This thesis sets out to identify physical and physiological differences between lambs of different birth ranks at birth, and to use this information to identify practical on-farm management strategies which could improve triplet-born lamb survival. Triplet-born lambs, especially the lightest-triplet-born lambs, not only had a greater capacity to lose heat but also had a reduced capacity to produce heat when compared to twin-born lambs. Due to their lighter birth weights, triplet-born lambs had lower plasma thyroxine (T4) concentrations within the first 24 hours of life than twin-born lambs, and within twin- and triplet-born litters, the lightest- and medium-triplet-born lambs had greater plasma lactate concentrations than all twin-born lambs and the heaviest-triplet-born lambs. Independent of lamb birth weight, triplet-born lambs had lower plasma tri-iodothyronine (T3) concentrations within five minutes of birth, and within twin- and triplet-born litters, the lightest- and medium-triplet-born lambs had lower plasma T4 and T3 concentrations within five minutes of birth than all twin-born lambs and the heaviest-triplet-born lambs. It was hypothesised that because triplet-born lambs had a lighter birth weight and lower plasma thyroid hormone concentrations, they would have inadequate thermoregulatory capabilities when compared to twin-born lambs. The lower rectal temperatures of triplet-born lambs within the first hour of life and the lower heat production on a per lamb basis at 24 to 36 hours of age, and the lack of difference in maximum heat production on a per kg of birth weight basis at 24 to 36 hours of age support this hypothesis. Two practical on-farm management strategies trialled in this thesis to improve triplet-born lamb thermoregulation were offering concentrate supplement during late pregnancy to improve lamb birth weights, and maternal iodine supplementation to improve lamb plasma thyroid hormone concentrations. While offering concentrate showed positive effects such as increasing lamb birth weights, colostrum uptake and triplet-born lamb heat production on a per kg of birth weight basis, the results were either inconsistent across experiments or between birth ranks suggesting additional work is required to determine the repeatability and cost effectiveness of these findings. Maternal iodine supplementation offered no iv | P a g e benefits in terms of lamb birth weights, plasma thyroid hormone concentrations or lamb heat production. Further investigations identified that lamb birth weights, thyroid hormone concentrations, glucose and NEFA concentrations are positively associated with maximum heat production at 24 to 36 hours of age. Practical on-farm management strategies which could target these physical and physiological factors may improve triplet-born lamb heat production, and therefore the survival rates of triplet-born lambs.

Dissertation (Ph.D.)--University of South Florida, 2009. / Includes vita. Includes bibliographical references. Also available online.

Evidências sugerem interação entre o hormônio tireoideano (T3) e os fatores de crescimento fibroblásticos (FGF) no desenvolvimento esquelético. Para estudarmos essa interação, avaliamos o efeito do T3 e do seu antagonista NH3 em células mesenquimais periósticas humanas de pacientes normais e portadores da Síndrome de Apert (SA), que é caracterizada por craniossinostose e causada por mutações no receptor de FGF tipo 2 (FGFR2). Nas células SA, o T3 aumentou o número de células e o NH3 bloqueou esse efeito do T3. O T3 e/ou NH3 aumentaram a atividade da fosfatase alcalina durante a diferenciação osteoblástica das células normais, mas não das mutadas. O T3 aumentou a diferenciação osteoblástica e o NH3 bloqueou esse efeito do T3 em células normais. Nas células mutadas, o NH3 limitou a diferenciação osteoblástica, enquanto o T3 não teve efeito. Concluímos que as células mesenquimais periósticas humanas normais e SA são responsivas ao T3 e NH3, e que o T3 e FGF podem atuar através de vias de sinalização comuns na regulação da diferenciação osteoblástica. / Evidence suggests that there is an interaction between the thyroid hormone (T3) and fibroblast growth factors (FGFs) in the skeletal development. To study this interaction, we evaluated the effect of T3 and its antagonist, NH3, in human periosteal mesenchymal cells from normal and Apert Syndrome (AS) patients, which is characterized by craniosynostosis and is caused by mutations in FGF receptor type 2 (FGFR2). In AS cells, the T3 increased the number of cells and NH3 blocked this effect of T3. T3 and/or NH3 increased the alkaline phosphatase activity in osteoblast differentiation of normal cells, but not in the mutated cells. T3 increased osteoblast differentiation and NH3 blocked this effect of T3 on normal cells. In the mutated cells, NH3 limited osteoblast differentiation while T3 had no effect. We concluded that normal and AS human periosteal mesenchymal cells are responsive to T3 and NH3, and T3 and FGF may act through common signaling pathways in the regulation of osteoblastic differentiation.

O presente estudo avaliou aspectos relacionados ao contexto inflamatório na hipertrofia cardíaca induzida pelos hormônios tiroideanos (HT) e o possível envolvimento do sistema renina-angiotensina (SRA) nesse processo, utilizando análises in vivo e com enfoque maior na abordagem in vitro. Os resultados mostraram algumas alterações em citocinas circulantes e cardíacas de animais tratados com HT. Além disso, as expressões de S100A8 e MyD88 foram aumentadas no coração de ratos submetidos ao hipertiroidismo e em cardiomiócitos em cultura estimulados com HT. S100A8 e MyD88 mediaram a ativação do fator nuclear NF-κB pelos HT, tendo papel crucial para o crescimento hipertrófico de cardiomiócitos tratados com HT. Por fim, a ação dos HT modulando a expressão de S100A8 e NF-κB foi mediada pelo SRA. Estes dados contribuem com o entendimento das bases moleculares da ação dos HT e da relação deste com o SRA na hipertrofia cardíaca. / The present study evaluated inflammation related aspects in cardiac hypertrophy induced by thyroid hormones (TH) and the possible involvement of the renin-angiotensin system (RAS) in this process, by using in vivo and in vitro analysis. The results showed alterations in circulating and cardiac cytokines from TH treated animals. The expression of S100A8 and MyD88 were increased in the heart of hyperthyroid rats and in cultured cardiomyocytes stimulated with TH. S100A8 and MyD88 mediated the nuclear factor NF-κB activation by TH and these factors presented crucial role to the hypertrophic growth of TH-treated cardiomyocytes. Finally, the action of TH on S100A8 and NF-κB expression was mediated by RAS. These data contribute to the knowledge of molecular basis of TH action and the relationship between TH and RAS in cardiac hypertrophy.

La métamorphose est une étape cruciale du cycle de vie de beaucoup de métazoaires. Elle est définie comme la transition d’une larve en un juvénile, souvent accompagnée de changements drastiques de morphologie, physiologie et de niche écologique. Le rôle des hormones thyroïdiennes a été mis en évidence dans la métamorphose des amphibiens, mais aussi de nombreux autres chordés comme certains poissons téléostéens ou l'amphioxus, suggérant que ce système était déjà présent et impliqué dans la métamorphose chez l'ancêtre commun de tous les chordés.Nous avons étudié le lien entre hormones thyroïdiennes et métamorphose selon deux axes. Premièrement, nous avons étudié la coordination entre la métamorphose et le recrutement larvaire, chez le chirurgien bagnard Acanthurus triostegus. Ce travail nous a permis de mieux comprendre le rôle de l’hormone thyroïdienne comme signal déclencheur d'une transition écologique importante. Dans le second axe de travail nous avons examiné l’origine de la signalisation thyroïdienne. Chez l’annélide Platynereis dumerili, nous avons identifié un récepteur fonctionnel des hormones et un rôle de l’hormone thyroïdienne dans son développement. Cela qui nous permet de démontrer que la signalisation thyroïdienne était présente à l’origine des bilateriens. Nous avons également mis en évidence un rôle des hormones thyroïdiennes dans le développement de cette espèce. Enfin dans le troisième axe nous nous sommes penchés sur l’origine de la synthèse de l’hormone thyroïdienne en retraçant l’évolution de la thyroglobuline, la protéine nécessaire à la production d’hormone chez les vertébrés. Son absence dans les autres taxons, alors que les dérivés de l’hormone y ont un rôle biologique, pose la question des mécanismes ancestraux de synthèse de ces hormones.Ces travaux explorent l’évolution de la signalisation thyroïdienne et proposent d’aborder la question du lien avec la métamorphose d’un point de vue évolutif et non-seulement développemental, afin de mieux comprendre la diversité des métamorphoses observées dans le monde animal. / Metamorphosis is a critical life step of many metazoans. It is defined as the transition between a larva and a juvenile. It comes with major changes of morphology, physiology and ecology. The role of thyroid hormone has been proven in the metamorphosis of amphibians but also in many other chordates such as teleost fishes or the amphioxus. This suggests that this system was functional and involved in the metamorphosis of the common ancestor of all chordates.We studied the link between thyroid hormones and metamorphosis according to two axes. First, we studied the coordination between metamorphosis and the larval colonization, using the convicted surgeon fish Acanthurus triostegus as a model. This work allowed us to better understand the role of thyroid hormones as a trigger signal of a major ecological transition. On the second axis, we investigated the origin of thyroid hormone signalization. In the annelid Platynereis dumerilii, we identified a functional thyroid hormone receptor and a role of TH in its development. This proves that thyroid hormone signaling was present at the basis of bilaterians. We also assessed the question of the origin of thyroid hormone synthesis by tracing back the evolution of the thyroglobulin, the protein mandatory for thyroid hormone synthesis in vertebrates. It absence in the other taxa, whereas thyroid hormone derivatives have a biological role, asks the question of the ancestral mechanisms of thyroid hormone synthesis.These works investigate the evolution of the thyroid hormone signalization and suggest to tackle the question of the link with metamorphosis from an evolutionary perspective, and not only from a developmental one, in order to better understand the diversity of metamorphosis overserved in the animal kingdom.

Thyroid hormones (THs; T4 and T3), secreted from thyroid gland, play an important role in human growth and development. T3 (3,5,3′-triiodothyronine) is the active hormone and the conversion of T4 (3,3′,5,5′-tetraiodothyronine) to T3 in cells is mediated by iodothyronine deiodinases enzymes (DIOs). DIOs are selenocysteine containing enzymes and are classified into three types (DIO1, DIO2 and DIO3). DIO1 catalyzes the outer-ring deiodination (ORD; T3 formation) and inner-ring deiodination (IRD; rT3 formation) reactions, involving in the activation (T4 to T3 conversion) and inactivation (T4 to rT3 conversion), respectively. DIO2 and DIO3 catalyse the ORD and IRD reactions, respectively. This homeostasis is regulated tightly and any deviation would lead to diseases like hyperthyroidism or hypothyroidism. Recently it is of interest to many research groups to develop iodothyronine deiodinase mimics and we have developed naphthalene-based peri-substituted thioselenol pair at 1,8-positions (1.25), which remove iodine selectively from inner-ring of T4. When selenium atom is substituted in place of sulfur (selenol-selenol pair; 1.26), the deiodination activity was ca. 90 times faster than with 1.25. This thesis deals with various aspects of the effect of substituents on the naphthalene-1,8-diselenol and solvent effect on the thyroid hormone deiodination by naphthalene-based iodothyronine deiodinase mimics. Figure 1. (A) Deiodination reactions by DIOs. (B) Chemical structure of 1.25 and 1.26. The thesis consists of five chapters. The first chapter provides a general overview about sialoproteins, thyroid hormone biosynthesis, thyroid hormone metabolism, halogen bonding, iodothyronine deiodinase mimics and proposed mechanisms for the deidoination of thyroid hormones. This chapter also introduces peri-naphthalene-1,8-diselenol (1.26), which is the key compound in this thesis and discusses about proposed mechanism for the deiodination of thyroxine involving co-operative halogen bonding and chalcogen bonding mechanism. Figure 2. (A) TH action. (B) Proposed mechanism for the deiodination of T4 by 1.26 involving cooperative halogen bonding and chalcogen bonding. Chapter 2 discusses about the synthesis, characterization and deiodination activity of a series of naphthalene-based peri-substituted-1,8-diselenols (Figure 3). These diselenols regioselectivity remove iodine from inner ring of thyroxine and other thyroid hormones, (T3 and 3, 5-T2). Substitution with different groups on the naphthalene ring did not change the regioselectivity of deiodination, indicating that the deiodination activity does not depend on the nature of substituents. Secondary or tertiary amine side chain group attached at the 2nd position of the naphthalene ring showed better activity. It is due to the secondary interaction, which facilitates the iodine removal. It was further confirmed with the substitutions at the 4th position of the ring to discriminate the possibility of electronic effect. The higher deiodination rate owing to the t-butyl group at second position of the ring also suggests that the steric effect may also play a role in the deiodination reaction (Figure 4). It is proposed that peri substituted naphthalene-1,8-diselenols remove iodine from thyroid hormones through halogen bonding-chalcogen bonding mechanism (Figure 2). The investigation of Se···Se bond distance from the crystal structures and through DFT calculation and NMR experiment showed that the stronger chalcogen bond could be the reason for the increase in the reactivity observed with substituted peri-naphthalene-1,8-diselenols. Figure 3. peri-substituted naphthalene-1,8-diselenols used for the study. Figure 4. Relative deiodinase activity of substituted-peri-naphthalene-1,8-diselenols with T4. In Chapter 3, we have discussed about the effect of chalcogen atom substitution in a series of deiodinase mimics on the deiodination of thyroid hormones. Moving from thiol-selenol pair (1.25) to selenol-selenol pair (1.26) in naphthalene based peri-substituted mimics, an increase in the activity was observed. In this chapter, we have shown that substituting with tellurium, as tellurium-thiol pair (3.3) and ditellurol (3.4) increases the reactivity of deiodination to several times and also regioselectivity of deiodination is changed from IRD in the case of 1.26 to both IRD and ORD for 3.3 and 3.4. The presence of two tellurol moieties (3.4) or a thiol-tellurol pair (3.3) can mediate sequential deiodination of T4, to produce all the possible thyroid hormone derivatives under physiologically relevant conditions (Figure 5). This study provided the first experimental evidence that the regioselectivity of the thyroid hormone deiodination is controlled by the nucleophilicity and the strength of halogen bond between the iodine and chalcogen atoms. Figure 5. (A) HPLC chromatograms of deiodination reaction of T4 with 3.3 and 3.4. (B) Chemical structure of 3.3 and 3.4. (C) Sequential deiodination reaction of T4 by 3.3 and 3.4. Chapter 4 describes the effect of alkyl conjugation at 4′-OH position of THs on the deiodination by iodothyronine mimics. In addition to the deiodination, iodothyronines undergo conjugation with sulfate and glucuronic acid group at 4′-hydroxyl position. Conjugation alters the physico-chemical properties of iodothyronines. For example, it is known that sulfate conjugation increases the rate of deiodination to a large extend. We have conjugated alkyl group at 4′-hydroxyl position of iodothyronines and investigated the deiodination reactions with reported peri-substituted naphthalene-1,8-diselenols. We observed that similar to sulfated thyroid hormones O-methylthyroxine also undergoes both phenolic and tyrosyl ring deiodination reactions and overall the rate of deiodination is increased at least by 5 times as compared with T4 under identical conditions. The phenolic iodine removal is favored by conjugation as compared to the tyrosyl ring iodine, which is similar to the observation made for T4S. Interestingly, when the acetamide group is conjugated at 4′-OH position, the regioselectivity of deiodination is changed exclusively to 5′-iodine. DFT calculations show that the positive potential on the iodine increase upon conjugation, which leads to stronger halogen bonding interaction with selenol, might be the reason for the change in the regioselectivity of deiodination. Figure 6. (A) HPLC chromatogram of deiodination reaction of T4(Me) with 1.26. (B) Initial rate comparison of T4 and T4(Me).(C) HPLC chromatogram of deiodination reaction of T4(AA) with 1.26 showing the formation of T3(AA) (ORD product). (D) Electron potential map of T4, T4(Me) and T4(AA) showing the increase in electro positive potential on 5′-iodine upon conjugation. Chapter 5 deals with the solvent effect on the deiodination reactions of THs by iodothyronine deiodinase mimics. As discussed in the earlier chapters, the deiodination reaction of thyroxine by naphthalene based-1,8-diselenols under physiological conditions produce, rT3 (IRD) as the only observable products. Surprisingly, when the deiodination reaction was performed in DMF or DMSO in the presence of 1.26, the regioselectivity of reaction was changed and the formation of both T3 (ORD) and rT3 was observed. In DMF or in DMSO, the deiodination reactivity of 1.26 was found to be 1000 fold higher than the reaction performed in phosphate buffer at pH 7.4. Figure 7. (A) HPLC chromatogram for the deiodination reaction of T4 in DMF by 1.26 showing both IRD and ORD. (B) A comparison of initial rate for the deiodination reactions of T4, T3 and 3,5-T2 in DMF and in DMSO by 1.26. (C) HPLC chromatograms for the deiodination reaction of T4 in DMF by 1.26 in the presence of TEMPO, showing the inhibition of deiodination (i) 0 mM TEMPO (ii) 10 mM of TEMPO (iii) 30 mM TEMPO. (D) HPLC chromatograms for the deiodination reaction of T4 in DMSO by 1.26 in the presence of TEMPO showing the inhibition of deiodination (i) 0 mM TEMPO (ii) 10 mM of TEMPO (iii) 30 mM TEMPO. 3,5-DIT was not denominated under physiological conditions, however, in DMF and in DMSO, 3,5-DIT was deiodinated by 2.4 to produce 3-MIT. We also observed that the control reactions in DMF or DMSO also showed a little deiodination activity. The very high reactivity observed in the presence of DMF or DMSO implied that the mechanism of denomination in these solvents may be different. It has been reported that DMSO or DMF radicals can be formed with small amounts of a base. Reaction mixture consisting of NaBH4 (for generating selenol from diselenide) and NaOH (T4 solution) may facilitate the radical formation. We also performed the reaction in the presence of TEMPO (free radical scavenger) and observed the inhibition of deiodination reaction. However, it is not clear whether the radical pathway could be one of the possible mechanisms of deiodination in these solvents by compounds 1.26 and 2.4. Further studies are required to propose a radical mechanism in different solvents such as DMF and DMSO.

No description available.

In recent times, endocrine disrupting chemicals (EDCs) have been associated with the rise in neurodevelopmental disorders such as autism, attention deficit hyperactivity disorder (ADHD) and decreased intelligence quotient (IQ) in children. This effect is suspected to be induced at pre-/peri-natal development, via an alteration in hormonal signaling, thus interfering with neuronal differentiation, with subsequent effect on normal brain development and function in exposed children. This issue increases the need for chemical screening for potential developmental neurotoxicity (DNT) effect. The current available EDC induced DNT test guideline is based on in-vivo testing that requires animal use. Here, a multipotent neural progenitor cell line, the C17.2 cell-line, generated from neural stem cells of the external germinal layer of mouse cerebellum, with potential to differentiate to neurons or astrocytes, is introduced for in-vitro EDC induced DNT testing. This project focused on optimizing the C17.2 cell-line for the detection of EDC-induced DNT with emphasis on the disruption of the oestrogen, androgen, and thyroid hormone systems. It aimed at validating the involvement of oestrogen, androgen, and thyroid hormone on molecular and cellular endpoints relevant for the differentiation of the C17.2 cells.  Herein, the cells were exposed to the hormonal agonist and antagonist at a range of concentrations for a 10-day differentiation period. After exposure, LDH, viability assay and morphological changes (percentage of neurons in culture and neurite outgrowth) were evaluated. The results showed no morphological changes induced by androgen receptor (AR) agonist/antagonist at relevant physiological concentrations. The thyroid receptor (TR) agonist and antagonist on the other hand showed a response in the form of increased neurite outgrowth in relation to the negative control at a concentration range of 40-200 nM and 40 nM respectively. The oestrogen receptor (ER) antagonist at 100 nM also increased percentage neuron in culture. Additionally, in-silico analysis of microarray and RNA sequencing data were used to map out target genes regulated by ER, AR and TR and involved in neurodevelopment. With this approach, 29 marker genes were identified. Validation of the marker genes by means of gene expression (qPCR) was carried out, ER and TR agonist/antagonist were observed to modulate the expression of examined genes. In summary, the model could not be established for detecting EDC induced DNT via androgenic and oestrogenic pathway, while it is a promising model for identifying DNT induced by thyroid hormone signalling disruption.

Dispatched 3 (Disp3), a thyroid hormone-regulated gene, is studied extensively in our laboratory. Phenotype of cells with overexpressed Disp3 and its expression pattern make it a perfect candidate for a molecular link between thyroid hormone action and cholesterol homeostasis in the brain. Moreover, we hypothesize that it might play a role in certain neurodegenerative disorders and brain tumours. This thesis is aimed at the process of regulation of this gene via thyroid hormone receptor (TR), specifically identification of responsive elements of the thyroid hormone receptor that are necessary for the regulation. Also, we searched for elements recognized by liver X receptor (LXR), as LXR binds to the same arrangement of repeats as TR and there are a number of genes regulated by both of them. We combined in silico analysis of the Disp3 locus with reporter luciferase assays. A cluster of six elements identified around the first exon with two of them being conserved among human and mice draw our attention. In order to analyze this sequence in more detail, reporter vectors of various truncations of 3 kb region around exon 1 were constructed and tested in reporter assays. Reporter assays did not reveal any substantial element activated by TR or LXR; on the other hand, region containing repressor element(s)...

Schilddrüsenhormone (TH) regulieren Metabolismus und Energiestoffwechsel. Der TH‐Metabolit (THM) 3,5‐T2 (3,5‐Diiod‐L‐Thyronin) aktiviert Fett‐Oxidation und mitochondriale Atmung. Der THM 3‐Iodothyronamin (3‐T1AM) beeinflusst zusätzlich glukoregulatorische Prozesse. THM können zur Reduktion von Körperfett beitragen. Um 3,5‐T2 im humanen Serum nachzuweisen sollte ein Immunoassay aufgebaut, validiert und angewendet werden. In intakten hepatozellulären (HepG2) sowie pankreatischen ß‐Zellen (MIN6) sollte untersucht werden ob THM durch Modulation der mitochondrialen Aktivität die zelluläre Substratverstoffwechslung (3,5‐T2) und Insulinsekretion (3‐T1AM) regulieren können. Der Immunoassay ist sensitiv, spezifisch und misst zuverlässig 3,5‐T2 im humanen Serum. Hyper‐ und Hypothyreose zeigen vergleichbare 3,5‐T2 Konzentrationen, jedoch akkumuliert 3,5‐T2 bei sekundären Erkrankungen der Schilddrüse und athyreoten Patienten unter Thyroxin‐Supplementation. In HepG2‐Zellen konnte die Aktivierung der mitochondrialen Atmung durch 3,3‘,5‐Triiod‐L‐Thyronin (T3), jedoch nicht durch 3,5‐T2 stimuliert werden. Die Expression von TH‐transporters (THT) war gering verglichen mit Maus‐Hepatozyten. MIN6 exprimiert THT vergleichbar mit Langerhansschen Inselzellen der Maus. 3‐T1AM wird in die Zelle aufgenommen, zu 3‐Iodothyroessigsäure (TA1) metabolisiert, und wieder exportiert. Nach 3‐T1AM Gabe ist die mitochondriale ATP‐Produktion sowie die Glukose‐stimulierte Insulinsekretion (GSIS) vermindert. 3,5‐T2 zirkuliert in euthyreoten Individuen, ist nicht an der zentralen Regulation der TH‐Achse beteiligt, wird extrathyroidal gebildet und niedrige T3‐Werte können durch erhöhtes 3,5‐T2 erklärt werden. HepG2 erwies sich als ungeeignetes Zellmodell, da wenige THT vorhanden sind, 3,5‐T2 die Plasmamembran wahrscheinlich nicht passieren kann und damit die Aktivierung der Mitochondrien aus bleibt. In MIN6 wurde gezeigt, dass die GSIS nicht ausschließlich an der Plasmamembran durch 3‐T1AM reguliert wird. / Thyroid hormones (TH) regulate metabolism and energy metabolism. The TH‐metabolite (THM) 3,5‐T2 (3,5‐diiodo‐L‐thyronine) activates fat oxidation and mitochondrial respiration. The THM 3‐T1AM (3‐iodothyronamine) influences in addition glucoregulatory processes. THM may support reduction in body fat mass. It was the idea to establish, validate and apply an immunoassay to determine 3,5‐T2 in human serum. Using intact hepatocellular (HepG2) as well as pancreatic ß‐cells (MIN6) it should be tested if THM can modulate mitochondrial activity, resulting in increased cellular substrate usage (3,5‐T2) as well as decreased insulin secreation (3‐T1AM). The established immunoassay is sensitive, specific and detects precisely 3,5‐T2 in human serum. Hyper‐ and hypothyroidism shows similar 3,5‐T2 concentrations, although 3,5‐T2 accumulates in secondary thyroidal illness as well as in athyreotic patients under thyroxine‐supplementation. Using HepG2 cells, mitochondrial respiration was stimulated by 3,3‘,5‐triiodo‐L‐thyronine (T3), but 3,5‐T2 had no effect. Expression of TH‐transporters (THT) was low compared to murine hepatocytes. In contrast, MIN6 express THT comparable to murine Langerhans islets. 3‐T1AM is taken up by the cell, metabolized to 3‐iodothyroacetic acid (TA1) and following export. After 3‐T1AM application mitochondrial ATP‐production as well as glucose‐stimulated insulin secretion (GSIS) was reduced. 3,5‐T2 circulates in euthyroid individuals, is not involved in central regulation of TH‐axis, is produced extrathyroidally and low T3 values can be explained by increased 3,5‐T2. HepG2 was shown to be an inappropriate cellmodel, because THT are merely expressed, suggesting that 3,5‐T2 is not able to pass the plasma membrane, thereby preventing mitochondrial activation. In addition, it was shown in MIN6 cells, that GSIS is not exclusively regulated at the plasma membrane level via 3‐T1AM.