Coated vesicles from thyroid cells carry iodinated thyroglobulin molecules. First indication for an internalization of the thyroid prohormone via a mechanism of receptor-mediated endocytosis

DUOX Defects and Their Roles in Congenital Hypothyroidism

Extracellular hydrogen peroxide is required for thyroperoxidase-mediated thyroid hormone synthesis in the follicular lumen of the thyroid gland. Among the NADPH oxidases, dual oxidases, DUOX1 and DUOX2, constitute a distinct subfamily initially identified as thyroid oxidases, based on their level of expression in the thyroid. Despite their high sequence similarity, the two isoforms present distinct regulations, tissue expression, and catalytic functions. Inactivating mutations in many of the genes involved in thyroid hormone synthesis cause thyroid dyshormonogenesis associated with iodide organification defect. This chapter provides an overview of the genetic alterations in DUOX2 and its maturation factor, DUOXA2, causing inherited severe hypothyroidism that clearly demonstrate the physiological implication of this oxidase in thyroid hormonogenesis. Mutations in the DUOX2 gene have been described in permanent but also in transient forms of congenital hypothyroidism. Moreover, accumulating evidence demonstrates that the high phenotypic variability associated with altered DUOX2 function is not directly related to the number of inactivated DUOX2 alleles, suggesting the existence of other pathophysiological factors. The presence of two DUOX isoforms and their corresponding maturation factors in the same organ could certainly constitute an efficient redundant mechanism to maintain sufficient H₂O₂ supply for iodide organification. Many of the reported DUOX2 missense variants have not been functionally characterized, their clinical impact in the observed phenotype remaining unresolved, especially in mild transient congenital hypothyroidism. DUOX2 function should be carefully evaluated using an in vitro assay wherein (1) DUOXA2 is co-expressed, (2) H₂O₂ production is activated, (3) and DUOX2 membrane expression is precisely analyzed.

ABSENCE OF A THYROID PHENOTYPE IN SORTILIN-DEFICIENT MICE

OBJECTIVE

The Vps10p family member sortilin is expressed in thyroid epithelial cells where it contributes to recycling of the thyroid hormone precursor thyroglobulin (Tg), a process that is thought to render hormone release more effective. Here we investigated the functional impact of sortilin in the thyroid gland using sortilin-deficient mice.

METHODS

We measured free T4, thyroid-stimulating hormone (TSH) and Tg serum levels and studied thyroid morphology in 14 sortilin-deficient (Sort1)(-/-)and 12 wildtype (WT) mice.

RESULTS

Serum free T4 levels did not differ between Sort1(-/-)and WT females but were significantly lower in Sort1(-/-)males compared with WT (P = .0424). Neither serum TSH nor Tg levels differed between Sort1(-/-)and WT mice, regardless of sex. On the same line, no thyroid histology differences were observed.

CONCLUSION

Our findings seem to exclude a role of sortilin in thyroid hormone secretion, although it is possible that the absence of sortilin may result in a thyroid phenotype if combined with other molecular defects of thyroid hormone synthesis and secretion or under iodine deficiency.

Intrinsic regulation of thyroid function by thyroglobulin

BACKGROUND

The established paradigm for thyroglobulin (Tg) function is that of a high molecular weight precursor of the much smaller thyroid hormones, triiodothyronine (T3) and thyroxine (T4). However, speculation regarding the cause of the functional and morphologic heterogeneity of the follicles that make up the thyroid gland has given rise to the proposition that Tg is not only a precursor of thyroid hormones, but that it also functions as an important signal molecule in regulating thyroid hormone biosynthesis.

SUMMARY

Evidence supporting this alternative paradigm of Tg function, including the up- or downregulation by colloidal Tg of the transcription of Tg, iodide transporters, and enzymes employed in Tg iodination, and also the effects of Tg on the proliferation of thyroid and nonthyroid cells, is examined in the present review. Also discussed in detail are potential mechanisms of Tg signaling in follicular cells.

CONCLUSIONS

Finally, we propose a mechanism, based on experimental observations of Tg effects on thyroid cell behavior, that could account for the phenomenon of follicular heterogeneity as a highly regulated cycle of increasing and decreasing colloidal Tg concentration that functions to optimize thyroid hormone production through the transcriptional activation or suppression of specific genes.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

Cathepsin C and plasma glutamate carboxypeptidase secreted from Fischer rat thyroid cells liberate thyroxin from the N-terminus of thyroglobulin

The release of a thyroid hormone from thyroglobulin is controlled by a complex regulatory system. We focused on the extracellular action of two lysosomal enzymes, cathepsin C (catC, dipeptidyl peptidase I) and PGCP (lysosomal dipeptidase), on thyroglobulin, and their ability to liberate the hormone thyroxin. Cathepsin C, an exopeptidase, removes dipeptides from the N-terminus of substrates, and PGCP hydrolyses dipeptides to amino acids. In vitro experiments proved that cathepsin C removes up to 12 amino acids from the N-terminus of porcine thyroglobulin, including a dipeptide with thyroxin on position 5. The newly formed N-terminus, Arg-Pro-, was not hydrolysed further by cathepsin C. Cell culture experiments with FRTL-5 cell line showed localization of cathepsin C and PGCP and their secretion into the medium. Secretion of the active cathepsin C from FRTL-5 cells is stimulated by TSH, insulin, and/or somatostatin. The released enzymes liberate thyroxin from porcine thyroglobulin added to media. The hormone liberation can be reduced by synthetic inhibitors of cysteine proteinases and metalloproteinases. Additionally, we show that TSH, insulin, and/or somatostatin induce up-regulation of N-acetylglucosaminyltransferase 1, the enzyme responsible for the initiation of biosynthesis of hybrid and complex N-glycosylation of proteins.

Sortilin is a putative postendocytic receptor of thyroglobulin

The Vps10p family member sortilin is involved in various cell processes, including protein trafficking. Here we found that sortilin is expressed in thyroid epithelial cells (thyrocytes) in a TSH-dependent manner, that the hormone precursor thyroglobulin (Tg) is a high-affinity sortilin ligand, and that binding to sortilin occurs after Tg endocytosis, resulting in Tg recycling. Sortilin was found to be expressed intracellularly in thyrocytes, as observed in mouse, human, and rat thyroid as well as in FRTL-5 cells. Sortilin expression was demonstrated to be TSH dependent, both in FRTL-5 cells and in mice treated with methimazole and perchlorate. Plasmon resonance binding assays showed that Tg binds to sortilin in a concentration-dependent manner and with high affinity, with Kd values that paralleled the hormone content of Tg. In addition, we found that Tg and sortilin interact in vivo and in cultured cells, as observed by immunoprecipitation, in mouse thyroid extracts and in COS-7 cells transiently cotransfected with sortilin and Tg. After incubation of FRTL-5 cells with exogenous, labeled Tg, sortilin and Tg interacted intracellularly, presumably within the endocytic pathway, as observed by immunofluorescence and immunoelectron microscopy, the latter technique showing some degree of Tg recycling. This was confirmed in FRTL-5 cells in which Tg recycling was reduced by silencing of the sortilin gene and in CHO cells transfected with sortilin in which recycling was increased. Our findings provide a novel pathway of Tg trafficking and a novel function of sortilin in the thyroid gland, the functional impact of which remains to be established.

General background on the hypothalamic-pituitary-thyroid (HPT) axis

This article reviews the thyroid system, mainly from a mammalian standpoint. However, the thyroid system is highly conserved among vertebrate species, so the general information on thyroid hormone production and feedback through the hypothalamic-pituitary-thyroid (HPT) axis should be considered for all vertebrates, while species-specific differences are highlighted in the individual articles. This background article begins by outlining the HPT axis with its components and functions. For example, it describes the thyroid gland, its structure and development, how thyroid hormones are synthesized and regulated, the role of iodine in thyroid hormone synthesis, and finally how the thyroid hormones are released from the thyroid gland. It then progresses to detail areas within the thyroid system where disruption could occur or is already known to occur. It describes how thyroid hormone is transported in the serum and into the tissues on a cellular level, and how thyroid hormone is metabolized. There is an in-depth description of the alpha and beta thyroid hormone receptors and their functions, including how they are regulated, and what has been learned from the receptor knockout mouse models. The nongenomic actions of thyroid hormone are also described, such as in glucose uptake, mitochondrial effects, and its role in actin polymerization and vesicular recycling. The article discusses the concept of compensation within the HPT axis and how this fits into the paradigms that exist in thyroid toxicology/endocrinology. There is a section on thyroid hormone and its role in mammalian development: specifically, how it affects brain development when there is disruption to the maternal, the fetal, the newborn (congenital), or the infant thyroid system. Thyroid function during pregnancy is critical to normal development of the fetus, and several spontaneous mutant mouse lines are described that provide research tools to understand the mechanisms of thyroid hormone during mammalian brain development. Overall this article provides a basic understanding of the thyroid system and its components. The complexity of the thyroid system is clearly demonstrated, as are new areas of research on thyroid hormone physiology and thyroid hormone action developing within the field of thyroid endocrinology. This review provides the background necessary to review the current assays and endpoints described in the following articles for rodents, fishes, amphibians, and birds.

Recombinant human thyroid-stimulating hormone: pharmacology, clinical applications and potential uses

The major functions of pituitary thyroid-stimulating hormone (TSH) are to maintain the biosynthesis and secretion of the thyroid hormones L-thyroxine (T4) and L-3,5,3'triidothyronine (T3). The TSH core contains two apoproteins, the alpha and beta subunits. The alpha subunit is identical to that of pituitary follitropin, pituitary lutropin and placental chorionic gonadotropin, whereas the beta subunit is unique. TSH is a glycoprotein; the glycoprotein components of the alpha and beta subunits account for more than 10% of their mass and are essential for normal thyrotropic action and intravascular kinetics. The hypothalamic tripeptide, TSH-releasing hormone (TRH) is required for optimum TSH biosynthesis, particularly as far as addition of the glycoprotein components is concerned. TRH deficiency is associated with secretion of TSH molecules that are appropriately measured in most assays but have reduced bioactivity. In previous years the TSH used in clinical practice was obtained and purified from bovine pituitaries. Bovine TSH was used to test thyroid function and to augment the uptake of radioiodine in patients with thyroid cancer. Bovine TSH has been largely abandoned as a clinical agent because of adverse immune reactions. A recombinant human TSH (rhTSH; Thyrogen), has been approved by the US FDA for diagnostic use in patients with thyroid cancer. The alpha and beta subunits of Thyrogen are identical to those of human pituitary TSH. Thyrogen has a specific activity of approximately 4 IU/mg and is a potent stimulator of T4, T3 and thyroglobulin (Tg) secretion in healthy volunteers. It also increases thyroid iodide uptake in patients with thyroid cancer or multinodular goitre and in volunteers, even those exposed to large amounts of stable iodide. Thyroid cancer patients who have been treated by thyroidectomy and radioiodine ablation but are at risk of harbouring residual thyroid cancer are candidates for Thyrogen administration to prepare them for whole body iodide scans and serum Tg measurements. In thyroidectomised thyroid cancer patients who are unable to secrete pituitary TSH upon thyroid hormone withdrawal, Thyrogen is the only acceptable method to prepare them for these procedures. Thyrogen has been used on a compassionate basis to prepare patients for radioiodine ablation. rhTSH, in addition to being useful in the management of patients with thyroid cancer, is potentially useful to test thyroid reserve and to aid in thyroid-related nuclear medicine procedures. In the future, TSH analogues that have superagonist or antagonist properties may become available as therapeutic agents.

Update on intrathyroidal iodine metabolism

The thyroid concentrates iodide from the serum and oxidizes it at the apical membrane, attaching it to tyrosyl residues within thyroglobulin (Tg) to make diiodotyrosine and monoiodotyrosine. Major players in this process are Tg, thyroperoxidase (TPO), hydrogen peroxide, pendrin, and nicotinamide adenine dinucleotide phosphate (NADPH). Further action of TPO, hydrogen peroxide (H2O2), and iodinated Tg produce thyroxine (T4) and triiodothyronine (T3). Hormone-containing Tg is stored in the follicular lumen, then processed, most commonly by micropinocytosis. The lysosomal enzymes cathepsins B, L, and D are active in Tg proteolysis. Tg digestion leaves T4 and T3 intact, to be released from the cell, while the 3,5'-diiodotyrosine (DIT) and 3-iodotyrosine (MIT) are retained and deiodinated for recycling within the thyroid. Some areas of especially active recent research include: (1) the role of molecular chaperones in directing properly folded TPO and Tg to the apical membrane; (2) details of proteolytic pathways; (3) modulation of iodine metabolism, not only by thyrotropin (TSH) but by iodine supply and by feedback effects of Tg, glutathione, and inhibitory elements in the N-terminal region of Tg; and (4) details of Tg structure and iodotyrosyl coupling. Despite general agreement on the major steps in intrathyroidal iodine metabolism, new details of mechanisms are constantly being uncovered and are greatly improving understanding of the overall process.

Role of thyroglobulin endocytic pathways in the control of thyroid hormone release

Thyroglobulin (Tg), the thyroid hormone precursor, is synthesized by thyrocytes and secreted into the colloid. Hormone release requires uptake of Tg by thyrocytes and degradation in lysosomes. This process must be precisely regulated. Tg uptake occurs mainly by micropinocytosis, which can result from both fluid-phase pinocytosis and receptor-mediated endocytosis. Because Tg is highly concentrated in the colloid, fluid-phase pinocytosis or low-affinity receptors should provide sufficient Tg uptake for hormone release; high-affinity receptors may serve to target Tg away from lysosomes, through recycling into the colloid or by transcytosis into the bloodstream. Several apical receptors have been suggested to play roles in Tg uptake and intracellular trafficking. A thyroid asialoglycoprotein receptor may internalize and recycle immature forms of Tg back to the colloid, a function also attributed to an as yet unidentified N-acetylglucosamine receptor. Megalin mediates Tg uptake by thyrocytes, especially under intense thyroid-stimulating hormone stimulation, resulting in transcytosis of Tg from the colloid to the bloodstream, a function that prevents excessive hormone release.

Binding of rat thyroglobulin to heparan sulfate proteoglycans

We previously showed that rat thyroglobulin (Tg) is a heparin-binding protein and that heparin inhibits Tg binding to megalin (gp330), an endocytic Tg receptor found on the apical surface of thyrocytes. Cooperation between cell surface receptors and heparin-like molecules, namely heparan sulfate proteoglycans (HSPGs), can facilitate cell surface binding of some heparin-binding proteins. Based on our previous findings indicating that heparin and megalin-binding sites of rat Tg are functionally related, here we investigated whether rat Tg binds to HSPGs, which are expressed by thyroid cells. We showed in solid phase assays that unlabeled rat Tg binds to a heparan sulfate (HS) preparation in a dose-dependent, saturable manner, with moderately high affinity (Kd approximately 19 nM, Ki approximately 25 nM). Binding was inhibited by heparin and by HS itself. We then studied the role of HSPGs in Tg binding to FRTL-5 cells, a differentiated Fisher rat thyroid cell line. As previously reported, after incubation of FRTL-5 cells with unlabeled rat Tg at 4 degrees C, heparin released virtually all the cell-bound Tg. Co-incubation of Tg with HS or with a preparation of HSPGs resulted in a reduction of binding by 35%-40%. When FRTL-5 cells were preincubated with heparitinase or heparinase I, which released 20%-30% of cell surface HSPGs, Tg binding was reduced to a similar extent. An antibody against a Tg heparin-binding site functionally related to a major megalin-binding site virtually abolished Tg binding to HS and to FRTL-5 cells, supporting the hypothesis that combined interactions of Tg with HSPGs and with megalin are involved in Tg binding to rat thyroid cells.

Role of megalin (gp330) in transcytosis of thyroglobulin by thyroid cells. A novel function in the control of thyroid hormone release

When thyroglobulin (Tg) is endocytosed by thyrocytes and transported to lysosomes, thyroid hormones (T4 and T3) are released. However, some internalized Tg is transcytosed intact into the bloodstream, thereby avoiding proteolytic cleavage. Here we show that megalin (gp330), a Tg receptor on thyroid cells, plays a role in Tg transcytosis. Following incubation with exogenous rat Tg at 37 degrees C, Fisher rat thyroid (FRTL-5) cells, a differentiated thyroid cell line, released T3 into the medium. However, when cells were incubated with Tg plus either of two megalin competitors, T3 release was increased, suggesting that Tg internalized by megalin bypassed the lysosomal pathway, possibly with release of undegraded Tg from cells. To assess this possibility, we performed experiments in which FRTL-5 cells were incubated with either unlabeled or (125)I-labeled Tg at 37 degrees C to allow internalization, treated with heparin to remove cell surface-bound Tg, and further incubated at 37 degrees C to allow Tg release. Intact 330-kDa Tg was released into the medium, and the amount released was markedly reduced by megalin competitors. To investigate whether Tg release resulted from transcytosis, we studied FRTL-5 cells cultured as polarized layers with tight junctions on permeable filters in the upper chamber of dual chambered devices. Following the addition of Tg to the upper chamber and incubation at 37 degrees C, intact 330-kDa Tg was found in fluids collected from the lower chamber. The amount recovered was markedly reduced by megalin competitors, indicating that megalin mediates Tg transcytosis. We also studied Tg transcytosis in vivo, using a rat model of goiter induced by aminotriazole, in which increased release of thyrotropin induces massive colloid endocytosis. This was associated with increased megalin expression on thyrocytes and increased serum Tg levels, with reduced serum T3 levels, supporting the conclusion that megalin mediates Tg transcytosis. Tg transcytosis is a novel function of megalin, which usually transports ligands to lysosomes. Megalin-mediated transcytosis may regulate the extent of thyroid hormone release.

Identification of a heparin-binding region of rat thyroglobulin involved in megalin binding

We recently showed that thyroglobulin (Tg) is a heparin-binding protein and that heparin inhibits binding of Tg to its endocytic receptor megalin (gp330). Here we have identified a heparin-binding region in the carboxyl-terminal portion of rat Tg and have studied its involvement in megalin binding. Rat thyroid extracts, obtained by ammonium sulfate precipitation, were separated by column fractionation into four Tg polypeptides, with apparent masses of 660, 330, 210, and 50 kDa. As assessed by enzyme-linked immunoadsorbent assays and ligand blot binding assays, megalin bound to intact Tg (660 and 330 kDa) and, to a even greater extent, to the 210-kDa Tg polypeptide. Furthermore, the 210-kDa Tg polypeptide inhibited megalin binding to intact Tg by approximately 70%. Solid phase assays showed binding of biotin-labeled heparin to intact Tg and to the 210-kDa Tg polypeptide. We characterized the 210-kDa Tg polypeptide by matrix-assisted laser desorption/ionization mass spectrometry analysis and found that it corresponds to the carboxyl-terminal portion of rat Tg. We developed a synthetic peptide corresponding to a 15-amino acid sequence in the carboxyl-terminal portion of rat Tg (Arg(689)-Lys(703)), containing a heparin-binding consensus sequence (SRRLKRP) and demonstrated heparin binding to this peptide. A rabbit antibody raised against the peptide recognized intact Tg in its native conformation and under denaturing conditions. This antibody markedly reduced heparin-binding to intact Tg, indicating that the region of native Tg corresponding to the peptide is involved in heparin binding. Furthermore, the anti-Tg peptide antibody almost completely inhibited binding of megalin to Tg, suggesting that the Tg region containing the peptide sequence is required for megalin binding. Physiologically, Tg binding to megalin on thyroid cells may be facilitated by Tg interaction with heparin-like molecules (heparan sulfate proteoglycans) via adjacent binding sites.

Selective endocytosis of thyroglobulin: a review of potential mechanisms for protecting newly synthesized molecules from premature degradation

In 1976 Cortese, Schneider and Salvatore (Eur. J. Biochem. 68 (1976) 121-129) showed that the thyroid gland protects newly synthesized, iodine and hormone poor thyroglobulin from immediate degradation. Since then there has been substantial progress in understanding the mechanism by which this selectivity of degradation occurs. Thyroglobulin in the follicular lumen is internalized mainly by receptor-specific endocytosis. Recycling of immature, poorly iodinated thyroglobulin back to the follicular lumen is the pathway most likely responsible for selectivity. Since additional carbohydrate groups are added to the immature thyroglobulin, it appears that this recycling occurs via the Golgi compartment. The molecular signal for recycling most likely involves the complex carbohydrates and probably is exposed GlcNAc groups. A thyroid-specific GlcNAc receptor has been identified and cloned. Other Tg-binding sites have been identified in the thyroid, but their physiological role remains to be determined.

Megalin (gp330) is an endocytic receptor for thyroglobulin on cultured fisher rat thyroid cells

We recently reported that megalin (gp330), an endocytic receptor found on the apical surface of thyroid cells, binds thyroglobulin (Tg) with high affinity in solid phase assays. Megalin-bound Tg was releasable by heparin. Here we show that Fisher rat thyroid (FRTL-5) cells, a differentiated rat thyroid cell line, can bind and endocytose Tg via megalin. We first demonstrated that FRTL-5 cells express megalin in a thyroid-stimulating hormone-dependent manner. Evidence of Tg binding to megalin on FRTL-5 cells and on an immortalized rat renal proximal tubule cell line (IRPT cells), was obtained by incubating the cells with 125I-Tg, followed by chemical cross-linking and immunoprecipitation of 125I-Tg with antibodies against megalin. To investigate cell binding further, we developed an assay in which cells were incubated with unlabeled Tg at 4 degrees C, followed by incubation with heparin, which released almost all of the cell-bound Tg into the medium. In solid phase experiments designed to illuminate the mechanism of heparin release, we demonstrated that Tg is a heparin-binding protein, as are several megalin ligands. The amount of Tg released by heparin from FRTL-5 and IRPT cells, measured by enzyme-linked immunosorbent assay (ELISA), was markedly reduced by two megalin competitors, receptor-associated protein (RAP) and 1H2 (monoclonal antibody against megalin), indicating that much of the Tg released by heparin had been bound to megalin ( approximately 60-80%). The amount inhibited by RAP was considered to represent specific binding to megalin, which was saturable and of high affinity (Kd approximately 11.2 nM). Tg endocytosis by FRTL-5 and IRPT cells was demonstrated in experiments in which cells were incubated with unlabeled Tg at 37 degrees C, followed by heparin to remove cell-bound Tg. The amount of Tg internalized (measured by ELISA in the cell lysates) was reduced by RAP and 1H2, indicating that Tg endocytosis is partially mediated by megalin.

Searching for pathogenic epitopes in thyroglobulin: parameters and caveats

Last year marked the 40th anniversary of the discovery that thyroglobulin (Tg) is a major autoantigen in autoimmune thyroiditis. The Tg molecule presents unique challenges for epitope mapping owing to its large size and extensive iodination. Consequently, pathogenic determinants have only recently been identified. Here, George Carayanniotis and Varada Rao summarize the approaches used to determine pathogenic Tg T-cell epitopes and discuss caveats in this unusual quest.

Morphology of thyroid follicular cells from adult marmosets (Callithrix jacchus)

The existence of follicles with a "super flat" epithelium and the large number of primary and especially secondary lysosomes and lipofuscin granules in these cells are characteristic features of the thyroid gland of adult marmosets (Callithrix jacchus, new-world monkeys). It is supposed that these morphological findings are indications of a relatively low function and early ageing.

Endocytosis of thyroglobulin is not mediated by mannose-6-phosphate receptors in thyrocytes. Evidence for low-affinity-binding sites operating in the uptake of thyroglobulin

Thyroglobulin, the major secretory product of thyrocytes, is the macromolecular precursor of thyroid hormones. After its synthesis, thyroglobulin follows a complex secretion, storage and recapture pathway to lysosomes. Porcine thyroglobulin was shown to carry the mannose 6-phosphate-(Man6P)-recognition marker on its N-linked glycans. Since the cation-independent Man6P receptor could also be found on the apical plasma membrane of porcine thyrocytes, we examined the significance of the Man6P signal for the transport of thyroglobulin. Here, we present data implying that Man6P receptors are not relevant for endocytosis of thyroglobulin in thyrocytes. Instead, we provide evidence for the existence of specific, low-affinity-binding sites for thyroglobulin on the apical plasma membrane of thyrocytes responsible for endocytosis of thyroglobulin. Binding studies with intact, polar-organized porcine thyrocytes grown on collagen-coated filters revealed cooperative and saturable binding of thyroglobulin to the apical-plasma-membrane domain at relatively high concentrations of thyroglobulin (20 microM). These observations show that low-affinity interactions between thyroglobulin and the apical plasma membrane play a key role in endocytosis of thyroglobulin and hormone formation in the thyroid. The data in this publication have been published as an abstract [Lemansky, P. and Herzog, V. (1991) J. Cell Biol. 115, 261a].