131I-labeled Thyroxine Research Articles

Localization of Human Thyroxine Absorption

The distribution of intestinal absorption of 131I-labeled thyroxine (T4*) was studied in 4 normal subjects after oral and i.v. T4*, given in separate experimental sessions. In addition to collection of time-activity curves for plasma T4* from the two sessions, distribution and transport of T4* through the gut was quantified by external imaging. Time-activity curves were obtained for the stomach, duodenum, and upper jejunoileum. A multicompartmental model for systemic T4, with three distribution compartments and a single exit route, was employed. Additional, gastrointestinal, compartments were introduced. The stomach data were fitted to a model with three compartments, two for transport and a small sink of gastric activity that does not interact with the absorptive sites. Transfer from the duodenum to the upper jejunoileum and from the upper to the lower jejunoileum was modeled from fits to the peak T4* activities in the images of the duodenum and upper jejunoileum. The rate of transfer from the lower jejunoileum into more distal intestinal sites was fixed, but the impact on the results of using various values for this parameter was analyzed. The model calculations of absorption (mean +/- SD for 3 of the subjects) are duodenum, 15 +/- 5%, upper jejunoileum, 29 +/- 14%, and lower jejunoileum, 24 +/- 11%. The fourth subject, whose global absorption was abnormally low for uncertain reasons, had 17% absorption from the duodenum, 9% from the upper jejunoileum and none from the lower jejunoileum. Model projections mimicking clinical gut abnormalities known to affect T4 absorption were compatible with the results of published studies.

Thyroxine secretion by isolated hog thyroid cells: A cyclic AMP independent pathway

The release of 131I-labeled thyroxine (T 4) from isolated hog thyroid cells was increased 1.5–2-fold by thyrotropin (TSH). Dibutyryl cyclic AMP failed to reproduce this TSH action. In this in vitro system another cell activity, T 4 synthesis, was stimulated in an essentially identical fashion by TSH and dibutyryl cyclic AMP (time course of action, dose-response relationship). 3-Isobutyl-l-methylxanthine (IBMX), 0.5 mM, did not alter the basal [ 131I]T 4 release whereas it enhanced the [ 131]T 4 synthesis. TSH, 60 mU/ml, increased the intracellular cyclic AMP concentration 3–4-fold. Chlorpromazine (5 × 10 −4 M) abolished the TSH stimulation of cyclic AMP accumulation but did not alter the TSH-induced increase in [ 131I]T 4 secretion. It is concluded that the TSH action on [ 131I]T 4 secretion by isolated thyroid cells is not mediated by the adenylate cyclase-cyclic AMP system.

Effect of propranolol on various aspects of thyroid function in the rat

The daily administration of propranolol (5–20 mg/kg) to the intact male rat does not effect the serum thyroxine concentration or the peripheral metabolism of 131I-labeled thyroxine. Except for a small decrease in 131I-labeled triiodothyronine, propranolol did not effect the intrathyroidal metabolism of 131I. These findings further suggest that the efficacious use of propranolol in the treatment of many of the signs and symptoms of thyrotoxicosis may not be associated with alterations in thyroid gland function or in the peripheral metabolism of thyroxine.

Deiodination of 131-I-labeled thyroxine in the fetal rat.

SummaryIn vitro deiodination of 131I-T4 as an index of T4 to T3 conversion, was assessed in homogenates of fetal and adult rat liver and kidney, to explore whether differences existed which might account for the low fetal concentrations of serum T3 observed by others. The deiodination rate per milligram of protein was greater for both kidney and liver homogenates in the fetus than in adult rats. These results suggest that fetal T4 deiodination is not decreased in comparison to the neonatal or adult rat. Thus, the mechanism for the low serum T3 concentration in the fetus does not appear to be a decreased production of T3 from T4, insofar as in vitro T4 deiodination may reflect in vivo T4 to T3 conversion.

Estimation of thyroxine production rates in non-steady states.

An equation for calculation of thyroxine production rate in non-steady state conditions has been developed. The method is based on the decay in serum thyroxine SA following the iv administration of 131I-labeled thyroxine. Thyroxine SA reflects changes in thyroxine secretion and serum total radioactivity reflects changes in thyroxine removal. Estimation of the former is, therefore, a more reliable index of thyroxine production rate in both steady and non-steady states. Following values were obtained: Normal males 79.3 ± 19.3 μg/24 hr (mean ± 1 SD) ; normal females 57.3 ± 16.6; hyperthyroid 563 ± 370 ; hypothyroid 14.3 ± 8.1; estrogen treatment 64.5 ± 29.8; salicylate treatment 66.6 ± 13.6; and Dilantin treatment 117.2 ± 24.9. Patients with thyroid dysfunction were distinctly separated from normal subjects with no overlapping of data.

Demonostration of a membrane-linked deiodination of thyroid hormones from human red cell membranes.

The deiodinaton of 131I-labeled thyroxine and triiodothyronine by red cell membranes prepared from healthy persons was examined. Under the experimental conditions (pH 6.0, 32°C) 0.5 mg membranes deiodinated 1.6 mµ moles triiodothyronine in 30 min. Iodide and a substance with an Rf of 0 in paper chromatography were the only radioactive deiodination products. The membrane system was shown to be heat stabile and dependent on an alkali labile nondialyzable factor. Iodide release is linked to membrane particles and can be demonstrated even after lipid extraction of the ghosts. The deiodinating system is highly active without the addition of any activators and is characterized by time dependency and a pH optimum of 6.0. Stimulators of the reaction are oxygen, Ca2+, Cu2+ and fructose 1,6-diphosphate. A peroxidative mechanism catalyzed by traces of hemoglobin and a non-enzymatic FMN light-induced iodide release could be excluded. The membrane-linked deiodination might play an important role in the peripheral metabolism of thyroid hormones.

Diurnal rhythm in total serum thyroxine levels.

The diurnal variation of serum thyroxine, plasma proteins, and hematocrit were studied in six patients (euthyroid, hypothyroid, and thyrotoxic) and in two obese subjects during total fast. Variations of 7% to 34% were observed in serum total protein, hematocrit, protein-bound iodine, total thyroxine, thyroxine binding globulin, and prealbumin binding capacity, and thyroxine dialyzable fraction, but not in the concentration of serum free thyroxine. Similar and parallel diurnal variation was observed during studies of the disappearance of 131I-labeled thyroxine and 125I-labeled albumin. The maximal daily concentration occurred between 10:00 a.m. and 2:00 p.m., and the minimum occurred around 2:00 a.m. These diurnal variations were not related to food intake or adrenocortical function but were influenced by posture. A reversed cyclic change could be produced by reversing the normal sleep-waking pattern. Because of reciprocal changes in the concentration of total serum thyroxine and thyroxine dialyzable fraction, the serum-free thyroxine concentration remained constant. These rhythmic changes are apparently caused by movement of fluid into and out of the vascular compartment. This factor may be an important cause of scatter in the observations recorded during studies of slowly diffusible plasma constituents if repeated blood sampling is not performed under the same conditions. The diurnal variation of serum thyroxine, plasma proteins, and hematocrit were studied in six patients (euthyroid, hypothyroid, and thyrotoxic) and in two obese subjects during total fast. Variations of 7% to 34% were observed in serum total protein, hematocrit, protein-bound iodine, total thyroxine, thyroxine binding globulin, and prealbumin binding capacity, and thyroxine dialyzable fraction, but not in the concentration of serum free thyroxine. Similar and parallel diurnal variation was observed during studies of the disappearance of 131I-labeled thyroxine and 125I-labeled albumin. The maximal daily concentration occurred between 10:00 a.m. and 2:00 p.m., and the minimum occurred around 2:00 a.m. These diurnal variations were not related to food intake or adrenocortical function but were influenced by posture. A reversed cyclic change could be produced by reversing the normal sleep-waking pattern. Because of reciprocal changes in the concentration of total serum thyroxine and thyroxine dialyzable fraction, the serum-free thyroxine concentration remained constant. These rhythmic changes are apparently caused by movement of fluid into and out of the vascular compartment. This factor may be an important cause of scatter in the observations recorded during studies of slowly diffusible plasma constituents if repeated blood sampling is not performed under the same conditions.

The Interaction of Thyroxine with Human Plasma Prealbumin and with the Prealbumin-Retinol-binding Protein Complex

Abstract Prealbumin was isolated from human plasma by chromatography on columns of diethylaminoethyl Sephadex and Sephadex G-200, followed by preparative polyacrylamide gel electrophoresis. The prealbumin was homogeneous in the analytical ultracentrifuge with an s20,w of 3.7 S and with a molecular weight of about 50,000. Prealbumin formed a protein-protein complex with plasma retinol-binding protein in a molar ratio of 1:1. The interaction of thyroxine with prealbumin was quantitatively studied by the method of equilibrium dialysis with 131I-labeled thyroxine. Studies were conducted at 24° and at 37° in phosphate buffer, pH 7.4, and in phosphate buffer containing 0.14 m NaCl. Prealbumin was found to possess a single binding site for 1 molecule of thyroxine. The association constant for the thyroxine-prealbumin interaction was approximately 1.6 x 107, and was only slightly affected by temperature change or by NaCl. The binding capacity and affinity of prealbumin for thyroxine were similar in the presence and absence of retinol-binding protein. Moreover, the binding of thyroxine to prealbumin did not interfere with the interaction of prealbumin with retinol-binding protein. The interaction of prealbumin with thyroxine appears to be independent of the prealbumin-retinol-binding protein interaction.

Biliary secretion of thyroxine in rats with thyroid-depressing hypothalamic lesions.

The 131I-labeled thyroxine (131I-T4) clearance and biliary secretion was studied in rats with confirmed depression of thyroid function following hypothalamic lesions. Biliary secretion and clearance of 131I-T4 over a 4-hr period were both significantly lower in rats with lesions than in controls, but the total volume of bile did not differ between groups. Plasma 131I-T4 levels were higher in the lesioned rats than in controls 3 1/2 hr after injection of labeled T4. (Endocrinology 84: 950, 1969)

Effect of Scurvy on Deiodination of 131I-labeled Thyroxine in Guinea Pigs

The effect of scurvy on metabolism of thyroxine (T4) and triiodothyronine (T3) was investigated. Experimental scurvy was produced in guinea pigs by feeding a diet low in vitamin C for 3 or 4 weeks. Control animals received the same diet and ascorbic acid injections. Deiodination of both thyroid hormones was greatly increased in scorbutic guinea pigs. This was shown by an increasing excretion of 131I in the urine and a decreasing retention of protein bound 131I in the plasma, liver and kidney after a single injection of 131I-labeled T4. Similar effects were observed after 131I-labeled T3 injection. Administration of nonradioactive T4 or cortisone to scorbutic animals failed to show any significant changes in the metabolism of 131I-labeled T4. Normal deiodinating activity was completely restored by daily injection of 15 mg of ascorbic acid for 7 days. Scurvy had no effect on the thyroid uptake of 131I in vivo, but increased the amount of 131I-labeled T3 to the erythrocyte in vitro. Both plasma protein-bound iodine and thyroxine levels were significantly decreased in scorbutic guinea pigs. These low values are probably due to a decrease of plasma protein binding capacity during scurvy.

The effects of p-chlorophenoxyisobutyrate on the turnover rate and distribution of thyroid hormone in the rat

A single dose of p-chlorophenoxyisobutyrate (CPIB) caused an apparent displacement of 131I-labeled thyroxine (T 4) from the plasma into the liver and it also increased the rate of T 4 disappearance from the plasma, liver, kidney, heart and skeletal muscle of the rat. Since T 4 disappeared at the same rate from all tissues, tissue T 4 must have been in equilibrium with plasma T 4. The effects of CPIB disappeared after 30–40 hr, and T 4 turnover rates regressed toward those seen in control animals. When CPIB was fed to animals for 2–3 wks, it caused the plasma PBI concentration to drop from 5·2 to 3·7 μg/100 ml. Although the absolute plasma “free T 4” concentration remained unchanged (2·5 mμg/100 ml), the relative amount of circulating hormone present as “free T 4” increased about 40 per cent when CPIB was given. The displaced T 4 was concentrated by the liver and to a lesser extent by the kidney, causing these organs to become “hyperthyroid” as evidenced by a large increase in the mitochondrial α-glycerolphosphate dehydrogenase activity. At the same time, the rest of the organism remained “euthyroid” as evidenced by a lack of change in metabolic rate and thyroid function.

Excretion of labeled thyroxine in the presence of cholesterol-cholic acid supplements.

The addition of cholesterol and cholic acid to a purified iodine deficient diet for rats greatly increased the degree of thyroid enlargement. Measurement of fecal and urinary radioactivity after injection of I131-labeled thyroxine demonstrated an increased fecal excretion of activity in the cholesterol-cholic acid supplemented group. Since the thyroid enlargement is prevented by iodine supplementation, the cholesterol-cholic acid supplement is thought to accentuate iodine depletion either by increasing biliary excretion of thyroxine or by interfering with the intestinal reabsorption of thyroxine. The addition of cholesterol and cholic acid to a purified iodine deficient diet for rats greatly increased the degree of thyroid enlargement. Measurement of fecal and urinary radioactivity after injection of I131-labeled thyroxine demonstrated an increased fecal excretion of activity in the cholesterol-cholic acid supplemented group. Since the thyroid enlargement is prevented by iodine supplementation, the cholesterol-cholic acid supplement is thought to accentuate iodine depletion either by increasing biliary excretion of thyroxine or by interfering with the intestinal reabsorption of thyroxine.

Thyroid function in the absence of vasopressin.

Thyroid function was assessed in rats with hereditary hypothalamic diabetes insipidus (DI). Vasopressin almost certainly is absent from these rats since they appear unable to synthesize the hormone. The urinary excretion of 131I following a single injection of 131I-labeled thyroxine (T4) was significantly greater in untreated DI rats than in normal animals of the same strain. However, this increase was not observed when urine flow was reduced in both groups either by administration of vasopressin or by deprivation of water. Furthermore, urinary excretion of 131I was also greater than normal in untreated DI rats after an injection of 131I-labeled sodium iodide and, as with T4, the increase was eliminated when urine flow was reduced. Diffences in urinary excretion of 131I were not observed between DI and normal rats when they were brought to isotopic equilibrium with 131I-labeled T4, and the deiodination of T4 in vitro in liver and kidney from DI rats was no greater than that from normal animals. It was thu...

Distribution and metabolism of I-131-labeled thyroxine and triiodothyronine in subcellular fractions of chicken myocardium.

Distribution and metabolism of I-131-labeled thyroxine and triiodothyronine in subcellular fractions of chicken myocardium.

Evidence for the Existence of Two Thyroxine-Binding Globulin Moieties: Correlation Between Paper and Starch-Gel Electrophoretic Patterns Utilizing Thyroxine-Binding Globulin-Deficient Sera

By comparing the starch-gel and paper electrophoretic patterns of normal serum, serum from pregnant women (TBGenriched), and TBG-deficient serum after equilibration with 131I-labeled thyroxine (T4*) and triiodothyronine (T3*), evidence has been derived for the existence of 4 major bands of radioactivity in starch gel; this confirms the work of Blumberg and Robbins. Evidence has also been obtained which indicates that TBPA has as its gel counterpart band 1, TBA has as its gel counterpart band 3, and TBG has 2 gel counterparts, bands 2 and 4 (herein designated TBG 1 and TBG 2). In addition to differences in their electrophoretic mobilities, possibly related to their molecular size or configuration, TBG 1 and TBG 2 appear to differ also in their binding affinities for T4* and T3*, a finding that further suggests that TBG 1 and TBG 2 are discrete entities.

Hypothalamic influence of thyroxine monodeiodination by rat anterior pituitary gland.

In order to determine if the monodeiodination of 131I-labeled thyroxine (T4*) to 3,5,3′-triiodothyronine (T3*) which occurs in the anterior pituitary is dependent upon hypothalamic influences, the metabolic fate of T4* in intraocular pituitary transplants has been examined chromatographically. Concentration of total radioactivity in the transplants was similar to that of the normal in situ pituitary, but the proportion of T3* was much reduced. In normal rat anterior pituitary, the relative amount of T3* was 17 % of the total radioactivity in 2 experiments; transplants, on the other hand, showed no T3* in one experiment and 9% in a second. In thyroidectomized rats the relative proportion of T3* in the anterior pituitary rose to 26 %. These findings are interpreted to indicate that monodeiodination of T4* to T3* is related to the functional activity of the anterior pituitary and that this activity depends upon intact hypothalamic-pituitary connections. (Endocrinology78: 302, 1966)

Antithyrotoxic Activity of Hemoglobin in the Rat

Dietary hemoglobin and the acid-insoluble precipitate obtained from an alkaline hydrolysate of hemoglobin had good antithyrotoxic activity, as measured by the α-glycerophosphate dehydrogenase (GPD) assay. Both interfered with the intestinal absorption of thyroxine, as measured by the level of radioactivity in blood after feeding 131I-labeled thyroxine to rats. Globin was inactive in both test systems, whereas crystalline hemin was very active in the absorption test, but only moderately active in the antithyrotoxic (GPD) assay. Hemin interfered with the catalytic role of thyroxine in the oxidation of DPNH by the horseradish peroxidase enzyme system; this indicated the formation of a complex between thyroxine and hemin. At least a part of the antithyrotoxic activity of hemoglobin can be attributed to an inhibition of the intestinal absorption of thyroxine by its iron porphyrin moiety.

Effect of Dinitrophenol Administration on the Transport and Metabolism of the Thyroid Hormones in Rats

Dinitrophenol administration to intact and thyroidectomized (L-thyroxine maintained) rats produced a 50 % decrease in the serum protein-bound iodine and a significant decrease in the amount of 131I-labeled thyroxine bound to the thyroxine-binding protein as determined by starch gel electrophoresis. The addition of dinitrophenol to rat sera produced a similar effect on thyroxine binding. Following the ip injection of 1 μc of 131Ilabeled thyroxine, the total retained radioactivity in thyroidectomized stabilized rats was measured 3 times daily for 14 consecutive days. This was done by using a large well liquid scintillation counter (Armac). The disappearance of radioactivity from the whole animal may be represented as 2 major components, an initial fast component with a biological half-life of 24 hr and a second and slower component with a half-life of 113 hr. The slower component became apparent on the fifth day, by which time 85 % of the administered radioactivity had been excreted. Dinitrophenol administr...

AN ULTRAMICRO PROTEIN-BOUND IODINE METHOD ON CAPILLARY BLOOD.

A dry ash ultramicro PBI assay is described using only 10 μl of serum easily obtained from a simple skin puncture. Experiments were performed to determine the loss of iodine at the various stages in the assay with the aid of 131I-labeled triiodothyronine and thyroxine. The relative value of potassium carbonate and sodium carbonate as the ashing agent was determined. The precision of the method does not differ significantly from that of the macro procedure. The nature of the brucine eerie complex was investigated. Optimum concentrations of the reactants, arsenious and eerie ions were determined to ensure greatest precision.

Metabolism of Thyroid Hormones in Frogs and Toads

The rate of turnover of 131I-labeled thyroxine and triiodothyronine in adult frogs and toads is temperature-dependent. The similar turnover rates for the 2 hormones and studies of plasma binding by equilibrium dialysis suggest that thyroxinebinding activity in amphibian plasma is low. Moreover, turnover of the hormones is slowed by injection of human plasma into the animal. A search for deiodinating systems in vitro revealed a heat-labile one only in frog skin from certain loci. Heat-stable deiodination proceeds rapidly in muscle and may account for the whole-body turnover of the hormones. Tetraiodothyroacetic acid appears in gut contents and in liver, particularly in toads, and its origin may be the skin which rapidly deaminates and decarboxylates the parent molecule. (Endocrinology 75: 157, 1964)