. The cellular uptake of, and response to, ioxynil were suppressed by rat serum greater than by tadpole serum. The cellular uptake of [125I]ioxynil competed strongly with phenols with a single ring, but not with THs. Our results suggested that ioxynil interferes with TH homeostasis in plasma and with a step of cellular TH-signaling pathway other than TH-uptake system, in a species-specific manner.

Synthesis of Thyroid Hormone Binding Proteins Transthyretin and Albumin by Human Trophoblast

Mechanisms regulating materno-fetal transfer of thyroid hormone are not well understood. Modulation of trophoblast type 3 iodothyronine deiodinase (D3) may play an important role. The objective of this study was to investigate trophoblast thyroid hormone binding proteins that may modulate interactions between D3 and T4. Placentas were obtained by informed consent from women delivering normal infants by repeat cesarean section at 38-40 wk gestation. T4 and T3 binding was examined in human placenta. Serum thyroid hormone binding proteins were identified by Western blotting, and their mRNA was examined by RT-PCR. Presence of these proteins in trophoblast was determined by immunocytochemistry and immunofluorescence. Cytosol was progressively purified to reveal additional thyroid hormone binding proteins that were identified by matrix-assisted laser desorption/ionization time of flight mass spectrometry. Effects of mefenamic acid on placental deiodination were examined by HPLC. We detected high-affinity T4 and T3 binding in human placental cytosol. All three major serum-binding proteins, T4 binding globulin (TBG), transthyretin (TTR), and albumin, were present in cytosol. TTR mRNA and albumin mRNA were detected in human placenta, and TTR and albumin were identified histochemically in syncytiotrophoblasts. Neither TBG mRNA nor TBG was detected, suggesting that plasma TBG had contaminated the cytosol preparation. Low-affinity thyroid hormone binding proteins alpha-1-antitrypsin and alpha-1-acid glycoprotein were also identified. Addition of mefenamic acid, a potent inhibitor of thyroid hormone binding, to placental cytosol significantly enhanced deiodination of T4 by D3. Placenta produces a series of thyroid hormone binding proteins that may modify thyroid hormone deiodination and materno-fetal thyroid hormone transport.

Selective consumption of thyroxine-binding blobulin during cardiac bypass surgery

A study of serum thyroid hormone binding proteins and thyroid hormone concentrations during and after coronary artery bypass graft (CABG) surgery shows a marked difference in the response of thyroxine binding globulin (TBG) and transthyretin (TTR). The effects of CABG on TBG and TTR were compared in 32 patients during the day of surgery. In a few of these patients, additional determinations were performed to 5 days. When corrected for dilution, TTR concentrations decline gradually after surgery, with no significant decrease over the first 24 hours. In contrast, a rapid decrease of TBG to a mean level of 60% of the preoperative control at 12 hours after the start of surgery appears to account for the concomitant decrease of serum T4. The rate at which the TBG concentration decreased far exceeds the reported fractional clearance of TBG and therefore implies accelerated consumption rather than inhibition of production. TBG is a member of the serine protease inhibitor (SERPIN) superfamily. We propose that its rapid consumption is due to protease cleavage at inflammatory sites. This may explain the previously observed accumulation of thyroxine iodine at such sites.

Clinical aspects and diagnosis of thyroid hormone transport protein anomalies.

The thyroid hormones T4 (thyroxine) and T3 (3, 5, 3′-triiodothyronine) circulate in the blood reversibly bound to proteins. The principal transport proteins are: T4-binding globulin (TBG), transthyretin (TTR), formerly called T4-binding prealbumin or TBPA, and albumin. These proteins differ widely in concentration and their affinity for the iodothyronines (Oppenheimer 1968; Robbins and Rall 1960; Robbins et al. 1986); collectively, they carry more than 99% of T3 and T4 in serum. Other thyroid hormone-binding proteins in serum are lipoproteins (Benvenga 1989; Benvenga et al. 1988,1989,1990), for example the 27 K protein, an apolipoprotein A-I, and the apolipoprotein B-100, which are both synthesized in the liver (Grimaldi et al. 1986; Bartalena et al. 1986a, b). Less important binding proteins are immunoglobulin M (Sviridov et al. 1991) and, in thyroid autoimmune disorders, thyroid hormone autoantibodies of the IgG class (Benvenga et al. 1987).

Familial dysalbuminaemic hyperthyroxinaemia and inherited partial TBG deficiency: first report

Abnormalities of the serum thyroid hormone binding proteins are not uncommon but, when properly assessed, they do not present diagnostic difficulties. In contrast, the presence of two inherited defects of thyroid hormone transport, of the type presented in the family described here, may cause a major problem in diagnosis and has not been described previously. All conventional thyroid function tests were carried out. In addition, thyroid hormone binding to serum proteins was assessed by agarose gel electrophoresis, and thyroxine binding globulin by immunoassays and by immunodiffusion. The affinity of TBG for thyroxine and its maximal binding capacity were assessed by Scatchard analysis. Tests carried out on 22 members of the family revealed familial dysalbuminaemic hyperthyroxinaemia in 10 family subjects. All five living siblings of the propositus had familial dysalbuminaemic hyperthyroxinaemia and two tested transmitted this trait to their children and grandchildren. This was not the case with the propositus. Partial thyroxine binding globulin deficiency only, inherited presumably from the propositus' mother, was found in two family members. Both thyroxine binding globulin deficiency and familial dysalbuminaemic hyperthyroxinaemia were detected in the propositus and in his male nephew, masking the typical laboratory abnormalities associated with each of these defects. Coexistence of two inherited defects of thyroid hormone transport proteins produce atypical thyroid function test abnormalities, which can be misinterpreted as thyroid hormone dysfunction.

The microheterogeneity of rat TBG

Isoelectric focusing (IEF) of native sera from immature or adult rats and of purified or partially purified rat serum thyroid hormone-binding proteins, demonstrates that rat TBG is a microheterogeneous protein. Autoradiography and radioactivity scans of the IEF plates show that it consists of at least four main isoforms, with bands at pH 4.35, 4.45, 4.5 and 4.55. This pattern is remarkably similar to that reported for human TBG. This is the first demonstration of the polymorphism of this recently discovered major binding protein of the rat.

Do residue levels of polychlorinated biphenyls (PCBs) in human blood produce mild hypothyroidism?

PCBs are nearly ubiquitous environmental contaminants, occurring in most human adipose tissue and blood samples. It has recently been recognized that PCBs and related compounds share important structural properties with thyroid hormones and can bind thyroid hormone binding proteins. It is reasonable that such specific binding interactions can modulate the distribution of these compounds in the body and alter hormone-protein interactions that are responsible for the maintenance of normal thyroid status. Most of the available evidence indicates that the levels of free thyroid hormones in plasma are a reflection of the maintenance of normal thyroid status in any individual. A theoretical model for the transport of thyroid hormones in blood has been extended to estimate the modulating effects of PCBs on free thyroid hormones. Using conservative assumptions based on experimental data, our calculations indicate that PCB concentrations normally found in humans can effect significant increases in free thyroxine levels in serum by competing with serum thyroid hormone binding proteins. Experimental data are discussed which support the proposal that antagonist binding of PCBs to thyroid hormone binding proteins in serum could produce varying degrees of hypothyroidism. The biological result is compatible with the "equilibrium hypothesis" in which thyroid hormone redistributes between specific and nonspecific binding proteins rather than emphasizing the importance of free hormone as the active moiety.

Seasonal changes in serum thyroid hormone binding proteins in the woodchuck (Marmota monax).

To evaluate the role of serum T4- and T3-binding proteins in the elevation of serum T4 and T3 concentrations in the woodchuck in the fall and winter, blood was collected from woodchucks during the four seasons of the year (seasonal study) and from 2-week fasted woodchucks in the summer (fasting study) and the serum concentrations of total T4 and T3 were measured. The distribution of [125I]4 and [125I]T3 tracers among the serum binding proteins and the serum T4-binding globulin (TBG) binding capacity for T4 was determined by polyacrylamide gel electrophoresis. Plasma concentrations of both T4 and T3 were highest in the winter. The major T4-binding protein in the woodchuck is TBG. There was an increase in both [125I]T4 and [125I]T3 tracer binding to serum TBG in fall and winter, and TBG binding capacity for T4 was 2-fold higher in winter than in summer. There were increases in TBG binding and in the TBG T4 binding capacity in the 2-week starved animals. The increased binding of T4 and T3 by TBG in the fall and winter may be partially responsible for the increased serum concentrations of T4 and T3 in the fall and winter in the woodchuck, a time when secretion of T4 and T3 by the thyroid gland is very low. This may be facilitated by the low or absent food consumption at these times of the year.

Antibodies to nuclear thyroid hormone-binding proteins: Antibody characterization and immunofluorescent localization

To further investigate the mechanism by which thyroid hormones regulate target cell function, we have prepared and partially characterized antibodies to highly purified nuclear thyroid hormone-binding proteins (NTBP). NTBPs were prepared from bovine liver nuclear extracts by bio-specific elution from an affinity gel containing immobilized 3,5,3′-triiodo- l-thyronine (T 3). Antibodies (Ab) raised to NTBP in BALB/c mice were assayed for Ab-NTBP complex formation on HPLC TSK SW3000 molecular exclusion gels and found to be species-specific and non-cross-reactive with serum thyroid hormone-binding proteins. Most of the antibody activity was directed against two fractions of molecular weight (MW) 89000 and 53000, which were associated with thyroxine (T 4)-binding activity. The 89000 D T 4-binding activity was shifted to a higher MW complex when incubated with specific antibody. Indirect immunofluorescence showed antibody activity against discrete, clumped chromatin sites, nuclear envelope and plasma membrane in hepatocytes. Intense fluorescence was also observed in the cells lining the hepatic sinusoids and in the cytoskeleton of bovine aortic endothelial cells in culture. The data suggest that thyroid hormone target cells contain extranuclear loci that share antigenic sites with NTBP and may also represent specific NTBP-like sites of thyroid hormone binding.

Low serum thyroxine and high serum triiodothyronine in nephrotic rats: etiology and implications for bioavailability of protein-bound hormone.

Traditionally, it has been thought that the bioavailable fraction of circulating serum hormones, i.e. that which is available for cellular uptake and is physiologically active, is limited to the free (nonprotein bound) hormone. However, recent evidence, based on acute organ uptake of labeled hormone, suggests that the amount of hormone which is bioavailable in vivo may exceed that which is calculated to be free in vitro. To explore the bioavailability of circulating protein-bound thyroid hormones under steady state conditions in vivo, we altered serum thyroid hormone-binding proteins in rats by inducing nephrotic syndrome with puromycin aminonucleoside. Nephrotic rats (serum albumin, 1.1 g/dl) were found to have a marked reduction in serum T4 [2.1 +/- 0.2 (SEM) vs. 6.5 +/- 0.3 microgram/dl; P less than 0.01] and an elevation of serum T3 [141 +/- 8 vs. 51 +/- 2 ng/dl; P less than 0.01]. Estimated T4 production rate was normal in nephrotic rats, and the 3- to 4-fold increase in T4 MCR appeared to account for the marked reduction in serum T4. By contrast, increased serum T3 levels in nephrotic rats reflected both a reduction (55%) in T3 MCR and an increased rate of peripheral conversion of T4 to T3. A circulating inhibitor of T4 binding to serum proteins appeared to be present in nephrotic rats. The changes in the various serum components of thyroid hormone [T4-binding prealbumin (TBPA)-bound, albumin-bound, free] produced by nephrotic syndrome were compared with the corresponding changes in indices of thyroid hormone bioavailability (MCR, urinary excretion, hepatic content, TSH suppression, single pass extraction by liver). These comparisons suggested that nephrotic syndrome results in increased bioavailability of circulating T4 and decreased bioavailability of circulating T3. The bioavailable fraction of circulating T3 in vivo seemed to include both free T3 and that which is albumin bound in vitro. The bioavailable fraction of circulating T4 resembled free T4 more than non-TBPA-bound T4 (= albumin bound + free), although a nephrosis-induced increase in bioavailability of TBPA-bound T4 was also possible. We conclude that nephrotic rats have low serum T4, which is related to accelerated T4 clearance, and high serum T3, which is related both to decreased T3 clearance and increased peripheral conversion of T4 to T3. Under steady state conditions in vivo, bioavailable circulating T3 appears to include both free T3 and the T3 that is bound to albumin in vitro.

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.

Dose related effects of betamethasone on iodothyronines and thyroid hormone-binding proteins in serum.

Earlier studies of the effect of glucocorticoids on thyroid function tests have given different results. This may be due to the fact that varying corticosteroid doses have been used. The present study examined four groups of healthy volunteers, who were given betamethasone in increasing doses (1.5, 3.0, 4.5 and 6.0 mg daily) for 5 days. Definite dose-response relations were found for rT3. The results obtained explain previous variable results. Oral corticosteroids also affected the concentrations of the thyroid hormone-binding proteins but no clear-cut dose-response relation were observed.

The effect of calorie restriction on serum thyroid hormone binding proteins and free hormone in obese patients.

In ten obese euthyroid subjects the concentration of thyroxine (T4), tri-iodothyronine (T3) and reverse T3 (rT3) were assayed in serum and of T4 and T3 in urine before and after 2 weeks of 2.1 MJ (500 Kcal) per day diet. Mean serum T4 was unchanged, while T3 decreased and rT3 increased. Urinary excretion of both T4 and T3 decreased after diet. In six subjects the concentrations of serum thyroxine binding globulin (TBG), thyroxine binding prealbumin (TBPA) and albumin were measured before and after diet. Free T4 and T3 were calculated using a formula based on measured concentration of hormones and their binding proteins. Calorie restriction resulted in a significant decrease in TBG and a greater decrease in TBPA, while albumin was unchanged. Calculation of free hormones showed a fall in absolute free T3 and rise in percentage free T3, but a rise in both absolute and per cent free T4. Our data indicate a selective effect of calorie restriction on the concentrations of TBPA and to a lesser extent TBG and confirm previous reports of altered peripheral monodeiodination of T4. Decreased urinary excretion of T4, despite a calculated increase of free T4 in serum, suggests intrarenal metabolic adjustment, the mechanism of which awaits elucidation.