Thyroid Hormone Binding Inhibitor Research Articles

The mechanism of thyroid hormone abnormalities in patients with diabetes mellitus

In order to clarify the mechanism of impaired thyroid hormone levels in patients with diabetes mellitus, thyroid hormone, thyroid hormone binding inhibitor (THBI), inhibitor of extrathyroidal conversion of T4 to T3 (IEC) and free fatty acid (FFA) were examined. In addition, TRH test was performed on 9 diabetic patients showing poor control of plasma glucose before and after glycemic control. Before glycemic control, fasting plasma glucose and HbA1c were significantly higher than after glycemic control (P < 0.05). T3 and the T3/T4 ratio significantly increased and rT3 significantly decreased after glycemic control (P < 0.05). THBI index and plasma FFA level significantly decreased and %T3 production (IEC) significantly increased after glycemic control (P < 0.05). The response of TSH to TRH significantly increased after glycemic control. In conclusion, (1) the presence of THBI, (2) the presence of IEC, and (3) dysfunction of the hypothalamo-hypophysial-thyroid axis are considered to be involved in abnormal thyroid function in diabetic patients.

Plasma free fatty acids, inhibitor of extrathyroidal conversion of T4 to T3 and thyroid hormone binding inhibitor in patients with various nonthyroidal illnesses.

In order to clarify the role of free fatty acid (FFA) in thyroid hormone abnormalities in patients with nonthyroidal illness, thyroid function, FFA, inhibitor of extrathyroidal conversion of T4 to T3 (IEC) and thyroid hormone binding inhibitor (THBI) were studied in 99 patients with various nonthyroidal illnesses including diabetes mellitus (DM) (n = 35), liver cirrhosis (LC) (n = 33), chronic obstructive pulmonary disease (COPD) (n = 17) and chronic heart failure (CHF) (n = 14). Patients were divided into three groups based on the level of serum T3: Group I (T3 < 50 ng/dl), Group II (50 < or = T3 < 80) and Group III (80 < or = T3). Serum T4, FT3 and the T3/T4 ratio decreased significantly in the order Group III, Group II and Group I (Group III > II > I). The plasma FFA level was 0.91 +/- 0.12 mmol/l in Group I (P < 0.05, vs. Group III), 0.65 +/- 0.06 in Group II and 0.54 +/- 0.04 in Group III, respectively. The incidence of positive IEC was 80.0% in Group I (P < 0.05, vs. Group III), 53.7% in Group II (P < 0.05, vs. Group III) and 34.2% in Group III. However, IEC was not correlated with the serum T3 concentration. The incidence of positive THBI was 80% in Group I (P < 0.05, vs. Group III), 68.3% in Group II and 47.4% in Group III, but THBI was not correlated with the serum T4 level. Positive correlations were observed among FFA, IEC and THBI (P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Development of Spontaneous Hypothyroidism in Patients with Graves' Disease Treated with Antithyroidal Drugs: Clinical, Immunological, and Histological Findings in 26 Patients

Graves' disease may result eventually in hypothyroidism in approximately 5-20% of patients. In a few such patients hypothyroidism was associated with TSH-blocking antibodies, but whether the frequency of TSH-blocking antibodies in such patients is as high as it is (21%) in patients with primary myxedema is not known. This study was undertaken to determine the presence of various immunoglobulins [TSH binding inhibitor immunoglobulins, thyroid-stimulating antibodies (TSAb), and TSH-blocking antibodies] in 26 patients with Graves' disease who developed hypothyroidism from 0.5-10 yr or more after discontinuation of antithyroid drug therapy. Eight of the 26 patients (31%) had TSH-blocking antibodies, 16 (61%) had TSAb, and 14 (54%) had thyroid hormone binding inhibitor immunoglobulins. Thyroid needle biopsies were performed in 9 patients. Three of 5 patients who had subclinical hypothyroidism had chronic lymphocytic thyroiditis, and all had positive TSAb titers. Three patients had the fibrous variant of chronic lymphocytic thyroiditis; their TSAb values were 902%, 431%, and 1290%. One patient had follicular hyperplasia. We conclude that TSH-blocking antibodies may account for hypothyroidism in approximately one third of patients with Graves' disease who were previously treated with antithyroid drugs, and that autoimmune thyroiditis is comparable for the hypothyroidism in the remaining two thirds of Graves' disease patients.

Thyroid hormone metabolism in nonthyroidal illnesses. II. A new method for measuring serum thyroid hormone binding inhibitor and its clinical application

It is well known that serum T3 and T4 concentrations are often decreased in patients with nonthyroidal illnesses (NTI). The purpose of this study was to elucidate the possible role of serum thyroid hormone binding inhibitor (THBI) in lowering serum T4 in patients with NTI using the originally developed method for measuring THBI. 125I.T4 and 0.05 micromilligram of pooled normal serum was applied to the Sephadex G.25 column equilibrated in advance by 0.1N NaOH, and 4 micromilligram of 0.075M veronal buffer, pH 8.6 (Buffer) was added. 125I.T4 thus adsorbed on the column was then eluted by adding the charcoal treated thyroxine-free serum (used as thyroxine binding protein (TBP) solution) with THBI which had been extracted from the serum by diethyl ether or standard sodium oleate solution (0.125 approximately 1.0 mumol/column). When the radioactivity of the eluate was counted, the gradual decrease in radioactivity was observed with increase in sodium oleate due to the inhibition of 125I.T4 and TBP binding. Thus, THBI in serum could be expressed as mumol sodium oleate equivalent from the dose response curve for binding inhibition. Coefficients of intrassay and interassay variations in this assay were 6 and 17%, respectively when sera from patients with acute myocardial infarction (AMI) were measured. THBI from 15 normal controls ranged from 0 to 0.27 (mean +/- SD: 0.11 +/- 0.08) mumol sodium oleate equivalent, the normal range being less than 0.27 mumoles. In 12 patients with AMI and 12 with end-stage malignancy (cancer), serum T4 levels were below the normal range in 9 and 8 cases, respectively. In these patients, serum THBI was positive in as many as 10 (83%) and 11 (92%) cases, respectively. Serum T4 and THBI were serially measured after admission in patients with AMI. Serum T4 concentrations showed their lowest values on the first, second or third hospital day and they gradually increased to the normal range by the seventh day. On the other hand, serum THBI showed their highest value on the first hospital day in 8 cases of AMI, and they gradually decreased thereafter. A statistically significant negative correlation was observed between serum T4 and THBI in these patients (r = -0.63, p less than 0.01). Moreover, statistically significant positive correlations were observed between serum THBI and FFA in both AMI and cancer patients (r = 0.89, p less than 0.01; r = 0.88, p less than 0.01, respectively).(ABSTRACT TRUNCATED AT 400 WORDS)

Alterations in thyroidal economy in a systemic illness induced by turpentine oil injection to the rat.

Injection of turpentine oil (5 microliter/g, sc) to Sprague Dawley rats was associated with a significant reduction in serum concentration of T4, T3 and TSH that lasted throughout 48 h of study. Dialyzable fraction of T4 and resin uptake of T3 did not change for 10 h after turpentine oil injection, but were increased significantly at 24 h and 48 h. Serum rT3 concentration was decreased significantly at 24 h and 48 h. The pituitary content of TSH in the experimental rat was significantly increased at 24 h after turpentine oil injection, but the TSH-beta subunit content was decreased significantly. Serum TSH response to TRH did not change in experimental rats. Thyroidal radioiodine uptake, serum T4, and serum T3 in experimental rats demonstrated little or no increase in response to exogenous bovine TSH. A thyroid hormone binding inhibitor (THBI) was detected in serum and iodothyronine 5'-monodeiodinating activity was decreased significantly in liver, kidney and heart of rats injected repeatedly with turpentine oil. Oxygen consumption of experimental rats did not change appreciably. Our data suggest that administration of turpentine oil to the rat leads, within a few hours of the injection, to a systemic illness associated with marked changes in thyroidal economy.

Thyroid functions before and after maintenance hemodialysis in patients with chronic renal failure.

To study the factors involved in the low thyroid hormone levels in patients with chronic renal failure (CRF), we investigated thyroid functions just before and after hemodialyses (HD) in 32 such patients who were on maintenance HD. In addition, we measured serum thyroid hormone binding inhibitor activities (THBI) in another set of 37 patients. None of the patients had been suspected of having thyroid diseases. HD duration and aging did not have a significant effect on the results of the thyroid function tests. Before each HD, the serum concentrations of T3, T4, FT3, FT4, rT3, PBI, FT3I, FT4I, FT3/T3, FT4/T4, T4/TBG, T4/TSH and FT4/TSH were lower, and those of TSH, TBG, and thyroglobulin (Tg) were higher in the patients than in normal controls. The thyroid hormone concentrations were negatively correlated with the BUN and creatinine levels. The Tg levels were positively correlated with the BUN levels. After each HD, almost all the thyroid function tests including T4/TBG ratio showed improvements, which indicated that hemodilution and a decrease in the T4-binding affinity of TBG with thyroid hormones were the major factors in the low thyroid hormone levels in CRF patients. However, even after HD, T3, FT3, rT3, T4/TSH and FT4/TSH were still lower and TSH and Tg were still higher in the patients. These data suggested that the CRF patients were in a subclinical hypothyroid state. THBI was high in patients with CRF and did not change following HD. NEFA did not seem to contribute to the high THBI before HD, because they were in the normal range. However, as NEFA became very high after HD and possessed THBI, we calculated the corrected THBI (C-THBI) by subtracting the effect of NEFA from total THBI. C-THBI was high before HD and decreased after HD. Therefore, it was suggested that this C-THBI contributed to the abnormalities in the affinity of TBG with thyroid hormones. From these studies, it is concluded that (1) the patients with CRF may be in a subclinical hypothyroid state, although hemodilution was seen to have a strong effect on the thyroid hormone concentrations, and (2) C-THBI may have an effect on the affinity of TBG with thyroid hormones and play an additional role in low thyroid hormone levels in these patients. The mechanisms of hypothyroidism and the nature of C-THBI remain to be clarified.

Hypothyroxinemia in Cardiac Arrest

Thyroid function was evaluated in cardiac arrest (CA), a condition associated with marked activation of the pituitary-adrenal axis. Blood samples were obtained in 24 patients immediately after diagnosis of CA and again ten minutes later. Samples were also obtained from 22 patients admitted consecutively to the intensive care unit (ICU). Abnormalities of thyroid indexes among patients on the ICU who had not experienced CA were low triiodothyronine (T3) in 45%, low thyroxine (T4) in 32%, low free T4 (equilibrium dialysis) in 21%, and elevated reverse T3 levels in 36%. The alterations of thyroid values were both more common and marked in patients with CA, with abnormally low T3 in 84% of the patients, low T4 in 65%, low free T4 in 65%, and high reverse T3 in 80%. Thyroxine-binding globulin and prealbumin concentrations were below the normal range in 40% and 21% of patients with CA. A thyroid hormone-binding inhibitor was detected in 38% of patients with CA. Thyroglobulin level was slightly high in patients with CA but not significantly different from controls on the ICU. The abnormalities present at zero minutes were further exaggerated ten minutes after CA. We conclude that abnormalities on tests measuring thyroid function are extremely common during the cardiovascular emergency of CA.

Serum thyroid hormone binding inhibitor in nonthyroidal illnesses

We have employed the recently developed competitive ligand binding assay (CLBA) to study thyroid hormone binding inhibitor (THBI) in ether extracts of sera of 25 patients admitted to the Medical Intensive Care Unit with a variety of nonthyroidal illnesses (NTI). THBI was detected in 60% ( 15 25 ) of patients using one sample/patient and in 88% ( 15 17 ) using multiple (two to six) samples from different days. Mortality rate and mean serum concentrations of total T 4, total T 3, and albumin were similar in THBI-positive and THBI-negative patients. There was a tendency for a higher frequency of low serum total T 4 in THBI-positive ( 10 15 ) than in THBI-negative ( 3 10 ) patients but the difference was not statistically significant ( P < 0.1 by Chi square). However, the mean dialyzable fraction of T 4 (DFT 4 0.11 ± 0.02%, n = 9 v 0.054 ± 0.004%, n = 10) and DFT 3 (0.54 ± 0.05% v 0.40 ± 0.032%) were both significantly ( P < 0.05) higher in THBI-positive patients than THBI-negative patients. There was a significant correlation between THBI and DFT 4 ( r = 0.55, P < 0.02) or DFT 3 ( r = 0.54, P < 0.02). Prior extraction of serum with ether reduced DFT 4 in NTI patients with high baseline DFT 4 but not in normal subjects or NTI patients with mildly abnormal baseline DFT 4. Addition to a normal serum (0.1 mL) of evaporated ether extract of a pooled NTI serum (0.10- to 3.0-mL equivalent) increased DFT 4 progressively from 0.025% to 0.14%. Similar extract of a pooled serum of normal subjects had little or no effect. THBI caused a 2.1 to 3.7 fold greater increase above baseline in DFT 4 than in DFT 3 of a pooled normal serum. Addition of THBI to a normal serum increased its DFT 4 by 35 to 377% but increased the resin uptake of T 3 (RT 3U) only by 18 to 37%. THBI reduced T 4 binding activity of TBG, albumin or prealbumin in the CLBA. The effect of THBI on T 4 binding activity of TBG was also studied in the competitive protein binding assay of Murphy and Patee. Scatchard plots of the data suggested that THBI reduces T 4 binding to TBG by reducing its affinity for T 4. In the CLBA, THBI activity of ether extract of NTI serum was increased ∼threefold by reducing the amount of albumin from 4.5 mg to 1.5 mg per assay tube. Similarly, THBI was more active when charcoal-treated NTI serum was used in place of charcoal-treated normal serum in the CLBA. The various data suggest that: (1) THBI is present very commonly in sera of NTI patients; (2) THBI inhibits T 4 binding to all three T 4-binding serum proteins; (3) THBI may contribute to high DFT 4, high DFT 3 and the frequently observed discrepancy between DFT 4 and RT 3U in NTI; and (4) Low serum albumin in NTI may enhance the activity of THBI.

Thyroid hormone-binding inhibitor in normal, pregnant, and lactating rat and postmenopausal human breast tissue.

The regulatory role of thyroid hormones in the synthesis of milk proteins and functional differentiation of the murine mammary gland has been demonstrated. Further, T4 and T3 and their metabolites are found in both rat and human milk. Since the enhanced metabolic demands of mammo- and lactogenesis may result in increased local hormonal requirements, we examined breast tissue from normal (virgin), pregnant, and lactating female rats, as well as postmenopausal human breast tissue for the presence of a thyroid hormone-binding inhibitor (THBI). This substance, which decreases binding of T4 to serum T4-binding globulin, has been described in rat liver, kidney, muscle, and intestine, and in the circulation of patients with nonthyroidal illnesses. THBI activity in the present study was assessed by equilibrium dialysis in whole tissue homogenates; the ether-soluble fraction was also analyzed for THBI activity by RIA. In a range of 0.8-8.0 mg tissue protein, we found lactating rat breast to contain elevated THBI levels when compared to those found in normal, virgin breast tissue. Half-maximum binding inhibition was achieved at 0.15 mg lactating breast tissue protein, vs. 0.23 mg breast tissue protein and 0.35 mg liver protein in homogenates from normal resting animals. Expressed in tissue milligram equivalents, half-maximum binding inhibition of lactating, 18-day pregnant, and normal rat breast tissue was 0.7, 2.6, and 4.8 mgeq, respectively. Thus, the THBI activity of lactating rat breast tissue was more than 3-fold greater than that of the pregnant rat and 7-fold greater than that found in resting breast tissue. Binding inhibition in postmenopausal human breast tissue was comparable to that of rat breast and liver tissue at low protein concentrations, but the maximum inhibition attainable (205 +/- 6%, n = 3) was significantly lower than that achieved by normal rat breast tissue (338 +/- 6%, n = 6; means +/- SEM). The ether-soluble fraction of breast tissue homogenates was also analyzed employing a THBI-RIA. THBI activity, expressed as percent binding inhibition, was elevated in extracts from lactating rat breast tissue in comparison to tissue from pregnant or normal animals at 6.0, 12.5, and 25 mgeq; the largest differences were observed at 25 mgeq tissue: lactating (57.8 +/- 2.3%) vs. pregnant (46.5 +/- 3.1%, P less than 0.01) vs. normal (41.7 +/- 2.3%, P greater than 0.05); n = 3 in all cases.(ABSTRACT TRUNCATED AT 400 WORDS)

Thyroid hormone binding inhibitor (THBI) mainly associated with serum oleic acid concentration.

Thyroid hormone binding inhibitor (THBI) mainly associated with serum oleic acid concentration.

Relationship between serum free fatty acids and thyroid hormone binding inhibitor in nonthyroid illnesses.

We measured the serum concentration and relative distribution of various free fatty acids (FFA) by gas/liquid chromatography in normal subjects and nonthyroid illness (NTI) patients with or without detectable serum thyroid-hormone binding inhibitor (THBI). The mean serum concentration of total FFA was 0.72 +/- 0.08 (SE) mM in eight normal subjects; it was similar (0.63 +/- 0.12) in eight THBI-negative NTI patients, but was significantly higher (3.1 +/- 1.0; P less than 0.05) in eight THBI-positive NTI patients (THBI index, 3.9 +/- 0.3 vs. 1.0 +/- 0.05 in normal subjects). Relative distribution of FFA in THBI-negative patients did not differ significantly from that in normal subjects. In THBI-positive patients, however, serum concentrations of palmitic, palmitoleic, stearic, and oleic acids were significantly above normal. Among fatty acids with appreciable THBI activity, oleic acid was most abundant in THBI-positive patients; its concentration of 1.3 +/- 0.32 mM in patients was about 6-fold higher than the corresponding normal value (0.21 +/- 0.02; P less than 0.005). The serum concentration of THBI correlated significantly with levels of both total FFA (r = 0.69; P less than 0.005) and oleic acid (r = 0.80; P less than 0.001). Addition of 0.33 mM oleic acid to THBI-negative NTI serum or 0.66 mM oleic acid to normal serum increased THBI activity to a level greater than 2 SD above the normal mean, i.e. 1.6. The various data suggest that 1) circulating FFA contribute importantly to THBI activity in sera of NTI patients; 2) oleic acid contributes more to THBI activity of NTI sera than do other FFA.

478 A THYROID HORMONE BINDING INHIBITOR (THBI) IN CORD SERA OF PREMATURE, TERM AND POST-TERM NEONATES

A thyroid hormone binding inhibitor (THBI) which affects the ability of thyroxin binding globulin (TBG) to bind T4 was first described by Chopra et al as a possible explanation for the low T4 levels observed in some adults patients with non-thyroidal illness. Because low T4 levels are likewise a common clinical feature of prematurity, we examined THBI activity in 30 neonates (GA 26-43 weeks, BW 930-4360 grams) using a competitive ligand binding assay in order to determine whether increased THBI levels might be responsible for the hypothyroxinemia of prematurity. Serum concentrations of T4 and TBG were measured by RIA and the free T4 index calculated. THBI activity was expressed as the % CPM T4 1125 displaced from serum binding protein in the absence of THBI. A THBI index was then calculated, normalizing patients' samples relative to a baseline (control value of 1.0). Significant positive correlations between THBI activity and birthweight (r=0.76, p< 0.0001) and gestational age (r=0.74, p< 0.0002) were noted in infants born between 26 and 39 weeks of gestation, but not in more mature infants. No significant correlations were observed between THBI activity and TBG, T4 and free T4 index in these infants. The results of this study indicate that: 1) increased THBI activity is not a likely etiology for the hypothyroxinemia of prematurity; 2) the activity of THBI increases with fetal maturation until term gestation, and then it appears to decline; 3) the relationship between THBI activity, birthweight, gestational age, and systemic illness needs to be further investigated.

Evidence for an inhibitor of extrathyroidal conversion of thyroxine to 3,5,3'-triiodothyronine in sera of patients with nonthyroidal illnesses.

To determine whether an inhibitor of extrathyroidal conversion (IEC) of T4 to T3 is present in sera of patients with nonthyroidal illness (NTI), we incubated rat liver homogenate (approximately 4 mg protein) with T4 (2.5 microM) and dithiothreitol (5 mM) in the presence of evaporated diethyl ether extracts of normal or NTI sera. The T3 produced was quantified by RIA. Extracts of NTI sera caused dose-dependent inhibition in the conversion of T4 to T3. T3 produced in the presence of 20 NTI sera (1.0 mleq aliquots) was 76 +/- 5.5% (mean +/- SE; range, 18-116) that of normal sera (100 +/- 4.1% n = 10; P less than 0.01); it was more than 2 SD below the normal mean in eight patients. Inhibition of T3 production by NTI sera was correlated highly significantly with the activity of a thyroid hormone-binding inhibitor (THBI) also present in ether extracts of these sera (r = 0.82; n = 20; P less than 0.001). Since THBI may be a lipid, we studied the effect of lipids on hepatic conversion of T4 to T3. Several fatty acids were potent inhibitors of the conversion in vitro. Doses (in micromoles) causing 50% inhibition in different experiments varied between 0.2-0.52 for arachidonic acid, 0.3-0.56 for linolenic acid, 0.38-0.40 for linoleic acid, and 0.8-0.9 for oleic acid. Other lipids had less or no inhibitory activity. The inhibition of hepatic T4 5'-monodeiodination by arachidonic acid was competitive in nature (Ki, approximately 0.11 mM). Pretreatment of rat liver with phospholipase A2 for 10-60 min led to a progressive reduction in the conversion of T4 to T3. Moreover, the evaporated ether extract of phospholipase A2-treated rat liver homogenate reduced T4 to T3 conversion by untreated rat liver homogenate. There was a significant correlation between serum concentrations of free fatty acids and IEC activity in NTI sera (r = 0.74; n = 10; P less than 0.02). The various data suggest that 1) many NTI sera contain a potent IEC; 2) some fatty acids are potent IECs; 3) like THBI, IEC may be a lipid moiety; and 4) activation of tissue phospholipases may contribute importantly to reduced extrathyroidal T3 production in NTI, presumably by releasing inhibitory fatty acids that act locally first and more generally after their release into the circulation.

Ether-extractable thyroid hormone binding inhibitor (THBI) in non-thyroidal illness

Journal Article Ether-extractable thyroid hormone binding inhibitor (THBI) in non-thyroidal illness Get access J Herrmann, J Herrmann Medizinische Kliniken C + E, Universität Düsseldorf Search for other works by this author on: Oxford Academic Google Scholar G Alasso, G Alasso Medizinische Kliniken C + E, Universität Düsseldorf Search for other works by this author on: Oxford Academic Google Scholar M Beyer, M Beyer Medizinische Kliniken C + E, Universität Düsseldorf Search for other works by this author on: Oxford Academic Google Scholar E Heinen, E Heinen Medizinische Kliniken C + E, Universität Düsseldorf Search for other works by this author on: Oxford Academic Google Scholar J Römisch, J Römisch Medizinische Kliniken C + E, Universität Düsseldorf Search for other works by this author on: Oxford Academic Google Scholar P Weyer, P Weyer Medizinische Kliniken C + E, Universität Düsseldorf Search for other works by this author on: Oxford Academic Google Scholar H L krüskemper H L krüskemper Medizinische Kliniken C + E, Universität Düsseldorf Search for other works by this author on: Oxford Academic Google Scholar Acta Endocrinologica (Norway), Volume 110, Issue 1_Supplement_a, Apr 1979, Pages S84–S85, https://doi.org/10.1530/acta.0.109S084-a Published: 01 April 1985

A competitive ligand binding assay for measurement of thyroid hormone-binding inhibitor in serum and tissues.

A competitive ligand-binding assay (CLBA) is described for measurement of an inhibitor(s) of serum binding of T4 in ether extracts of serum and in homogenates and extracts of tissues. The CLBA is based on the effect of thyroid hormone binding inhibitor (THBI) on partition of a constant amount of radiolabeled ligand [(125I]T4) between fixed amounts of serum and an anti-T4 antibody. The method is convenient, rapid, sensitive, and reproducible. The coefficient of variation averaged 8.9% within an assay and 12.8% between assays. Several fatty acids, e.g. arachidonic acid, lauric acid, linolenic acid, and linoleic acid, had potent THBI activity in the CLBA; arachidonic acid was more potent than the other fatty acids. Since oleic acid cross-reacted substantially with T4-binding sites on anti-T4, its THBI activity was examined by an equilibrium dialysis method; it was about 77% as potent as arachidonic acid. Arachidic, myristic, palmitic, and stearic acids, choleserol, various phospholipids and triglycerides (triolein and tripalmitin) had little or no THBI activity in the CLBA. THBI activity was detected in the sera of 50% (60% when serum T4 was low and 42% when it was normal) of 34 patients with nonthyroid illnesses (NTI) when studied by CLBA and in 59% (67% when serum T4 was low and 53% when it was normal) of patients when determined by the inhibitory ratio (normalized dialysis ratio/normalized binding ratio). THBI values obtained by the CLBA correlated significantly (r = 0.58; P less than 0.001) with those obtained by the inhibitory ratio method. The dose-response curve of an ether extract of pooled sera of hospitalized patients was parallel to that of arachidonic acid in the CLBA. Among various rat tissues, the small intestine had the most THBI activity in both homogenates and ether extracts of homogenates. Ether (2 vol) extracted about 63% of the THBI activity in small intestine homogenate at pH 5.2. THBI activity was demonstrable in all particulate fractions (especially mitochondria and endoplasmic reticulum) of small intestine homogenate; cytosol contained little or no THBI activity. THBI activity changed little after treatment of small intestine homogenate with trypsin or protease inhibitors.(ABSTRACT TRUNCATED AT 400 WORDS)