The kinetics of distribution between plasma and liver of 131-I-labeled L-thyroxine in man: observations of subjects with normal and decreased serum thyroxine-binding globulin

A minimal human physiologically based kinetic model of thyroid hormones and chemical disruption of plasma thyroid hormone binding proteins

The thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3), are under homeostatic control by the hypothalamic-pituitary-thyroid axis and plasma TH binding proteins (THBPs), including thyroxine-binding globulin (TBG), transthyretin (TTR), and albumin (ALB). THBPs buffer free THs against transient perturbations and distribute THs to tissues. TH binding to THBPs can be perturbed by structurally similar endocrine-disrupting chemicals (EDCs), yet their impact on circulating THs and health risks are unclear. In the present study, we constructed a human physiologically based kinetic (PBK) model of THs and explored the potential effects of THBP-binding EDCs. The model describes the production, distribution, and metabolism of T4 and T3 in the Body Blood, Thyroid, Liver, and Rest-of-Body (RB) compartments, with explicit consideration of the reversible binding between plasma THs and THBPs. Rigorously parameterized based on literature data, the model recapitulates key quantitative TH kinetic characteristics, including free, THBP-bound, and total T4 and T3 concentrations, TH productions, distributions, metabolisms, clearance, and half-lives. Moreover, the model produces several novel findings. (1) The blood-tissue TH exchanges are fast and nearly at equilibrium especially for T4, providing intrinsic robustness against local metabolic perturbations. (2) Tissue influx is limiting for transient tissue uptake of THs when THBPs are present. (3) Continuous exposure to THBP-binding EDCs does not alter the steady-state levels of THs, while intermittent daily exposure to rapidly metabolized TBG-binding EDCs can cause much greater disruptions to plasma and tissue THs. In summary, the PBK model provides novel insights into TH kinetics and the homeostatic roles of THBPs against thyroid disrupting chemicals.

Intrathyroidal feedforward and feedback network regulating thyroid hormone synthesis and secretion

Thyroid hormones (THs), including T4 and T3, are produced and released by the thyroid gland under the stimulation of thyroid-stimulating hormone (TSH). The homeostasis of THs is regulated via the coordination of the hypothalamic-pituitary-thyroid axis, plasma binding proteins, and local metabolism in tissues. TH synthesis and secretion in the thyrocytes-containing thyroid follicles are exquisitely regulated by an elaborate molecular network comprising enzymes, transporters, signal transduction machineries, and transcription factors. In this article, we synthesized the relevant literature, organized and dissected the complex intrathyroidal regulatory network into structures amenable to functional interpretation and systems-level modeling. Multiple intertwined feedforward and feedback motifs were identified and described, centering around the transcriptional and posttranslational regulations involved in TH synthesis and secretion, including those underpinning the Wolff-Chaikoff and Plummer effects and thyroglobulin-mediated feedback regulation. A more thorough characterization of the intrathyroidal network from a systems biology perspective, including its topology, constituent network motifs, and nonlinear quantitative properties, can help us to better understand and predict the thyroidal dynamics in response to physiological signals, therapeutic interventions, and environmental disruptions.

The Retinol Circulating Complex Releases Hormonal Ligands During Acute Stress Disorders

Intensive care workers actively participate in very hot debates aiming at defining the true metabolic, hormonal and nutritional requirements of critically ill patients, the contributory roles played by thyroid and retinoid ligands being largely underestimated. The present article makes up for redressing the balance on behalf of these last hormonal compounds. The retinol circulating complex is transported in the bloodstream in the form of a trimolecular edifice made up of transthyretin (TTR), retinol-binding protein (RBP) and its retinol ligand. TTR reflects the size of the lean body mass (LBM) and is one of the 3 carrier-proteins of thyroid hormones whereas RBP is the sole conveyor of retinol in human plasma. In acute inflammatory disorders, both TTR and RBP analytes experience abrupt cytokine-induced suppressed hepatic synthesis whose amplitude is dependent on the duration and severity of the inflammatory burden. The steep drop in TTR and RBP plasma values releases thyroxine and retinol ligands in their physiologically active forms, creating free pools estimated to be 10-20 times larger than those described in healthy subjects. The peak endocrine influence is reached on day 4 and the freed ligands undergo instant cellular overconsumption and urinary leakage of unmetabolized fractions. As a result of these transient hyperthyroid and hyperretinoid states, helpful stimulatory and/or inhibitory processes are set in motion, operating as second frontlines fine-tuning the impulses primarily initiated by cytokines. The data explain why preexisting protein malnutrition, as assessed by subnormal LBM and TTR values, impairs the development of appropriate recovery processes in critically ill patients. These findings have survival implications, emphasizing the need for more adapted therapeutic strategies in intensive care units.

Plasma Transthyretin as a Biomarker of Lean Body Mass and Catabolic States

Plasma transthyretin (TTR) is a plasma protein secreted by the liver that circulates bound to retinol-binding protein 4 (RBP4) and its retinol ligand. TTR is the sole plasma protein that reveals from birth to old age evolutionary patterns that are closely superimposable to those of lean body mass (LBM) and thus works as the best surrogate analyte of LBM. Any alteration in energy-to-protein balance impairs the accretion of LBM reserves and causes early depression of TTR production. In acute inflammatory states, cytokines induce urinary leakage of nitrogenous catabolites, deplete LBM stores, and cause an abrupt decrease in TTR and RBP4 concentrations. As a result, thyroxine and retinol ligands are released in free form, creating a second frontline that strengthens that primarily initiated by cytokines. Malnutrition and inflammation thus keep in check TTR and RBP4 secretion by using distinct and unrelated physiologic pathways, but they operate in concert to downregulate LBM stores. The biomarker complex integrates these opposite mechanisms at any time and thereby constitutes an ideally suited tool to determine residual LBM resources still available for metabolic responses, hence predicting outcomes of the most interwoven disease conditions.

Estimation of thyroxine distribution in man

A group of 13 normal subjects were evaluated for their extrathyroidal thyroxine distribution. The method employed the measurement of the acute plasma disappearance of a thyroxine-(131)I tracer and its concomitant uptake into the liver and forearm. The analysis of these parameters allowed the theoretical construction of a four compartmental mathematical model system comprised of the plasma, extracellular fluid, hepatic, and extrahepatic thyroxine pools. The results of this analysis revealed that the exchange of thyroxine from the plasma into the hepatic and extrahepatic cellular fluid spaces appeared, in general, to be rapid, while the uptake into the extrahepatic tissues was relatively slow. The calculated distribution of thyroxine at equilibrium was estimated to be 14% in liver, 34% in extrahepatic tissues, and 26% each in the plasma and extracellular fluid pools in this group of normal subjects.

Streptokinase treatment of deep venous thrombosis of the lower extremity. Clinical, phlebographic and plethysmographic evaluation of early and late results

The investigation comprises 19 patients with acute deep vein thrombosis of the leg treated with streptokinase. The acute clinical symptoms rapidly subsided in 15 patients. Phlebography, performed immediately after treatment, revealed complete thrombus regression in 7 cases, restoration of venous flow but remnants of thrombi in 4 and no effect in 7. One woman was not evaluated radiologically due to pregnancy. The phlebographic restoration seemed to be correlated to the duration of the thrombotic symptoms. Follow-up examinations 6-50 months after the thrombotic incident demonstrated normal phlebograms in 8 patients, all of whom were also free from post-thrombotic symptoms. Venous occlusion plethysmography confirmed that these patients had a normal venous outflow capacity and valvular function in the relevant limbs. By contrast, the remaining 11 patients, with more or less extensive remnants of thrombi at the follow-up phlebography, were found to have plethysmographic signs of venous obstruction and sometimes also valvular insufficiency. The results indicate that thrombolytic treatment is able to give a complete and lasting anatomical and functional restitution of the deep veins after an acute thrombosis in the leg, provided that treatment is induced early enough.

The stressful condition as a nutritionally dependent adaptive dichotomy

The injured body manifests a cascade of cytokine-induced metabolic events aimed at developing defense mechanisms and tissue repair. Rising concentrations of counterregulatory hormones work in concert with cytokines to generate overall insulin and insulin-like growth factor 1 (IGF-1), postreceptor resistance and energy requirements grounded on lipid dependency. Salient features are self-sustained hypercortisolemia persisting as long as cytokines are oversecreted and down-regulation of the hypothalamo-pituitary-thyroid axis stabilized at low basal levels. Inhibition of thyroxine 5'-deiodinating activity (5'-DA) accounts for the depressed T3 values associated with the sparing of both N and energy-consuming processes. Both the liver and damaged territories adapt to stressful signals along up-regulated pathways disconnected from the central and peripheral control systems. Cytokines stimulate liver 5'-DA and suppress the synthesis of transthyretin (TTR), causing the drop of retinol-binding protein (RBP) and the leakage of increased amounts of T4 and retinol in free form. TTR and RBP thus work as prohormonal reservoirs of precursor molecules which need to be converted into bioactive derivatives (T3 and retinoic acids) to reach transcriptional efficiency. The converting steps (5'-DA and cellular retinol-binding protein-I) are activated by T4 and retinol, themselves operating as limiting factors of positive feedback loops. Healthy adults with normal macrophage functioning and liver parenchymal integrity, who submitted to a stress of medium severity, are characterized by TTR-RBP plasma levels reduced by half and an estimated ten-fold increase in free ligand disposal to target cells during the days ensuing injury. This transient hyperthyroid and hyperretinoid climate creates a second defense line strengthening and fine-tuning the effects primarily initiated by cytokines. The suicidal behavior of thyroxine-binding globulin (TBG), corticosteroid-binding globulin (CBG), and IGFBP-3 allows the occurrence of peak endocrine and mitogenic influences at the site of inflammation. The production rate of TTR by the liver is the main determinant of both the hepatic release and blood transport of holoRBP, which explains why poor nutritional status concomitantly impairs thyroid- and retinoid-dependent acute-phase responses, hindering the stressed body to appropriately face the survival crisis. The prognostic significance of low TT4 blood levels may be assigned to the exhaustion of extrathyroidal hormonal pools normally stored in liver and plasma but markedly shrunken in protein-depleted states. These data offer new insights into the mechanisms whereby preexisting malnutrition and stressful complications are interrelated, emphasizing the pivotal role played by TTR in that context.

Hormone-specific alterations of T4, T3, and reverse T3 metabolism with recent ethanol abstinence in humans

Effects of recent alcoholic withdrawal on thyroxine (T4), 3,5,3'-triiodothyronine (T3), and reverse T3 (rT3) metabolism were determined by serum tracer kinetic studies in recently abstinent alcoholics without overt hepatocellular injury or caloric deprivation. Data were compared with those of normal subjects using a three-pool model, with rapidly and slowly equilibrating pools exchanging with serum. Significant differences included 1) reduced serum total rT3 levels (to 69% of normal) and rT3 degradation rates (to 61%); 2) increased rT3 binding in rapidly (to 557%) but reduced binding in slowly (to 13%) equilibrating tissues, with opposite effects on rT3 fractional transfer rates to serum from rapidly (to 7.5%) and slowly equilibrating sites (to 669%); 3) increased T4 fractional transfer rates from serum to rapidly equilibrating tissues (to 122%); and 4) increased T4 binding to both rapidly (to 195%) and slowly (to 190%) equilibrating tissues. T3 kinetics were not significantly altered. Thus recently abstinent alcoholics have hormone-specific alterations of T4, T3, and rT3 transfer, distribution, and metabolism distinct from other nonthyroidal illnesses or caloric deprivation. Furthermore, these data indicate separate transfer processes for T4, T3, and rT3 from serum to tissue sites and hormone-specific tissue binding characteristics in humans in vivo.

T4 uptake into the perfused rat liver and liver T4 uptake in humans are inhibited by fructose

Recently, we described a two-pool model for 3,5,3'-triiodothyronine uptake and metabolism in the isolated perfused rat liver. Here, we applied this model to investigate transmembrane thyroxine (T4) transport and its possible ATP dependence in vivo. These studies are performed in perfused rat livers during perfusion with or without fructose in the medium, as it has been shown that intracellular ATP is decreased after fructose loading. Furthermore, we studied serum T4 tracer disappearance curves in four human subjects before and after intravenous fructose loading. In the perfused rat liver, we found a decrease in liver ATP concentration and a decrease in medium T4 disappearance and T4 uptake in the liver pool after fructose. Furthermore, it was shown that, when corrected for differences in the medium free hormone concentration, only transport to the metabolizing liver pool was decreased after fructose perfusion, whereas uptake in the nonmetabolizing pool was unaffected. Disposal, corrected for differences in transport into the metabolizing pool, was also not affected after fructose. In the human studies, intravenous fructose administration induced a rise in serum lactic acid and uric acid, indicating a decrease in liver ATP. This was observed concomitant with a decrease in serum tracer T4 disappearance during the first 3 h after fructose administration. These results suggest ATP dependence of transport of iodothyronines into the liver in vivo and show that, in the rat liver and in humans, uptake of T4 may be regulated by intracellular energy stores; in this way the tissue uptake process may affect intracellular metabolism and bioavailability of thyroid hormone.

Reduced tissue thyroid hormone levels in fatal illness

Patients with severe nonthyroidal illnesses (NTIs) frequently have decreased serum concentrations of triiodothyronine (T3) and less often of thyroxine (T4) without clear evidence of hypothyroidism. To determine whether T3 and T4 levels are also reduced in the tissues, we analyzed autopsy samples from 12 patients dying of NTI and 10 previously healthy individuals dying suddenly from trauma. Mean serum T3, T4, and free T4 index values were lower by 79%, 71%, and 49%, respectively, in the NTI group than in controls, but serum thyrotropin (TSH) values did not differ significantly. Mean T3 concentrations in cerebral cortex, hypothalamus, pituitary, liver, kidney, and lung were lower in the NTI group than in controls by 43% to 76%, but mean values in heart and skeletal muscle did not differ significantly between the groups. The mean liver T4 concentration was 66% lower in the NTI group, but mean T4 concentrations in the cerebral cortex were similar in the two groups. These results indicate that many tissues may be deficient in thyroid hormones in patients with fatal NTI, although the severity of the reduction in thyroid hormone concentrations may vary from one organ to another.

Thyroid gland size and function in patients with cirrhosis of the liver

Thyroid dysfunction has long been reported in patients with liver disease, but limited information is available on thyroid gland size in cirrhosis. Most studies were carried out on small, selected series of patients, and no study has measured thyroid volume in relation to the etiology of liver disease. Thyroid volume was measured at ultrasound in 118 consecutive patients with cirrhosis of different etiology and 48 healthy subjects matched for age and sex. No subjects had evidence of overt thyroid disease. The mean volume was increased by 17% (from 16.0 [SD 5.2] ml in controls to 18.8 [7.6] in cirrhosis; P less than 0.025), and thyroid enlargement (antero-posterior diameter greater than 20 mm) was present in 38% of cases, in the presence of hormone values indicative of low-T3 syndrome. No significant differences in thyroid gland size were observed in relation to the extent of liver dysfunction or to the etiology of liver disease. The prevalence of thyroid nodules was similar in controls and in patients with cirrhosis. In only 8% of cases were laboratory values indicative of hypothyroidism, with low free triiodothyronine and raised thyroid-stimulating hormone levels; in these patients thyroid volume was decreased on average by 26%. This was mainly the case with patients with primary biliary and alcoholic cirrhosis. The largest mean thyroid volume was observed in patients with HBsAg + ve postnecrotic cirrhosis, whose thyroid volume was increased on average by 37%, and 53% of subjects had thyroid enlargement. This finding raises the question of a possible direct involvement of the thyroid in hepatitis B virus infection.

Modeling thyroxine transport to liver: rejection of the "enhanced dissociation" hypothesis as applied to thyroxine

Three models for the hepatic uptake of thyoxine (T4) from human plasma were considered: 1) uptake occurs exclusively via the pool of free T4 after spontaneous dissociation of T4-plasma-protein complexes, 2) uptake occurs primarily via the pool of bound T4 by the interaction of one or more binding proteins with the cell-surface membrane, and 3) uptake occurs primarily by "enhanced dissociation" of T4 from one or more of its binding proteins within the sinusoids. Each of these models was examined in relation to well-accepted unidirectional uptake and steady-state kinetics data that indicate that 1) between 4 and 24% of the T4 in normal human serum is taken up unidirectionally by the liver in a single pass, and 2) the in vivo disposal rate of T4 is unaffected by primary changes in the plasma concentration of thyroid hormone-binding globulin. Both analytical and numerical techniques were used. The first two models were found to be compatible with both the steady-state kinetics data and the unidirectional uptake data, given certain assumptions in each of the models. Although theoretically distinguishable on the basis of unidirectional uptake data, uncertainty over the true uptake (influx) rate constant for free T4 prevented resolution between these two models. In contrast, the third model, that of enhanced dissociation [W. M. Pardridge, Am. J. Physiol. 252 (Endocrinol. Metab. 15): E157-E164, 1987], was found, as currently formulated with respect to T4, to be incompatible with both the steady-state kinetics data and the unidirectional uptake data.(ABSTRACT TRUNCATED AT 250 WORDS)

Uptake of thyroxine by the perfused rat liver: implications for the free hormone hypothesis

To investigate the mechanism by which thyroxine (T4) in plasma enters hepatic cells, we measured the rate constants for uptake of free T4 by the perfused rat liver and for dissociation of T4 from its plasma binding proteins. Quantitative autoradiography of liver lobules after perfusion with [125I]T4 indicated an apparent rat constant for removal of free T4 from the sinusoids of at least 1.1 +/- 0.2 s-1. Single-pass extraction of T4 from human serum was 10.6 +/- 1.7% at physiological flow rates (1 ml.min-1.g liver-1). Rate constants for dissociation of T4 from plasma binding proteins at 37 degrees C (determined by rapid filtration) were 0.017 +/- 0.002 s-1 for human thyroid hormone-binding globulin, 0.080 +/- 0.015 s-1 for human thyroid hormone-binding prealbumin, and greater than 0.5 s-1 for human albumin. To investigate the factors that determine the concentration of T4 within hepatic cells, we analyzed the above data together with data reported in the literature on the equilibrium-binding constants and the rate constant for cellular metabolism of T4. Analysis of all of these data using a previously published mathematical model leads to the following conclusions for the physiological state: 1) metabolism, not uptake, is rate limiting to removal of T4 from plasma by the liver; 2) binding equilibrium is present in the intrahepatic plasma; 3) intracellular T4 is in equilibrium with the free T4 pool in plasma (and maintenance of this equilibrium may be an important function of plasma thyroid hormone-binding proteins); and 4) the concentration of T4 within the liver is proportional to the concentration of free T4 in the plasma. Our data do not allow us to determine definitively whether hepatic uptake of T4 occurs only from the free T4 pool in plasma or also from the protein-bound pool by interaction of one or more of the binding proteins with the liver cell. However, mathematical analysis indicates that this distinction is irrelevant to steady-state intracellular hormone concentrations when equilibrium exists between the plasma and cytosolic pools of hormone.

Plasma protein-mediated transport of steroid and thyroid hormones

The free hormone or free drug hypotheses have traditionally assumed that the concentration of cellular exchangeable hormone (i.e., the pool that drives cellular hormone or drug receptor occupancy) can be reliably estimated by in vitro measurements of unbound hormone concentrations. The corollary of this view is that the large reservoir of bound hormone in blood is passively transported by plasma proteins and is physiologically inactive. However, when these assumptions are subjected to direct empiric testing with either in vivo or perfused organ techniques, it is found that the large pool of bound hormone in blood is operationally available for transport across microcirculatory barriers without the plasma protein, per se, significantly exiting the plasma compartment. This process is believed to involve a mechanism of enhanced dissociation of hormone or drug from the plasma protein caused by transient conformational changes about the ligand binding site within the microcirculation: The biochemical mechanism of the interaction of the plasma protein with the surface of the microcirculation may involve receptor, charged selectivity, or local inhibitor mechanisms.

The diagnostic challenge of euthyroid hyperthyroxinemia

In euthyroid hyperthyroxinemia high levels of thyroxine (T4) may be either transient or persistent, associated with high, normal, or low levels of tri-iodothyronine (T3). Euthyroid hyperthyroxinemia may occur: as a response to abnormal plasma binding (thyroxine binding globulin, albumin, prealbumin, or autoantibodies), because of hormone resistance, after exposure to drugs such as amiodarone, cholecystographic contrast agents, or propranolol, during acute psychiatric illness or stress, and in hyperemesis gravidarum. In some instances the cause of persistent hyperthyroxinemia still remains obscure. No single investigation (including free hormone measurement and the response of thyrotropin to its releasing hormone) can distinguish all of these entities from true hyperthyroidism. Hence, re-evaluation in cases of diagnostic uncertainty should begin with clinical reassessment. Techniques are now available to identify easily some causes of euthyroid hyperthyroxinemia, allowing us to recognise patients and relatives who are at risk of inappropriate treatment. Because measurement of serum T4 remains the key investigation for diagnosis of thyroid dysfunction, it is important to appreciate the full range of conditions that compromise its specificity.

Peripheral serum thyroxine, triiodothyronine and reverse triiodothyronine kinetics in the low thyroxine state of acute nonthyroidal illnesses. A noncompartmental analysis

The low thyroxine (T(4)) state of acute critical nonthyroidal illnesses is characterized by marked decreases in serum total T(4) and triiodothyronine (T(3)) with elevated reverse T(3) (rT(3)) values. To better define the mechanisms responsible for these alterations, serum kinetic disappearance studies of labeled T(4), T(3), or rT(3) were determined in 16 patients with the low T(4) state and compared with 27 euthyroid controls and a single subject with near absence of thyroxine-binding globulin. Marked increases in the serum free fractions of T(4) (0.070+/-0.007%, normal [nl] 0.0315+/-0.0014, P < 0.001), T(3) (0.696+/-0.065%, nl 0.310+/-0.034, P < 0.001), and rT(3) (0.404+/-0.051%, nl 0.133+/-0.007, P < 0.001) by equilibrium dialysis were observed indicating impaired serum binding. Noncompartmental analysis of the kinetic data revealed an increased metabolic clearance rate (MCR) of T(4) (1.69+/-0.22 liter/d per m(2), nl 0.73+/-0.05, P < 0.001) and fractional catabolic rate (FCR) (32.8+/-2.6%, nl 12.0+/-0.8, P < 0.001), analogous to the euthyroid subject with low thyroxine-binding globulin. However, the reduced rate of T(4) exit from the serum (Kii) (15.2+/-4.6 d(-1), nl 28.4+/-3.9, P < 0.001) indicated an impairment of extravascular T(4) binding that exceeded the serum binding defect. This defect did not apparently reduce the availability of T(4) to sites of disposal as reflected by the increased fractional disposal rate of T(4) (0.101+/-0.018 d(-1), nl 0.021+/-0.003, P < 0.001). The decreased serum T(3) binding was associated with the expected increases in MCR (18.80+/-2.22 liter/d per m(2), nl 13.74+/-1.30, P < 0.05) and total volume of distribution (26.55+/-4.80 liter/m(2), nl 13.10+/-2.54, P < 0.01). However, the unaltered Kii suggested an extravascular binding impairment comparable to that found in serum. The decreased T(3) production rate (6.34+/-0.53 mug/d per m(2), nl 23.47+/-2.12, P < 0.005) appeared to result from reduced peripheral T(4) to T(3) conversion because of decreased 5'-deiodination rather than from a decreased T(4) availability. This view was supported by the normality of the rT(3) production rate. The normal Kii values for rT(3) indicated a comparable defect in serum and extravascular rT(3) binding. The reduced MCR (25.05+/-6.03 liter/d per m(2), nl 59.96+/-8.56, P < 0.005) and FCR (191.0+/-41.19%, nl 628.0+/-199.0, P < 0.02) for rT(3) are compatible with an impairment of the rT(3) deiodination rate. These alterations in thyroid hormones indices and kinetic parameters for T(4), T(3), and rT(3) in the low T(4) state of acute nonthyroidal illnesses can be accounted for by: (a) decreased binding of T(4), T(3), and rT(3) to vascular and extravascular sites with a proportionately greater impairment of extravascular T(4) binding, and (b) impaired 5'-deiodination activity affecting both T(4) and rT(3) metabolism.

Inhibition of hepatic binding of thyroxine by cholecystographic agents

Subsequent to studies indicating that cholecystographic agents and sulfobromophthalein (BSP) inhibit uptake of thyroxine (T(4)) by rat liver slices, the effect of such compounds on hepatic storage of T(4) in man has been examined. After intravenous administration of [(125)I]T(4) to five normal subjects, hepatic radioactivity, estimated by external gamma counting, rose to a peak in approximately 4 h and then declined in parallel with serum radioactivity. When a 6-g dose of the cholecystographic agent, tyropanoate (Bilopaque), was administered orally 3 d later, estimated hepatic extravascular radioactivity fell 50-60% within 4 h and then rose toward the pretyropanoate value. Concomitant with the fall in hepatic radioactivity, serum radioactivity rose 57-70%, as did stable T(4) levels in serum, suggesting that hormone discharged from the liver entered the serum. Both uptake of T(4) and discharge by tyropanoate were much less in two patients with liver disease. Ipodate (Oragrafin), 12 g orally in two subjects, caused much smaller changes in hepatic and serum radioactivity. However, ipodate also caused a doubling of the percent free T(4) in the serum as judged by equilibrium dialysis, and the ratio of hepatic radioactivity to free [(125)I]T(4) in serum declined markedly after ipodate, similar to the fall in hepatic:serum (125)I ratios after tyropanoate. BSP, 20 mg/kg i.v. in three subjects, caused a smaller change; the decline in hepatic T4 content averaged 19%. We conclude that these organic anions, tyropanoate, ipodate, and BSP, all can displace T(4) from the liver. The results imply a link between T(4) transport and organic anion transport, and indicate a mechanism for altering hepatic T(4) content in man that could be the site of physiologic regulation or of disease. A method is described whereby analysis of the change in hepatic and plasma radioactivity after tyropanoate could be employed to estimate hepatic T(4) content in diverse clinical circumstances.

Regulation of the conversion of thyroxine to triiodothyronine in the perfused rat liver

This study was undertaken to determine what factors control the conversion of thyroxine (T(4)) to triiodothyronine (T(3)) in rat liver under conditions approximating those found in vivo. Conversion of T(4) to T(3) was studied in the isolated perfused rat liver, a preparation in which the cellular and structural integrity is maintained and that can perform most of the physiologic functions of the liver. The perfused liver readily extracted T(4) from perfusion medium and converted it to T(3). Production of T(3) by the perfused liver was a function of the size of the liver, the uptake of T(4) by the liver, and the presence of T(4)-5'-deiodinase activity. Production of T(3) was increased by increasing the uptake of T(4) by liver, which could be accomplished by increasing the liver size, by increasing the perfusate T(4) concentration, or by decreasing the perfusate albumin concentration. These changes occurred without altering the conversion of T(4) to T(3). The liver had a large capacity for extracting T(4) and for T(4)-5'-deiodination to T(3), which was not saturated at a T(4) concentration of 60 mug/dl. Production of T(3) was decreased by inhibiting hepatic T(4)-5'-deiodinase with propylthiouracil, which decreased T(3) production by decreasing the conversion of T(4) to T(3). Propylthiouracil did not alter hepatic T(4) uptake. Fasting resulted in a progressive decrease in hepatic T(4) uptake to 42% of control levels by the 3rd d of fasting; this was accompanied by a proportionate decrease in T(3) production. The rate of conversion of T(4) to T(3) did not change during fasting. When T(4) uptake in 2-d-fasted rat livers was raised to levels found in fed rats by increasing the perfusate T(4) concentration from 10 to 30 mug/dl, T(3) production returned to normal. Again, no change in the rate of conversion of T(4) to T(3) was observed. These results indicate that the decreased hepatic T(3) production during fasting primarily results from decreased hepatic uptake of T(4), rather than from changes in T(4)-5'-deiodinase activity. Thus, these studies have delineated a new mechanism that functions independently of enzyme quantity or activity whereby production of T(3) from T(4) is regulated.

Thyroid hormone kinetics: improved method for quantitative separation and measurement of the various radioiodothyronine injection

An accurate and reproducible method for measurement of radioactive species in blood after in vivo injection of labelled iodothyronines is described. By extraction with high-affinity antisera, radioactive reverse T3 and T3 are separated from serum quantitatively. Radioiodide is quantitatively separated from radio-thyronine species and serum proteins by Sephadex G50 filtration. The residual mixture of radio-T4 and iodoprotein is quantitatively resolved by ion-exchange adsorption. Minimal misclassification of radiospecies occurs, and can be corrected for. Mean recoveries of various radiospecies added to serum were: radioiodide 98.9%, radio-rT3 87.6%, radio-L-T3 94.5%, radio-T4 98.0% and radioiodoprotein 94.5%. The performance of the method is superior to that of chemical methods such as trichloracetic acid precipitation, ion-exchange or alkaline Sephadex extraction, and chromatographic separation.

Factors influencing the early plasma disappearance rate and liver uptake of thyroxine

The early disappearance from plasma (T 1/2) of i.v. administered thyroxine (T4) labelled with 131I was studied in order to find a simple biochemical parameter with which it could be correlated in health and disease. The radioactivity of the liver was also measured by surface counting to determine the time for the peak liver count rate. The T 1/2 for the 131I-T4 showed a correlation of borderline significance with T4 in plasma but a good correlation was found with T4-binding globulin (TBG) in plasma and an even better with the T3-test. The T3-test showed a significant correlation with TBG but when the T4-binding capacity of prealbumin (TBPA) was taken into account the correlation was even better indicating that the result of the T3-test was not only dependent on the TBG concentration in plasma but also on the TBPA. The T 1/2 correlated best with the time for the liver peak count rate. The study supports the hypothesis that in the early distribution phase of T4, the free binding sites extracellularly and the available intracellular binding locations compete for T4 until a state of equilibrium is reached.

Reduced peripheral conversion of thyroxine to triiodothyronine in patients with hepatic cirrhosis

The role of liver in the peripheral conversion of thyroxine (T4) to triiodothyronine (T3) was studied in normal subjects and patients with alcoholic liver disease by measurement of thyrotrophin (TSH) and total and free T4 and T3 in randomand serial serum samples. Also, T4 to T3 conversion rates and T3 disposal rates were compared by noncompartmental analysis. While the mean total serum T4 values were similar for the two groups, 8.6 and 8.1 mug/kl, the mean free T4 value was significantly higher in the cirrhotic patients (3.3 ng/dl) than in the normal subjects (2.1 ng/dl, P less than 0.001). The mean serum T3 value, 85 ng/dl, was significantly reduced in the hepatic patients as compared to a mean serum T3 value of 126 ng/dl in the normal subjects (P less than 0.001), while the free T3 value was 0.28 ng/dl in both groups. The reduction of the serum total and free T3 values were closely correlated with the degree of liver damage, as indicated by elevation of serum bilirubin (r equal -0.547) and reduction of serum albumin (r equal 0.471). The mean serum TSH level was 3.1 muU/ml in the normals and 7.1 muU/ml in the cirrhotic aptients ( less than 0.001). 15% of the hepatic patients had serum TSH values above 10 muU/ml, which, however, did not correlate with any of the four liver function tests studied. Serial blood sampling from two convalescing patients with alcoholic hepatitis showed a gradual normalization of serum TSH and T3 levels as the liver function improved. After oral T4 administration, 0.25 mg/day for 10 days, three of four cirrhotic patients studied failed to raise their serum T3 values. The mean T4 to T3 conversion rate of seven normal subjects was 35.7%. The mean T4 to T3 conversion rate of four cirrhotic patients studied was significantly reduced to 15.6% (P less than 0.001). The mean disposal rates of T4 and T3 of the normal subjects were 114 and 34 mug/day, respectively. The ratio of T4 disposal to T3 disposal was 3.5. In contrast, the mean T4 disposal rate, 82 mug/day, and the mean T3 disposal rate, 10 mug/day, were both reduced in the cirrhotic patients. Their ratio of T4 disposal to T3 disposal was 7.9. These findings suggest that impairment of T4 conversion in patients with advanced hepatic cirrhosis may lead to reduced T3 production and lowered serum T3 level. Therefore, the liver is one of the major sites of T4 conversion to T3.

Estimation of thyroxine and triiodothyronine distribution and of the conversion rate of thyroxine to triiodothyronine in man

Studies on peripheral metabolism of simultaneously administered 125-I-labeled L-thyroxine ([125-I]T4) and 131-I labeled L-trilodothyronine ([131-I]T3) were performed in five normal subjects, in four patients with untreated hypothyroidism, and in 3 hypothyroid patients made euthyroid by the administration of T4. The fractional turnover rate (lambda 03) of thyroid hormones irreversibly leaving the site of degradation and the volumes of pool 1 (serum V1) of pool (interstitial fluid, V2), and of pool 3 (all tissues, V3)were obtained by using a three-compartment analysis. In addition to the turnover studies, the ratios for the in vivo T4 to T3 conversion were determined by paper chromatographic study in sera obtained 4, 7, and 10 daysafter the injection. The rate (K12) of the extrathyroidal conversion of T4 to T3 was also estimated by the compartment analysis. The T3 distribution volume (V3) of pool 3, in which T3 is utilized and degraded, was about 60% of totaldistribution volume (V=V1+V2+V3) in normal subjects, whereas only about 25% of the extrathyroidal T4 pool was in the intracellular compartment, indicating that T3 is predominantly an intracellular hormone..

Relative rates of transcapillary movement of free thyroxine, protein-bound thyroxine, thyroxine-binding proteins, and albumin

The rate of appearance of labeled thyroxine (T4) and albumin in lymph from various areas after simultaneous i.v. injection of the labeled substances in conscious ambulatory sheep has been used to estimate the relative rates of transcapillary movement of stable T4 and albumin. Labeled T4 appeared in hepatic lymph at the same rate as albumin. In intestinal and leg lymph, labeled T4 appeared eight and four times as rapidly as albumin indicating that T4 crosses capillaries in these areas independently of and much more rapidly than albumin and other proteins having similar distribution kinetics. The lymph:plasma ratios for all the T4-binding proteins including albumin were very similar in any one area showing that the relative fractional rates of transcapillary movement of these proteins were very similar. Therefore in extrahepatic areas, transcapillary movement of T4 in the protein-bound form was quantitatively much less important than in the free form. The findings support earlier views, recently questioned, that free T4 is of considerable physiological significance.

Study of four new kindreds with inherited thyroxine-binding globulin abnormalities. Possible mutations of a single gene locus

Five families with inherited thyroxine-binding globulin (TBG) abnormalities were studied. On the basis of serum thyroxine (T(4))- binding capacity of TBG in affected males, three family types were identified: TBG deficiency, low TBG, and high TBG capacity. In all families evidence for X-linked inheritance was obtained and in one family all criteria establishing this mode of inheritance were met. Only females were heterozygous, exhibiting values intermediate between affected males and normals. Overlap in heterozygotes was most commonly encountered in families with low TBG. QUANTITATIVE VARIATION IN THE SERUM CONCENTRATION OF FUNCTIONALLY NORMAL TBG WAS DEMONSTRATED BY: (a) failure of serum from TBG-deficient subjects to react with anti-TBG antibodies; (b) normal kinetics of T(4) and triiodothyronine-binding to TBG in sera from subjects with low TBG and high TBG capacity; (c) concordance of estimates of TBG concentration by T(4) saturation and by immunological methods; and (d) normal rate of heat inactivation of TBG. No abnormalities in serum transport of cortisol, testosterone, aldosterone, or thyroxine bound to prealbumin could be detected. These observations suggest that all the TBG abnormalities thus far observed reflect mutations at a single X-linked locus involved in the control of TBG synthesis.

Evidence for enhanced cellular uptake and binding of thyroxine in vivo during acute infection with Diplococcus pneumoniae

Previous work has demonstrated that acute pneumococcal infections in man and in the rhesus monkey are accompanied by accelerated metabolic disposal of L-thyroxine (T(4)). In order to study the influence of acute pneumococcal infection on the kinetics of hormone distribution, the early cellular uptake of T(4) (CT(4)), reflecting the net effect of plasma and cellular binding factors, was assessed in rhesus monkeys from the differences in instantaneous distribution volumes of T(4)-(131)I and albumin-(125)I during the first 60 min after their simultaneous injection. Hepatic and renal uptakes of (131)I were also determined. Plasma binding of T(4) was assessed by measuring the per cent of free T(4) (% FT(4)) in serum. Six monkeys were studied 12 hr (INF-12) and seven 24 hr (INF-24) after intravenous inoculation with Diplococcus pneumoniae; seven controls were inoculated with a heat-killed culture. CT(4) at 60 min as per cent administered dose was 31.5 +/-2.0 (mean +/-SE) in INF-12 and 33.0+/-0.8 in INF-24, values significantly greater than control (22.4+/-1.3). By contrast, mean% FT(4) was identical in control and INF-12 (0.028 +/-0.002 and 0.028 +/-0.001) and variably increased in INF-24 (0.034 +/-0.003). Thus, in the infected monkeys CT(4) and% FT(4) were not significantly correlated. The increased CT(4) in the infected monkeys could not be ascribed to an increase in vascular permeability and did not correlate with the magnitude of fever. Although the increased CT(4) could not be accounted for by increased hepatic or renal uptake of hormone, hepatic and renal T(4) spaces were increased, results consistent with increased binding by these tissues. Our data indicate that the cellular uptake of T(4) is increased early in acute pneumococcal infection and suggest that this results from a primary enhancement of cell-associated binding factors for T(4).

Effects of norethandrolone on the transport and peripheral metabolism of thyroxine in patients lacking thyroxine-binding globulin. Observations on the physiological role of thyroxine-binding prealbumin

Studies of the effect of norethandrolone on the transport and peripheral metabolism of thyroxine were carried out in four patients lacking thyroxine-binding globulin. Before norethandrolone administration, values for serum protein-bound iodine (PBI) were decreased (1.8 +/-0.5 mug/100 ml) and the proportion of free thyroxine increased (0.036 +/-0.008%). As a result, values for the absolute concentration of free thyroxine iodine were at the lower end of the normal range (0.63 +/-0.12 mmug/100 ml). During the control thyroxine-turnover study, the thyroxine distribution space was strikingly increased (18.2 +/-7.9 liters) and the fractional rate of thyroxine turnover moderately increased (17.1 +/-11.3%/day), as compared to the expected mean values for normal subjects. Therefore, calculated values for the daily rate of thyroxine clearance were increased even more, ranging between 255 and 500% of normal values. However, owing to the low PBI in these patients, the daily disposal of thyroxine iodine was similar to that expected in normals on the basis of age and weight. During the administration of norethandrolone, the thyroxine-binding capacity of the thyroxine-binding prealbumin increased strikingly in all patients, values averaging 162% of those found during the control period. This increase was associated with a highly significant increase in PBI (133% of control values) and a small but significant decrease in the proportion of free thyroxine, resulting in no significant change in the absolute concentration of free thyroxine iodine. In all four patients, administration of norethandrolone was associated with a pronounced decrease in the thyroxine distribution space to values which averaged 69% of those found during the control period. Values for the fractional rate of thyroxine turnover increased slightly. As a result, thyroxine-clearance rate decreased in all patients. Owing to the reciprocal changes in clearance rate and PBI, no significant change in total daily thyroxine disposal was observed. The present studies reveal that when the thyroxine-binding prealbumin is increased in patients lacking thyroxine-binding globulin, several indices of peripheral thyroxine transport and metabolism are altered. However, these changes were small, even in the absence of thyroxine-binding globulin. It is suggested, therefore, that the effect of changes in thyroxine-binding prealbumin would be even smaller in individuals in whom thyroxine-binding globulin is present.

The distribution kinetics of triiodothyronine: studies of euthyroid subjects with decreased plasma thyroxine-binding globulin and patients with Graves' disease

The kinetics of distribution of 3,3',5-triiodo-L-thyronine (T(3)) have been studied employing both a single-injection and a continuous infusion of T(3-) (131)I. External monitoring of radioactivity in the liver during the infusion permitted estimation of the hepatic distribution volume (V(H)) and the one-way hepatic clearance (C(H)) of the hormone. Among 10 euthyroid control subjects, V(H) averaged 2.07 liters +/-0.50 (SD), and the mean value for C(H), 231 ml of plasma per min (+/-64). In three euthyroid men whose plasma showed decreased binding capacity by thyroxine-binding globulin (TBG) abnormally high V(H) and C(H) values were found, the increase in C(H) being proportional to the decrease in binding activity by plasma proteins. Among all 13 subjects, there was a high correlation (+ 0.86) between C(H) and the proportion of free hormone in plasma, measured in vitro. In four patients with hyperthyroid Graves' disease V(H) ranged from 3.2 to 4.2 liters and C(H) was elevated in every case, averaging 989 ml/min. The increase in C(H) in this group was out of proportion to the elevation of free hormone fraction in plasma. Seven patients who were either euthyroid or hypothyroid after treatment of Graves' disease showed a slight but significant increase in C(H) compared with the euthyroid controls without Graves' disease. The percentage of free hormone in the plasma of the treated group was normal or low and therefore could not explain the persistent elevation in unidirectional hepatic clearance observed. The rate of accumulation of labeled T(3) in the tissues of the thigh during the interval from 10 to 60 min of the sustaining infusion of tracer was slow compared to the rate of equilibration in the liver and did not differ significantly among the various groups studied. These latter findings suggest that in slowly equilibrating tissues such as the thigh the kinetics of T(3) distribution are relatively insensitive to alterations in hormone-binding activity by plasma proteins.

The effect of diphenylhydantoin on thyroxine metabolism in man

The effect of 5,5'-diphenylhydantoin on thyroxine metabolism was examined in five normal volunteers. Intravenous injection of radiothyroxine was followed by a 10-12 day control and subsequent 9-14 day treatment periods. During oral administration of diphenylhydantoin, plasma thyroxine concentration decreased to about 80% of its pretreatment level and the plasma radiothyroxine disappearance rate increased a maximum of 20% over control estimates. These changes were a result of increases in both urinary and fecal excretion of radioisotope.A minimum plasma thyroxine was apparent after 10-12 days of diphenylhydantoin administration. In two of the subjects, treatment was sufficiently prolonged to achieve this new steady state. In these subjects, the decrease in total body thyroxine was balanced by the increase in the fractional turnover rate. As a result, absolute thyroxine degradation during diphenylhydantoin administration was unchanged from the pretreatment values. Plasma ultrafiltration was used to estimate the free thyroxine fraction at regular intervals during the control and treatment periods. During diphenylhydantoin treatment, there was little or no change in this fraction and therefore, absolute free thyroxine decreased. Thyroxine-binding globulin and thyroxine-binding prealbumin capacities remained constant. These results indicate that thyroxine degradation can proceed at a normal rate in subjects receiving diphenylhydantoin despite decreases in plasma free thyroxine concentration. If free thyroxine is the only portion of the hormone available for cellular utilization, then free thyroxine clearance must be increased in these subjects. This increase in clearance could represent either a direct stimulation of peripheral thyroxine metabolism by diphenylhydantoin, or it could reflect the response of intrinsic regulatory systems to a diphenylhydantoin-mediated displacement of thyroxine from thyroxine-binding globulin. Whatever the mechanism for this effect, a decreased free thyroxine value in patients receiving diphenylhydantoin may not imply hypothyroidism.

Differences in primary cellular factors influencing the metabolism and distribution of 3,5,3'-L-triiodothyronine and L-thyroxine

Administration of phenobarbital, which acts exclusively on cellular sites, results in an augmentation of the liver/plasma concentration ratio of L-thyroxine (T4) in rats but no change in the liver/plasma concentration ratio of L-triiodothyronine (T3). Whereas phenobarbital stimulates the fecal clearance rate both of T3 and T4, it increases the deiodinative clearance rate of T4 only. These findings suggest basic differences in the cellular metabolism of T3 and T4. Further evidence pointing to cellular differences was obtained from a comparison of the distribution and metabolism of these hormones with appropriate corrections for the effect of differential plasma binding. The percentage of total exchangeable cellular T4 within the liver (28.5) is significantly greater than the corresponding percentage of exchangeable cellular T3 within this organ (12.3). Extrahepatic tissues bind T3 twice as firmly as T4. The cellular metabolic clearance rate (= free hormone clearance rate) of T3 exceeds that of T4 by a factor 1.8 in the rat. The corresponding ratio in man, 2.4, was determined by noncompartmental analysis of turnover studies in four individuals after the simultaneous injection of T4-(125)I and T3-(131)I. The greater cellular metabolic clearance rate of T3 both in rat and man may be related to the higher specific hormonal potency of this iodothyronine.

The permanence of tissue binding of thyroxine in the rat

1. Male rats were given radiothyroxine and after 24 hr, approximately half the blood volume was replaced by exchange transfusion.2. Plasma concentration of radiothyroxine 2 hr after exchange averaged 81.1% of the value before exchange. This compared with a predicted mean of 97.1% assuming a free and rapid exchange of thyroxine throughout the thyroxine distribution space. The difference between the observed and expected value was highly significant.3. The effective exchangeable pool of thyroxine was found to be approximately twice the plasma volume.4. From these and other observations it is suggested that the liver and plasma constitute most of the exchangeable pool of thyroxine.5. This experiment supports a model of thyroxine metabolism which envisages essentially permanent binding of thyroxine by tissues other than the liver.

The effects of an acute load of thyroxine on the transport and peripheral metabolism of triiodothyronine in man

In order to examine the question of whether thyroxine-binding globulin (TBG) influences significantly the peripheral metabolism of 3,3',5-triiodo-L-thyronine (T(3)) in vivo, paired studies of the effects of a large intravenous load of L-thyroxine (T(4)) on the kinetics of (131)I-labeled T(3) metabolism were carried out in five normal subjects. After the T(4) load, both the early distributive loss of labeled T(3) from serum and the volume of T(3) distribution, observed after distribution equilibrium had been attained, were greatly increased. These alterations were consistent with those to be expected from displacement of T(3) from its extracellular binding sites. After the T(4) load, however, the fractional rate of T(3) turnover was decreased. This finding is ascribed either to competition between T(3) and T(4) for common intracellular pathways of degradation or excretion or to displacement of T(3) from sites of more rapid to sites of less rapid metabolism. These effects of alterations in the binding activity of TBG on the peripheral metabolism of T(3), together with those previously reported by others, are consistent with the interpretation that T(3) is significantly bound by TBG in vivo. However, it is suggested that the effects of alterations in the T(3)-TBG binding interaction on the metabolism of T(3) are obscured by alterations in the extracellular-cellular partitioning of T(4) that would result from concurrent alterations in T(4)-binding by TBG.

Albumin synthesis in cirrhotic subjects with ascites studied with carbonate-14C

The synthesis of serum albumin was measured in 19 patients with cirrhosis of the liver and ascites. Carbonate-(14)C was used to label the guanido carbon of arginine in albumin.18 of the patients had the diagnosis of cirrhosis confirmed by biopsy and/or by the presence of esophageal varices. Seven patients with albumin synthesis rates of 42-105 mg/kg per day demonstrated the lowest serum cholesterol esters and highest serum glutamic oxalacetic transaminase (SGOT) levels, while the seven patients whose albumin synthesis rates ranged from 203-378 mg/kg per day, had significantly higher cholesterol ester levels and significantly lower values for SGOT. The serum albumin levels were equally depressed in all patients. In patients with cirrhosis and ascites albumin production was found to be normal or elevated in seven of the 19 subjects, and only markedly depressed in seven patients, in spite of the fact that the serum albumin level was depressed in all patients.

Studies of peripheral thyroxine distribution in thyrotoxicosis and hypothyroidism

Compartmental analysis of the peripheral distribution of labeled thyroxine was applied to various groups of subjects with thyrotoxicosis and hypothyroidism. It was observed that the hepatic incorporation of thyroxine was augmented in subjects with Graves' disease when compared to non-Graves' disease control groups at all levels of thyroid function. Decreased values of hepatic incorporation occurred in primary hypothyroid subjects. These lowered values were not acutely corrected by elevation of the serum thyroxine level, but were observed to be rectified after several months' therapy with exogenous thyroid hormone. These alterations of the hepatic thyroxine-(131)I incorporation were independently verified by direct quantitative liver scintiscan determinations. Employing a dual thyroxine tracer system, we were able to demonstrate that during the early phases of equilibration of a tracer dose of thyroxine, alterations in the rate of deiodination were observed to be present in the various thyroid disease states. Increased deiodination rates were found in subjects with Graves' disease and the reverse was noted in patients with primary hypothyroidism. Kinetic analysis of thyroxine compartmental distribution during this early phase of equilibration of a labeled thyroxine tracer indicated that the primary tissue uptake occurred in the liver. These findings supported the contention that the amount of labeled thyroxine incorporated in the liver may be directly related to the deiodination rate of thyroxine by that organ. The pathogenetic basis of these alterations is presently unknown.

Thyroid hormone transport in the serum of patients with thyrotoxic Graves' disease before and after treatment

Multiple indices of thyroid hormone binding have been studied in sera obtained from patients with thyrotoxic Graves' disease, before and after treatment, and in sera from a group of carefully matched normal controls. Specimens from patients with thyrotoxicosis displayed a decrease in the thyroxine (T(4))-binding capacities of T(4)-binding globulin (TBG) and T(4)-binding prealbumin (TBPA), an increase in serum protein-bound iodine (PBI), and an increase in both the proportion and absolute concentration of free T(4). In addition, a smaller than normal proportion of (131)I-labeled T(3) was associated with TBG during filter paper electrophoresis. After treatment of thyroxicosis, the only residual abnormality detected was a very slight persistent decrease in the T(4)-binding capacity of TBPA, which did not appear to influence the overall thyroid hormone-plasma protein interaction significantly, regardless of whether this was assessed under basal conditions or after enrichment of specimens with stable T(4). It is concluded that the persistent abnormalities in the peripheral metabolism of T(4), previously reported to occur in some patients with treated Graves' disease, probably do not stem from residual abnormalities in the transport of T(4) in the plasma but must arise from abnormalities in T(4) accumulation or metabolism within the tissues themselves.