Coupling Of Iodotyrosines Research Articles

SAT488 Wolff-Chaikoff’s Revenge: There-and-Back Again Graves Thyrotoxicosis

Abstract Disclosure: A. Akofu: None. S.A. Brietzke: None. N. Sheung: None. A. Uddin: None. N. Akabogu: None. S. Dejhansathit: None. F. marium: None. Grave’s disease is due to autoantibodies activating TSH receptors, driving downstream effects of iodine organification, iodotyrosine coupling, and release of T4 and T3 from intracellular stores. Severe thyrotoxicosis is traditionally treated with high dose thionamides to block thyroid hormone synthesis, iodides to slow iodine transport and block thyroid hormone release (collectively known as the Wolff-Chaikoff effect), and beta-blockers to slow heart rate and relieve tremulousness. We report the case of a health-conscious patient who had taken OTC iodide for years, and whose thyrotoxic condition abruptly worsened with discontinuation of his iodide supplement. A 67-year-old man was found in a delirious state wandering in the woods and was taken to the hospital ED. He had afib with RVR and evidence of high-output CHF. Diffuse goiter twice normal size was obvious on exam, and he was tremulous and diaphoretic. Serum TSH was <0.005 mcU/ml, with FT4 > 7.77 ng/dl and FT3 7.8 pg/ml. Initial treatment was metoprolol 25 mg TID, propylthiouracil (PTU) 300 mg TID, cholestyramine 4g BID and Lugol’s iodine 10 drops TID. Encephalopathy resolved after 2 days and FT4 decreased to 3.72 ng/dl after 8 days. Cholestyramine and Lugol’s iodine were thus discontinued. 2 days later, FT4 further declined to 2.75 ng/dl and he was switched from PTU to methimazole 30 mg twice daily. After 3 days, FT4 had increased to 5.17 ng/dl, and in response methimazole was increased to 40 mg TID. 3 days later FT4 increased further to 6.43 ng/dl, prompting a switch from methimazole back to PTU 300 mg TID. Because of further increase in FT4 and FT3, cholestyramine and Lugol's iodine were resumed. 3 days later, FT4 had declined from > 7.7 to 5.4 ng/dl, and he was discharged with plan to perform thyroidectomy once FT4 and FT3 reached nearly normal levels. Near the end of his hospitalization, he divulged that he had taken iodide drops for “thyroid health” for many years independent of medical advice, but hadn’t taken it for several days during the period of being lost in the woods prior to admission. Possible explanations for resistance of thyrotoxicosis to aggressive prescribed treatment include medication non-adherence, drug malabsorption, hyperaccelerated drug metabolism, anti-drug antibodies and antagonism of thionamide action by excessive intrathyroidal iodine. In this case, we hypothesize that his use of iodide serendipitously attenuated severity of Graves disease, by giving him a prolonged Wolf-Chaikoff effect. Discontinuation of iodide over a matter of days allowed reversal of this phenomenon and prevented control of his hyperthyroid condition until the deliberate restoration of the Wolf-Chaikoff effect with therapeutically prescribed Lugol’s solution. Whenever response to usual hyperthyroidism treatment is ineffective or paradoxical, as in this case patients should be specifically queried regarding use of OTC iodine preparations. Presentation Date: Saturday, June 17, 2023

Carbimazole-Induced Cholestatic Jaundice with Heterozygous Mutant of H63D for Hemochromatosis

Carbimazole belongs to thionamide group of drugs and inhibits thyroid hormone synthesis by preventing organification of iodine to tyrosine residue in thyroglobulin as well as coupling of iodotyrosines. Although most of the side effects are minor such as rash, serious side effects such as hepatotoxicity and agranulocytosis have also been reported. The occurrence of carbimazole-induced hepatotoxicity has overall risk of <0.5%. Before confirming the diagnosis of cholestatic jaundice secondary to carbimazole, other causes has to be excluded as well. We report an interesting as well as rare case of cholestatic jaundice in a 50-year-old female who is a known case of hyperthyroidism who presented with icterus, generalized itching for 15 days duration and later on found to have hyperferritinemia, increased transferrin saturation, heterozygous mutant for H63D gene for hemochromatosis.

Creatine Kinase Elevation during Antithyroid Treatment of a Patient with Graves’ Disease: A Case Report and Review of Literature

Thionamides (methimazole and propylthiouracil), which have been used in the treatment of graves’ disease since 1940, inhibit the organification of iodine and coupling of iodotyrosines, thus blocking the synthesis of hormones. Myalgia is a rare side effect of these drugs. CK is the muscle specific kinase. The measurement of serum concentration of CK is useful to estimate the muscles’ breakdown. We present a young male patient with Graves’ disease who had abnormal increase of CK level during treatment with methimazole (MMI). He experienced myalgia and elevated CK level 1 month after initiation of MMI. In the beginning of the myalgia, his free T4 level decreased to normal range. After dose reduction of MMI, CK level decreased and his symptoms were resolved. Although the mechanisms for this effect are not yet clear, it is thought that rapid decrease in thyroid hormone by antithyroid treatment in susceptible patients with graves’ disease can be the cause of CK elevation. Measuring CK level in Graves disease patients presented with myalgia during treatment with antithyroid drugs would be a useful diagnostic tool.

Cloning of TPO gene and associations of polymorphisms with chicken growth and carcass traits

Thyroid peroxidase (TPO), which located on the apical membrane surface of thyrocytes, is the key enzyme involved in thyroid hormone synthesis, mainly catalyses the iodination of tyrosine residues and the coupling of iodotyrosines on thyroglobulin to form thyroxine and triiodothyronine. The objectives of this study were to identify genetic polymorphisms of the chicken TPO gene and to analyze potential association between single nucleotide polymorphisms (SNPs) and growth and carcass traits in chicken. Partial sequences of TPO gene were cloned firstly. The nucleotide sequence was found to have 72 % identity with that of humans. The chicken TPO amino acid sequence was 71 %. Through polymerase chain reaction-restriction fragment length polymorphism and DNA sequencing methods, three novel mutations of the chicken TPO gene were detected in the F2 resource population from Gushi chickens and Anka broilers. The association analysis indicated that all of the three SNPs showed association with chicken growth at different periods. The g.29996C>T polymorphisms was significantly associated with body weight, breast bone length, pectoral angle at 12 weeks, claw weight and leg muscle weight (P < 0.05). In addition, individuals with the TT genotype had higher value for almost all the traits than CC and CT genotype. Meanwhile for CLW, the additive effects were significant (P < 0.05). Hence, we suggest that genotype TT can be regarded as a potential molecular marker for later growth and carcass traits in chicken.

The effects of lithium therapy on thyroid and thyrotropin-releasing hormone.

Lithium is used in the prophylaxis of bipolar depressive disorder in augmentation treatment of depression and in the therapy of some cases of unipolar depression. Lithium affects cell function via its inhibitory action on adenosine triphosphatase (ATPase) activity, cyclic adenosine monophosphate (cAMP), and intracellular enzymes. The inhibitory effect of lithium on inositol phospholipid metabolism affects signal transduction and may account for part of the action of the cation in manic depression. Lithium also alters the in vitro response of cultured cells to thyrotropin-releasing hormone (TRH) and can stimulate DNA synthesis. Lithium is concentrated by the thyroid and inhibits thyroidal iodine uptake. It also inhibits iodotyrosine coupling, alters thyroglobulin structure, and inhibits thyroid hormone secretion. The latter effect is critical to the development of hypothyroidism and goiter. Effects on brain deiodinase enzymes and alterations in thyroid hormone receptor concentration in the hypothalamus are under investigation in relation to the therapeutic effect of lithium. The ion affects many aspects of cellular and humoral immunity in vitro and in vivo. This accounts for a rise in antithyroid antibody titer in patients having these antibodies before lithium administration whereas there is no induction of thyroid antibody synthesis de novo. Goiter, due to increased thyrotropin (TSH) after inhibition of thyroid hormone release, occurs at various reported incidence rates from 0%-60% and is smooth and nontender. Subclinical and clinical hypothyroidism due to lithium is usually associated with circulating anti-thyroid peroxidase (TPO) antibodies but may occur in their absence. Iodine exposure, dietary goitrogens, and immunogenetic background may all contribute to the occurrence of goiter and hypothyroidism during long-term lithium therapy. It is currently unclear whether the reported association of lithium therapy and hyperthyroidism are causal, although there is suggestive epidemiological evidence. Finally, lithium therapy is associated with exaggerated response of both TSH and prolactin to TRH in 50%-100% of patients, although basal levels are not usually high. It is probable that the hypothalamic pituitary axis adjusts to a new setting in patients receiving lithium.

Dityrosine bridge formation and thyroid hormone synthesis are tightly linked and are both dependent on N-glycans

Formation of dityrosine bridges is a ubiquitous process mainly attributed to oxidative stress leading to protein degradation and cellular damages. Here we show that dityrosine formation is involved in a physiological process, thyroid hormone synthesis, and is strictly dependent on structural characteristics, namely N-glycans, presented by the protein acting as the prothyroid hormone. We used two isoforms of the N-terminal thyroid hormone forming domain (NTD) of human thyroglobulin: one without N-glycan (19 kDa isoform) and the other with high mannose type structures (25 kDa isoform). Both isoforms were able to form iodotyrosines after in vitro iodination. However, iodotyrosine coupling to form thyroxine did not occur with the unglycosylated 19 kDa NTD. In contrast, the 25 kDa isoform formed thyroxine. Strikingly, thyroxine synthesis was accompanied by dimerization of the 25 kDa isoform and formation of a dityrosine bridge; none of this was observed with the 19 kDa isoform. Taken as a whole, our results indicate that dimerization through dityrosine bridging accompanies and could have a role in thyroid hormone synthesis.

N-Glycans Modulate in Vivo and in Vitro Thyroid Hormone Synthesis: STUDY AT THE N-TERMINAL DOMAIN OF THYROGLOBULIN

Thyroglobulin (Tg) is the substrate for thyroid hormone biosynthesis, which requires tyrosine iodination and iodotyrosine coupling and occurs at the apical membrane of the thyrocytes. Tg glycoconjugates have been shown to play a major role in Tg routing through cellular compartments and recycling after endocytosis. Here we show that glycoconjugates also play a direct role in hormonosynthesis. The N-terminal domain (NTD; Asn1-Met171) of human Tg, which bears the preferential hormonogenic site, brings two N-glycans (Asn57 and Asn91). NTD preparations were purified from Tg with low and mild iodine content in vivo and from poorly iodinated Tg after in vitro iodination and coupling. NTD separated from poorly iodinated Tg was also submitted to iodination and coupling after desialylation and deglycosylation. The various NTD isoforms were analyzed for their N-glycan structures and hormone contents. Our results show that 1) in vivo as well as in vitro unglycosylated isoforms did not synthesize hormones, whereas fully or partially (at Asn91) glycosylated isoforms did; 2) high mannose type structures enhanced the hormone content; and 3) desialylation did not affect in vitro hormone synthesis. Evidence of a direct involvement in hormonosynthesis adds to the role of N-glycans in Tg function and opens the way to new mechanisms for regulation (e.g. TSH modulation of N-glycan) or alteration (e.g. Asn91 mutation) of thyroid hormone synthesis.

Tyrosine iodination and iodotyrosyl coupling of the N-terminal thyroid hormone forming site of human thyroglobulin modulate its binding to auto- and monoclonal antibodies

The present work was aimed at studying the interaction of autoantibodies (aAb) and monoclonal antibodies (mAb) with the N-terminal thyroid hormone forming site of human thyroglobulin (TG). Obtained by CNBr treatment of TG, the peptide (22 kDa) containing the complete major hormonogenic site of human TG was purified in three forms according to the degree of iodination and iodotyrosine coupling: the native, poorly iodinated form (n-22K), the iodinated form containing iodotyrosine but not hormone residues (i-22K) and the form containing thyroid hormone (t-22K). We report that aAb from some patients with autoimmune thyroid diseases showed significant binding to both iodinated 22 kDa forms. Furthermore, a detailed study using mAb evidenced that iodination and coupling induced changes in the antigenicity of the molecule, some occurring without direct implication of iodine or thyroid hormones. The 22 kDa peptide appears as an interesting model to study the antigenic changes induced by the structural modifications in the course of thyroid hormone synthesis. This observation could be relevant to the etiopathogenic process of thyroid autoimmune diseases.

The molecular biology of thyroid peroxidase: cloning, expression and role as autoantigen in autoimmune thyroid disease.

I. Introduction THYROID peroxidase (TPO) is the primary enzyme involved in thyroid hormone synthesis, catalyzing iodide oxidation, iodination of tyrosine residues, and coupling of iodotyrosines to generate the iodothyronines T3 and T4 (1). It is a membrane-bound glycoprotein with a heme prosthetic group. In addition to the pivotal physiological role of TPO, there was strong immunological evidence, even before its molecular cloning, that this enzyme was the elusive “thyroid microsomal antigen” in autoimmune thyroid disease (discussed below). The molecular cloning of the complementary DNA for a protein provides powerful new opportunities for studying protein structure and function at an unprecedented level of detail. In t h e past decade, three of the four major thyroid-specific proteins have been cloned, namely thyroglobulin (TG) (2–4), TPO (see below), and the TSH receptor (5–8). Molecular cloning of the thyroid iodide transporter may occur soon (9). As might be expected, these advances have been followed...

Hyposialylated thyroglobulin in a patient with congenital goiter and hypothyroidism.

A large family (14 children) with congenital goiter whose parents are first cousins was studied. Thyroid tissue was obtained, after 125I in vivo labeling, from one of the siblings (JBM). Gel filtration of thyroid proteins indicated that thyroglobulin (Tg) eluted as a single symmetrical peak in the same position as authentic 19S Tg. Gel electrophoresis in a 7.5% sodium dodecyl sulfate-polyacrylamide gel revealed a major band with the same mobility and immunoreactivity as normal 19S Tg. Hydrolysis of the patient's Tg indicated that most of the radioactivity was mono- and diiodotyrosines. The yield of T4 from JBM Tg (26 pmol/mg protein) was 5-fold less than normal thyroid tissue (140 pmol/mg protein) and approximately half of that in thyroid tissue from endemic goiter (51 pmol/mg). Total T3 released from JBM Tg was similar to the other two tissues. When the carbohydrate content of normal and patient Tg was analyzed, there was no differences in glucosamine, galactose or mannose content. However, unlike normal and endemic-goiter Tg, that had a mean sialic acid content of 7.3 and 5.6 micrograms/mg protein, respectively, the sialic acid concentration of the patients Tg was only 0.3 microgram/mg. Sialyltransferase activity was readily demonstrated in homogenate from normal thyroid or endemic goiter, but no sialyltransferase activity was detectable in a homogenate of JBM-thyroid tissue. We conclude that the finding of severely hyposialylated Tg is linked to a defect in iodotyrosine coupling seen in this patient with a possibly abnormal migration of Tg into the follicular lumen.

Coupling of iodotyrosine catalyzed by human thyroid peroxidase in vitro.

The coupling of iodotyrosine (coupling reaction) is one of the least studied in the formation of thyroid hormone, particularly in human thyroid diseases. This paper describes a method of measuring iodotyrosine coupling catalyzed by human thyroid peroxidase (TPO) in vitro. There were two important requirements to demonstrate the coupling reaction: 1) thyroglobulin with a low thyroid hormone content, and 2) partially purified TPO. Thyroglobulin with low thyroid hormone content was obtained from Grave's and follicular adenoma tissues after propylthiouracil (PTU) therapy and L-T4 therapy, respectively. TPO was prepared from Graves' thyroid by solubilizing the 100,000 X g pellet of thyroid homogenate with sodium deoxycholate and trypsin, followed by Sephacryl S-300 gel filtration. Before the coupling reaction, thyroglobulin was iodinated with chloramine-T and potassium iodide, followed by dialysis. The coupling reaction was carried out by incubating newly iodinated thyroglobulin with TPO, diiodotyrosine, a coupling stimulator, and a H2O2-generating system (glucose and glucose oxidase) for 20 min at 37 C. After thyroglobulin was digested with Pronase, the thyroid hormone content of the thyroid digest was measured by RIA. Coupling activity was measured by the amount of newly formed T3 (nanograms of T3 per mg thyroglobulin). The time course of coupling reaction showed a progressive increase in coupling activity up to 30 min, and the reaction was temperature and pH dependent, with a pH optimum of 7.0. Coupling activity in the presence of H2O2 and TPO was 43 +/- 5.0 ng T3/mg thyroglobulin (mean +/- SD of triplicate samples), and addition of diiodotyrosine to the H2O2-TPO system caused a nearly 3-fold increase in coupling activity. This method has potential utilization for measurement of peroxidase coupling activity, since there was a linear relationship between the measured coupling activity and the amount of added TPO when the TPO concentration was over 3 micrograms/300 microliter. Methimazole (MMI) and PTU had similar potencies in inhibiting the TPO-catalyzed coupling reaction, whereas MMI was distinctly more potent than PTU as an inhibitor of TPO-mediated iodination in vitro. The different potencies of MMI in the two reactions suggest that different inhibitory mechanisms may be involved in iodination and coupling. The reducing agent, sodium metabisulfite, was also found to be a more potent inhibitor of the TPO-mediated coupling reaction than of the TPO-mediated iodination reaction. The method of iodotyrosine coupling described here may be useful to investigate the coupling step of thyroid hormone formation in human thyroid diseases.

Effect of cycloheximide on iodothyronine formation in vitro

The unique inhibitory effect of cycloheximide (CH) on the coupling of iodotyrosines was examined in vitro. Rat thyroid lobes were incubated for 8 h under our improved condition. In the presence of 10(-4) - 10(-3) M CH, the per cent uptake of 131I decreased, proportionate synthesis of [131I]MIT increased slightly, and that of [131I]T4 or [131I]T3 decreased markedly. The incorporation of medium 127I into T4 or T3 during the 8 h incubation period decreased markedly, but was fairly constant into MIT and only slightly decreased into DIT. Thus the inhibitory effect of CH seemed more prominent on iodothyronine formation than on iodotyrosine formation in this in vitro system. Inhibition of formation of newly labelled iodothyronines seemed to occur almost in parallel with the inhibition of [3H]amino acid incorporation into the thyroidal soluble protein. However, the coupling of iodotyrosines prelabelled in the absence of CH did not seem to be affected by CH. The presence of 10(-4) M CH induced the Wolff-Chaikoff effect at a lower iodide concentration than that which occurred in the absence of CH, suggesting that CH sensitized the Wolff-Chaikoff effect. However, the organification of 127I and T4 synthesis were markedly reduced in the presence of CH even before the apparent Wolff-Chaikoff effect was initiated. These results give further support to out contention that prethyroglobulin is more important for organification of iodide than pre-existing thyroglobulin. We conclude that CH reduces coupling efficiency indirectly, probably by inhibiting the formation of prethyroglobulin with a favourable structure for coupling.

Studies on the mechanism of inhibition of thyroid function by lithium.

The effect of dietary lithium on the rate of thyroglobulin degradation was investigated using both in vivo and in vitro experimental models. The in vivo experiments utilized a modification of the double isotopic method of estimation of protein turnover (Arias, I.M., Doyle, D and Schminke, R.T. (1969) J. Biol. Chem. 244, 3303–3315) injecting isotopes of iodine, 131 I and 125 I, to label thyroglobulin 27 h and 3 h respectively before killing. Faster rates of thyroglobulin hydrolysis were associated with lower activity of 131 I as compared to 125 I and thus resulted in higher 125 I/ 131 I isotopic ratios in thyroglobulin. The isotopic ratio in the 1000 × g supernatant fraction of thyroid gland homogenate closely approximated the isotopic ratio in the isolated thyroglobulin and was used as an estimate of the rate of thyroglobulin degradation. Treatment with lithium had no direct effect on the rate of thyroglobulin degradation. The only exception was observed in states of extreme iodine deficiency, presumably through suppression of thyrotropin release by the pituitary. Experiments in vitro were carried out by labeling of the thyroglobulin pool in vivo with an injection of 131 I and subsequent culture of the isolated glands for 6 h in vitro under conditions where deiodination of iodotyrosines was prevented (Bagchi, N., Brown, T.R., Shivers, B. and Mack, R.E. (1977) Endocrinology 100, 1002–1008). The rate of thyroglobulin degradation, as measured by the percent 131 I released as iodotyrosines and iodothyronines, was not altered by lithium administration. However, lithium-treated animals demonstrated a marked decrease in the rate of release of 131 I-iodothyronines. The effect of lithium on thyroglobulin iodination was investigated in parallel in vivo experiments. Lithium administration resulted in a decreased content of iodothyronines and a lower moniodotyrosine/diiodotyrosine ratio. These studies suggest that lithium acts primarily by inhibition of the coupling of iodotyrosines and has no direct effect on thyroglobulin degradation. Hydrolysis of such iodothyronine-poor thyroglobulin would account for the observed decreases of hormonal release from the gland.

Biosynthesis of thyroid hormone: Basic and clinical aspects

Thyroid hormone formation requires the coincident presence of peroxidase, H 2O 2, iodide, and acceptor protein at one anatomic locus in the cell. The peroxidase enzyme appears to be a protoporphyrin IX containing heme protein, with binding sites for both iodide and tyrosine. It is probable that both iodide and tyrosine are oxidized to free radical forms which unite to form iodotyrosine. The peroxidase is also involved through an uncertain mechanism in iodotyrosine coupling and probably in oxidation of sulfhydryl bonds in thyroglobulin. H 2O 2 may be supplied by microsomal NADPH-cytochrome c reductase or NADH-cytochrome b 5 reductase. Other possible intracellular H 2O 2 generating systems include monoamine oxidase and xanthine oxidase. The usual acceptor for iodide is thyroglobulin, which is currently believed to be iodinated within apical secretory vesicles at the cell border just prior to liberation into the colloid, or possibly after liberation into the colloid. Other soluble and insoluble proteins are also iodinated within the gland. The peroxidase is present in numerous cellular structures, but iodination activity occurs primarily, if not only, at the apical cell border. The controls of iodination are imperfectly known. Thyrotrophin modulation of iodide uptake, H 2O 2 generation, thyroglobulin synthesis, and peroxidase enzyme level obviously are the main regulations. Many of these actions are thought to involve mediation of adenyl cyclase and subsequent activation of intracellular phosphokinases. Antithyroid drugs of the thiocarbamide group are competitive inhibitors of iodination under some circumstances, but if much iodide is present, they react with the oxidized iodine intermediate and are irreversibly inactivated themselves. Clinical problems involving defective peroxidase function are among the most frequent hereditary defects of thyroid hormone formation. Recognized abnormalities include deficient peroxidase, abnormality in binding of the peroxidase apoprotein to its prosthetic group, and other less well-identified abnormalities in peroxidase structure and function. Peroxidase is typically elevated in thyroid tissue from patients with hyperthyroidism, sometimes deficient in cold thyroid nodules, and frequently diminished in tissue from patients with Hashimoto's thyroiditis.

Thyroidal triiodothyronine and thyroxine in Graves' disease: correlation with presurgical treatment, thyroid status, and iodine content.

To evaluate the potential contribution of thyroidal secretion to the relative excess of triiodothyronine (T3) production in hyperthyroidism and to investigate the effects of treatment, iodine (127I), T3 and thyroxine (T4) were measured in digests of thyroid tissue obtained at surgery from 13 patients with Graves' disease. In 11 normal human thyroid glands, 127I content was 630 +/- 60 (all values mean +/- SE in mug/ wet weight) T4, 254 +/- 39 and T3 21 +/-3. The T4I was 26 +/- 3% of the total iodine and the molar ratio T4/T3 was 11 +/- 1. The 13 patients with Graves' disease were divided into three groups. Eleven were clinically euthyroid (Groups I and II) and had received either iodide or iodide plus a thiourea derivative before surgery. Two subjects (Group III) received only propranolol. In Group I (n = 8), mean thyroidal 127I content was 320 +/- 50, T4 was 115 +/- 9 and T3 22 +/- 4. The molar ratio T4/T3 was 5.9 +/- 1 and T4I was 26 +/- 2% of the total. Group II patients (3) had the lowest preoperative serum T4 (less than 2.5 mug/dl) and T3 (less than ng/dl) concentrations with TSH elevated in only one (7 muU/ml). Thyroidal 127I was 100 +/- 26, T49 +/- and T3 1.3 +/- 0.3. The % T4I was 5 +/-2. The two chemically hyperthyroid subjects had a mean tissue 127I of 450; T4, 295 and T3, 56, the T4/T3 ratio was 4.5 and % T4I was 42. There was no correlation between tissue 127I and T4/T3 within either the normal or Graves' disease group. Since adequate clinical and chemical control of hyperthyroidism with antithyroid drugs and iodine was attained in the 8 Group I subjects without a decrease in the % T4I or T3I below that of normal thyroids, it suggests that inhibition of iodotyrosine coupling is not required for this effect. The % T4I was below normal only in patients with marked suppression of serum T4 and T3 concentraions. The lack of correlation between tissue 127I and T4/T3 ratio in the treated patients suggests that the lower T4/T3 ratio in Graves' thyroids is independent of intrathyroidal iodine concentrations. This hypothesis is strengthened by the similarly low T4/T3 ratio in untreated subjects with near normal tissue 127I content. Assuming that the thyroid hormones are secreted in the ratios present in these digests, one can estimate that direct secretion by the thyroid could contribute most, of not all, of the excess T3 production in Graves' disease.

Thyroid Function of Chicks after Withdrawal of (—)-5-Vinyl-2-oxazolidinethione, a Goitrogen in Rapeseed

Thyroid function of chicks after withdrawal of goitrin, a goitrogen in rapeseed, from the diet following 21 days feeding was investigated. Increased thyroid weight (about 4 times that of the control) and thyroidal 131 I uptake (about 2.7 times that of the control) induced by 21 days feeding of 0.05% goitrin were decreased gradually after withdrawal of goitrin. Other synthesizing steps of thyroid hormone in the gland, such as iodination of tyrosines and coupling of iodotyrosines and synthesizing of thyroxine, and plasma level of thyroid hormone were completely recovered within 2 days after withdrawal of goitrin. Distinct rebound phenomena were observed in circulating level of thyroid hormone and synthesis of thyroxine in the gland after the withdrawal. These results suggest that depression in metabolism of thyroid hormone induced by administration of goitrin may be restored in a relatively short time after withdrawal of goitrin, despite hypertrophy of thyroid gland which persist for relatively longer time.

Thyroid hormone biosynthesis in agnatha and protochordata

In the present review the problem of the evolutionary level at which the biosynthesis of thyroid hormones first appeared in the zoological scale is discussed. Thyroid hormones are certainly present in all Chordata, Urochordata, Cephalocordata, and Vertebrata. However, although concentration and organification of iodine are widespread processes occurring throughout the animal kingdom, the synthesis of thyroid hormones, i.e., iodothyronines, has not been clearly established below the Chordata. The biochemical mechanism leading to hormone synthesis follows the same general pattern from its first appearance in the Protochordata. The coupling of iodotyrosines to yield iodothyronines occurs within the polypeptide chains of specific proteins. In vertebrates, from the cyclostomes up to the mammals, the polymerization of similar, but perhaps not identical, subunits, leads to the formation of a family of thyroid proteins of increasing molecular size, i.e., 12 S, 19 S (thyroglobulin), and 27 S iodoproteins.

Antithyroid effects of 3-amino-1,2,4-triazole.

SummaryThe effects of 3-amino-1,2,-4-triazole (ATZ) on weight, content of iodine, and histology of the thyroid in rats were determined and compared to the effects of propylthiouracil (PTU), and KCIO4. In an 83-day period of treatment, ATZ and PTU produced goiters of comparable size. The KCIO4 treatment resulted in slightly greater depletion of iodine in the thyroid, smaller goiters, and less thyroxine depletion of the thyroid. The effect of ATZ treatment on intrathyroid metabolism of iodine in rats was studied and compared to similar studies in rats without treatment. In the ATZ-treated group, there was a progressive decrease in T3 and T4 and a slight increase in the proportion of MIT. The biologic half-life of 131I in the thyroid of the control and treated groups was 4.9 and 1.3 days, respectively. These findings are similar to those found previously for PTU and show that ATZ, like PTU, not only interferes with organification of iodine but also inhibits the coupling of iodotyrosines to form iodothyronines.

Identification of Thyroidal Iodoamino Acids by Gel Filtration and Automated127I Determination

Hydrolysates of thyroid tissue from 4 patients with diverse thyroid disease have been subjected to gel filtration on Sephadex G-25. Iodide (I-), monoiodotyrosine (MIT), diiodotyrosine (DIT), triiodothyronine (T3) and thyroxin (T4) were determined both by an automated chemical analysis of m I and by 131I counting. Detection of these compounds by 127I analysis reliably reflects their presence in the thyroid gland and under certain circumstances may allow a more accurate evaluation of possible iodotyrosine coupling defects than 131I counting. A portion of the thyroid gland from one of the patients was stored for a prolonged period at −4 C. Gel filtration of a hydrolysate of this stored thyroid tissue gave results similar to those obtained immediately post thyroidectomy. Gel filtration of thyroid hydrolysates offers a simple, reliable and highly reproducible means of separating iodide and the thyroidal iodoamino acids.

ANTITHYROID EFFECT OF NONCALORIGENIC CONGENERS OF SALICYLATE, WITH OBSERVATIONS ON THE INFLUENCE OF SERUM PROTEINS ON THE POTENCY OF ANTITHYROID AGENTS.

Several congeners of salicylate, previously shown to inhibit the binding of thyroxine in serum and to alter its peripheral metabolism in man, have been found to inhibit thyroid hormone synthesis. The most potent were 2,3-dihydroxybenzoic acid, 3,5- dihydroxybenzoic acid, 2,4-dihydroxybenzoic (beta-resorcylic) acid, and 2,6-dihydroxybenzoic (gamma-resorcylic) acid. In vitro, these compounds inhibit the organic binding of thyroidal iodide. In rats, gamma-resorcylic acid also inhibited both the iodination of monoiodotyrosine to form diiodotyrosine and the coupling of iodotyrosines to form iodothyronines. As might be expected from their effects on thyroxine binding, which suggest that they themselves are bound to serum proteins, the antithyroid activity of these compounds was decreased in vitro by the presence of human serum. This observation led to a study of the effects of serum on the in vitro activity of the traditional antithyroid agents. In protein-free media, propylthiouracil was much more potent than ...