Special features of Graves' disease in early childhood

Bone development in fetuses with intrauterine growth restriction caused by maternal endocrine-metabolic dysfunctions

Intrauterine growth restriction (IUGR) affects a large proportion of infants, particularly in underdeveloped countries. Among the main causes of IUGR, maternal endocrine-metabolic dysfunction is highlighted, either due to its high incidence or due to the severity of the immediate and mediated changes that these dysfunctions cause in the fetus and the mother. Although the effects of endocrine and metabolic disorders have been widely researched, there are still no reviews that bring together and summarize the effects of these conditions on bone development in cases of IUGR. Therefore, the present literature review was conducted with the aim of discussing bone changes observed in fetuses with IUGR caused by maternal endocrine-metabolic dysfunction. The main endocrine dysfunctions that occur with IUGR include maternal hyperthyroidism, hypothyroidism, and hypoparathyroidism. Diabetes mellitus, hypertensive disorders, and obesity are the most important maternal metabolic dysfunctions that compromise fetal growth. The bone changes reported in the fetus are, for the most part, due to damage to cell proliferation and differentiation, as well as failures in the synthesis and mineralization of the extracellular matrix, which results in shortening and fragility of the bones. Some maternal dysfunctions, such as hyperthyroidism, have been widely studied, whereas conditions such as hypoparathyroidism and gestational hypertensive disorders require further study regarding the mechanisms underlying the development of bone changes. Similarly, there is a gap in the literature regarding changes related to intramembranous ossification, as most published articles only describe changes in endochondral bone formation associated with IUGR. Furthermore, there is a need for more research aimed at elucidating the late postnatal changes that occur in the skeletons of individuals affected by IUGR and their possible relationships with adult diseases, such as osteoarthritis and osteoporosis.

Vitamin D inhibits bone loss in mice with thyrotoxicosis by activating the OPG/RANKL and Wnt/β-catenin signaling pathways

OBJECTIVE

Vitamin D and thyroid hormones have crucial roles in bone metabolism. This study aims to explore the effects of vitamin D on bone metabolism in mice with thyrotoxicosis and its mechanisms.

METHODS

12-week-old mice were randomly divided into 6 groups (6 mice/group), the control (CON) group, vitamin D (VD) group, low-dose LT4 (Low LT4) group, low-dose LT4+VD (Low LT4+VD) group, high-dose LT4 (High LT4) group, high-dose LT4+VD (High LT4+VD) group, LT4 was provided every day and vitamin D3 every other day for 12 weeks. Thyroid function, 25-hydroxy vitamin D, type I collagen carboxy-terminal peptide (CTX), and type I procollagen amino-terminal peptide were determined. In addition, microcomputed tomography, bone histology and histomorphometry, a three-point bending test, and the mRNA expression of osteoprotegerin (OPG), receptor activator of nuclear factor-κB ligand (RANKL) and β-catenin in bone were conducted.

RESULTS

The BMD of lumbar vertebrae and femur decreased and the bone microstructure was destroyed significantly in thyrotoxicosis mice. Addition of vitamin D improved the BMD and bone microstructure only in the low LT4+VD group. Mice with thyrotoxicosis had a significantly higher level of CTX (P<0.05), which was decreased by treatment with vitamin D (P<0.05). The eroded surface per bone surface (Er. S/BS) of the cancellous bone and elongated surface/endocortical perimeter (Er. S/E Pm) of the cortical bone significantly increased in the Low LT4 and High LT4 groups (P<0.05). Treatment with vitamin D significantly decreased the Er. S/BS and Er. S/E Pm. But, treatment with vitamin D did not significantly improve the toughness and rigidity of bones. The ratio of OPG to RANKL and mRNA expression of β-catenin in the Low LT4+VD group were higher than that in the Low LT4 group (P<0.05).

CONCLUSION

In mice with thyrotoxicosis, treatment with vitamin D can inhibit bone resorption and improve the BMD and trabecular bone architecture by increasing the ratio of OPG to RANKL and upregulating the expression of Wnt/β-catenin.

Hashimoto's Thyroiditis and Graves' Disease in Genetic Syndromes in Pediatric Age

Autoimmune thyroid diseases (AITDs), including Hashimoto’s thyroiditis (HT) and Graves’ disease (GD), are the most common cause of acquired thyroid disorder during childhood and adolescence. Our purpose was to assess the main features of AITDs when they occur in association with genetic syndromes. We conducted a systematic review of the literature, covering the last 20 years, through MEDLINE via PubMed and EMBASE databases, in order to identify studies focused on the relation between AITDs and genetic syndromes in children and adolescents. From the 1654 references initially identified, 90 articles were selected for our final evaluation. Turner syndrome, Down syndrome, Klinefelter syndrome, neurofibromatosis type 1, Noonan syndrome, 22q11.2 deletion syndrome, Prader–Willi syndrome, Williams syndrome and 18q deletion syndrome were evaluated. Our analysis confirmed that AITDs show peculiar phenotypic patterns when they occur in association with some genetic disorders, especially chromosomopathies. To improve clinical practice and healthcare in children and adolescents with genetic syndromes, an accurate screening and monitoring of thyroid function and autoimmunity should be performed. Furthermore, maintaining adequate thyroid hormone levels is important to avoid aggravating growth and cognitive deficits that are not infrequently present in the syndromes analyzed.

Graves' disease: developments in first-line antithyroid drugs in the young

Introduction: First-line treatment for most young people with Graves' disease (GD) will include the administration of a thionamide antithyroid medication (ATD); Carbimazole (CBZ), Methimazole (MMZ), or rarely, propylthiouracil (PTU). GD is a challenge for families and clinicians because the likelihood of remission following a course of ATD is lower in young people when compared to adults, yet the risk of adverse events is higher. An overall consensus regarding the optimal ATD treatment regimen is lacking; how ATD are prescribed, for how long and how the associated risk of adverse events is managed varies between clinicians, units and nations. This partly reflects clinician and family uncertainty regarding outcomes.Areas covered: This review will focus on some of the key articles published in the field of thionamide ATD in children. It will highlight key issues that need to be discussed with families as well as addressing the approach and controversies in the treatment of GD. This article does not reflect a formal systematic review of the literature.Expert opinion: New strategies in areas such as immunomodulation may see the development of new antithyroid drug treatments that, either in isolation or in combination with thionamide therapy, may increase the likelihood of long-term remission.

The Role of Dickkopf-1 in Thyroid Hormone-Induced Changes of Bone Remodeling in Male Mice

Thyroid hormones regulate bone homeostasis, and exogenously induced hyperthyroidism and hypothyroidism in mice was recently found to be associated with an altered expression of the Wnt inhibitor Dickkopf-1 (Dkk1), a determinant of bone mass. Here, we assessed the role of Dkk1 in thyroid hormone-induced changes in bone using conditional Dkk1 knockout mice. Male mice with a global (Dkk1fl/fl;Rosa26-CreERT2) or osteocyte-specific (Dkk1fl/fl;Dmp1:Cre) deletion of Dkk1 were pharmacologically rendered hypothyroid or hyperthyroid. The bone phenotype was analyzed using micro-CT analysis, dynamic histomorphometry, and serum concentrations of bone turnover markers. Hypothyroid and hyperthyroid Cre-negative mice of either Cre line revealed the expected changes in bone volume with hypothyroid mice displaying a 40% to 60% increase in vertebral trabecular bone volume, while hyperthyroid mice lost 45% to 60% of bone volume. Similar changes were observed at the spine. Interestingly, Cre-positive mice of both lines did not gain or lose as much bone at the femur when rendered hypothyroid or hyperthyroid. While Cre-negative hypothyroid mice gained 80% to 100% bone volume, Cre-positive hypothyroid mice only increased their bone volume by 55% to 90%. Similarly, Cre-negative hyperthyroid mice lost 74% to 79% bone, while Cre-positive hyperthyroid mice merely lost 40% to 54%. Despite these site-specific differences, both global and osteocyte-specific Dkk1 knockout mice displayed similar changes in bone turnover as their Cre-negative controls in the hypothyroid and hyperthyroid states. While osteoblast and osteoclast parameters were increased in hyperthyroidism, hypothyroidism potently suppressed bone cell activities. Loss of Dkk1 is not sufficient to fully reverse thyroid hormone-induced changes in bone mass and bone turnover.

Analysis of Physiological Responses to Thyroid Hormones and Their Receptors in Bone

Thyroid hormone has profound effects on skeletal development and adult bone maintenance. Here, we review the current literature concerning thyroid hormone action in bone and cartilage in relation to human disease and animal models. We describe state-of-the-art imaging and biomechanical methods used to determine structural and functional parameters in the skeletal phenotyping of mouse models.

The role of triiodothyronine hormone and mechanically-stressed endothelial cell paracrine signalling synergism in gene reprogramming during hBMSC-stimulated osteogenic phenotype in vitro

We therefore investigated whether there is synergism between triiodothyronine (T3) hormone and trophic molecules released from mechanically-stressed endothelial cells (EC-enriched medium) in osteogenic phenotype by mapping classical repertory of genes. Although there are studies reporting the efficiency of T3 hormone on bone cells, it is scarce considering their effect in conjunction with other physiologically active molecules, such as those released by the active endothelial cells. To address this issue, human bone marrow-derived mesenchymal stem cells (hBMSCs) were treated with EC-enriched medium subjected to shear-stress up to 72 h in vitro, in conjunction or not with T3 hormone. Although our results found an important synergism considering these parameters on modulating key bone-related gene markers, such as on the alkaline phosphatase (ALP) behavior (at both mRNA and protein content), contributing for osteoblast differentiation, important genes such as OSTERIX and RUNX2 were significantly down-expressed, while a over-expression of RANKL was found when the conjunction effect of T3 and endothelial paracrine signaling was considered. In addition, T3 hormone over expressed both OCT4 and NANOG genes in a DNA epigenetic-independent manner. However, we observed a dynamic reprogramming of DNMT1, DNMT3A, DNMT3B and TET1, important DNA-related epigenetic markers. Specifically, T3 hormone alone up-modulated TET2 transcripts profile. Complimentarily, expression of microRNA (miRs) processing-related genes also was modulated, as well as miR-10b, miR-22, miR-21, miR-143 and miR-145 transcriptional related profiles. Altogether, our results suggested a positive effect of mechanically-stressed endothelial cells-induced paracrine signaling on T3 hormone-obtaining osteogenic phenotype, contributing to understanding the paradoxal effect of T3 hormone on the bone physiology.

Sexual precocity in a girl with early-onset Graves’ disease

We describe the case of a girl diagnosed with Graves’ disease (GD) at 2 yr of age, who developed early puberty. Preoperative examination for craniosynostosis revealed thyrotoxicosis. While she was tall and her bone age was advanced at GD onset, her linear growth attenuated after commencement of anti-thyroid treatment. However, at approximately 6 yr of age, breast budding was recognized. Hormonal analysis revealed pubertal levels of LH response to a GnRH stimulation test and serum E2. Gonadal suppression therapy with GnRH agonist was initiated, and her adult stature slightly exceeded the genetic potential. Although accelerated growth and skeletal maturation are often reported to occur at GD onset in prepubertal patients, early puberty is unusual, and this is the first reported case of sexual precocity in a girl with GD.

Thyroid diseases and bone health

Thyroid hormones are essential for skeletal development and are important regulators of bone maintenance in adults. Childhood hypothyroidism causes delayed skeletal development, retarded linear growth and impaired bone mineral accrual. Epiphyseal dysgenesis is evidenced by classic features of stippled epiphyses on X-ray. In severe cases, post-natal growth arrest results in a complex skeletal dysplasia. Thyroid hormone replacement stimulates catch-up growth and bone maturation, but recovery may be incomplete dependent on the duration and severity of hypothyroidism prior to treatment. A severe phenotype characteristic of hypothyroidism occurs in children with resistance to thyroid hormone due to mutations affecting THRA encoding thyroid hormone receptor α (TRα). Discovery of this rare condition recapitulated animal studies demonstrating that TRα mediates thyroid hormone action in the skeleton. In adults, thyrotoxicosis is well known to cause severe osteoporosis and fracture, but cases are rare because of prompt diagnosis and treatment. Recent data, however, indicate that subclinical hyperthyroidism is associated with low bone mineral density (BMD) and an increased risk of fracture. Population studies have also shown that variation in thyroid status within the reference range in post-menopausal women is associated with altered BMD and fracture risk. Thus, thyroid status at the upper end of the euthyroid reference range is associated with low BMD and increased risk of osteoporotic fragility fracture. Overall, extensive data demonstrate that euthyroid status is required for normal post-natal growth and bone mineral accrual, and is fundamental for maintenance of adult bone structure and strength.

Thyroid Hormone Signaling in the Development of the Endochondral Skeleton

Thyroid hormone (TH) is an established regulator of skeletal growth and maintenance both in clinical studies and in laboratory models. The clinical consequences of altered thyroid status on the skeleton during development and in adulthood are well known, and genetic mouse models in which elements of the TH signaling axis have been manipulated illuminate the mechanisms which underlie TH regulation of the skeleton. TH is involved in the regulation of the balance between proliferation and differentiation in several skeletal cell types including chondrocytes, osteoblasts, and osteoclasts. The effects of TH are mediated primarily via the thyroid hormone receptors (TRs) α and β, ligand-inducible nuclear receptors which act as transcription factors to regulate target gene expression. Both TRα and TRβ signaling are important for different stages of skeletal development. The molecular mechanisms of TH action in bone are complex and include interaction with a number of growth factor signaling pathways. This review provides an overview of the regulation and mechanisms of TH action in bone, focusing particularly on the role of TH in endochondral bone formation during postnatal growth.

Excess maternal and postnatal thyroxine alters chondrocyte numbers and the composition of the extracellular matrix of growth cartilage in rats

Purpose/Aim: The aim of this study was to evaluate the effects of excess maternal and postnatal thyroxine on chondrocytes and the extracellular matrix (ECM) of growth cartilage.

MATERIALS AND METHODS

We used 16 adult female Wistar rats divided into two groups: thyroxine treatment and control. From weaning to 40 days of age, offspring of the treated group (n = 8) received L-thyroxine. Plasma free T4 was measured. Histomorphometric analysis was performed on thyroids and femurs of all offspring. Alcian blue histochemical staining and real-time reverse transcription polymerase chain reaction measurements of gene expression levels of Sox9, Runx2, Aggrecan, Col I, Col II, Alkaline phosphatase, Mmp2, Mmp9, and Bmp2 were performed. Data were analyzed for statistical significance by student's t-test.

RESULTS

Excess maternal and postnatal thyroxine reduced the intensity of Alcian blue staining, altered the number of chondrocytes in proliferative and hypertrophic zones in growth cartilage, and reduced the gene expression of Sox9, Mmp2, Mmp9, Col II, and Bmp2 in the growth cartilage of all offspring. Additionally, excess thyroxine altered the gene expression of Runx2, Aggrecan and Col I, and this effect was dependent on age.

CONCLUSIONS

Excess thyroxine in neonates suppresses chondrocyte proliferation, stimulates chondrocyte hypertrophy and changes the ECM composition by reducing the amount of proteoglycans and glycosaminoglycans (GAGs). Prolonged exposure to excess thyroxine suppresses chondrocyte activity in general, with a severe reduction in the proteoglycan content of cartilage and the expression of gene transcripts essential for endochondral growth and characteristics of the chondrocyte phenotype.

Sclerostin Blockade and Zoledronic Acid Improve Bone Mass and Strength in Male Mice With Exogenous Hyperthyroidism

Hyperthyroidism in mice is associated with low bone mass, high bone turnover, and high concentrations of sclerostin, a potent Wnt inhibitor. Here, we explored the effects of either increasing bone formation with sclerostin antibodies (Scl-Ab) or reducing bone turnover with bisphosphonates on bone mass and strength in hyperthyroid mice. Twelve-week-old C57BL/6 male mice were rendered hyperthyroid using l-thyroxine (T4; 1.2 µg/mL added to the drinking water) and treated with 20 mg/kg Scl-Ab twice weekly or 100 µg/kg zoledronic acid (ZOL) once weekly or phosphate-buffered saline for 4 weeks. Hyperthyroid mice displayed a lower trabecular bone volume at the spine (-42%, P < 0.05) and the distal femur (-55%, P < 0.05) compared with euthyroid controls. Scl-Ab and ZOL treatment of hyperthyroid mice increased trabecular bone volume at the spine by threefold and twofold, respectively. Serum bone formation and resorption markers were increased in hyperthyroid mice and suppressed by treatment with ZOL but not Scl-Ab. Trabecular bone stiffness at the lumbar vertebra was 63% lower in hyperthyroid mice (P < 0.05) and was increased fourfold by Sci-Ab (P < 0.001) and threefold by ZOL treatment (P < 0.01). Bone strength based on ultimate load, which was 10% lower in hyperthyroidism, was increased by Scl-Ab by 71% and ZOL by 22% (both P < 0.001). Increased proportion of low mineralized bone seen in hyperthyroid mice was restored by treatment with Scl-Ab and ZOL. Thus, bone-forming and antiresorptive drugs prevent bone loss in hyperthyroid mice via different mechanisms.

Effects of long-term oral administration of methimazole on femur and tibia properties in male Wistar rats

Physiological concentrations of thyroid hormones are crucial for skeletal growth and development, physiological bone turnover and bone homeostasis maintenance. Methimazole (1-methyl-2-mercaptoimidazole) is an antithyroid drug used for the treatment of the hyperthyroidism in humans and animals. The aim of the study was to determine effects of long-term oral methimazole treatment in male Wistar rats on biochemical bone metabolism markers, as well as morphological, geometric, densitometric and mechanical properties of femur and tibia. Experimental rats were subjected to 90-day-long oral treatment with 0.05% water solution of methimazole and were kept under identical environmental conditions and received the same diet ad libitum as the control group. Serum concentration of osteocalcin (OC) and C-terminal telopeptides of type I collagen (CTX-I) was determined. Femur and tibia were evaluated using quantitative computed tomography (QCT), peripheral QCT (pQCT) and three-point bending test. Final body weight of the experimental group was significantly decreased by 30% (P=0.01). Methimazole treatment significantly decreased serum OC concentration by 21% (P=0.02) and increased CTX-I concentration by 17% (P=0.06). Methimazole decreased morphological, geometric and densitometric parameters of femur and tibia in rats. Mechanical evaluation of bones has shown significantly decreased maximum elastic strength and ultimate strength of femur in rats treated with methimazole by 36% and 40% when compared to the control group (P<0.05). In conclusion, this study has shown that long-term treatment with methimazole inhibits bone formation and accelerates bone resorption processes. The observed negative effects of methimazole treatment on body weight gain and skeletal properties may be considered as additional possible side effects in living organisms to those reported in the previous studies. It may be suggested that long-term antithyroid treatment should be combined with prevention of the negative effects of methimazole on bone tissue and whole body metabolism.

Excesso de tiroxina materna associado ao hipertireoidismo pós-natal reduz o crescimento ósseo e o perfil proliferativo e angiogênico das cartilagens de crescimento de ratos

RESUMO Foram estudados os efeitos do excesso da tiroxina materna associado ao hipertireoidismo pós-natal sobre o crescimento ósseo e o perfil proliferativo e angiogênico das cartilagens. Dezesseis ratas Wistar adultas foram distribuídas nos grupos tratados com L-tiroxina e controle. A prole do grupo tratado recebeu L-tiroxina do desmame até 40 dias de idade. Ao desmame, foi realizada dosagem plasmática de T4 livre nas mães. Na prole, foram realizados: dosagem plasmática de T3 total e T4 livre, morfometria das tireoides, mensuração do comprimento e largura do fêmur. Nas cartilagens, foi avaliada a expressão imuno-histoquímica e gênica de CDC-47, VEGF, Flk-1, Ang1, Ang2 e Tie2. As médias entre grupos foram comparadas pelo teste T de Student. As concentrações de T4 livre das mães tratadas e de T3 total e T4 livre da prole foram significativamente mais elevadas. A largura do fêmur foi menor nos animais tratados. Houve também redução da imunoexpressão de CDC-47 e de VEGF e dos transcritos gênicos para VEGF e Ang1 nas cartilagens. Conclui-se que o excesso de tiroxina materna associado ao hipertireoidismo pós-natal reduz a largura da diáfise femoral, a proliferação celular e a expressão de VEGF e de Ang1 nas cartilagens de crescimento de ratos.

Fetal neonatal hyperthyroidism: diagnostic and therapeutic approachment

Fetal and neonatal hyperthyroidism may occur in mothers with Graves' disease. Fetal thyrotoxicosis manifestation is observed with the transition of TSH receptor stimulating antibodies to the fetus from the 17th-20th weeks of pregnancy and with the fetal TSH receptors becoming responsive after 20 weeks. The diagnosis is confirmed by fetal tachycardia, goiter and bone age advancement in pregnancy and maternal treatment is conducted in accordance. The probability of neonatal hyperthyroidism is high in the babies of mothers that have ongoing antithyroid requirement and higher antibody levels in the last months of pregnancy. Clinical manifestation may be delayed by 7-17 days because of the antithyroid drugs taken by the mother. Neonatal hyperthyroidism symptoms can be confused with sepsis and congenital viral infections. Herein, the diagnosis and therapeutic approach are reviewed in cases of fetal neonatal hyperthyroidism.

Emerging role of thyroid hormone metabolites

Thyroid hormones (THs) are essential for the regulation of development and metabolism in key organs. THs produce biological effects both by directly affecting gene expression through the interaction with nuclear receptors (genomic effects) and by activating protein kinases and/or ion channels (short-term effects). Such activations can be either direct, in the case of ion channels, or mediated by membrane or cytoplasmic receptors. Short-term-activated signalling pathways often play a role in the regulation of genomic effects. Several TH intermediate metabolites, which were previously considered without biological activity, have now been associated with a broad range of actions, mostly attributable to short-term effects. Here, we give an overview of the physiological roles and mechanisms of action of THs, focusing on the emerging position that TH metabolites are acquiring as important regulators of physiology and metabolism.

Role of Thyroid Hormones in Skeletal Development and Bone Maintenance

The skeleton is an exquisitely sensitive and archetypal T3-target tissue that demonstrates the critical role for thyroid hormones during development, linear growth, and adult bone turnover and maintenance. Thyrotoxicosis is an established cause of secondary osteoporosis, and abnormal thyroid hormone signaling has recently been identified as a novel risk factor for osteoarthritis. Skeletal phenotypes in genetically modified mice have faithfully reproduced genetic disorders in humans, revealing the complex physiological relationship between centrally regulated thyroid status and the peripheral actions of thyroid hormones. Studies in mutant mice also established the paradigm that T3 exerts anabolic actions during growth and catabolic effects on adult bone. Thus, the skeleton represents an ideal physiological system in which to characterize thyroid hormone transport, metabolism, and action during development and adulthood and in response to injury. Future analysis of T3 action in individual skeletal cell lineages will provide new insights into cell-specific molecular mechanisms and may ultimately identify novel therapeutic targets for chronic degenerative diseases such as osteoporosis and osteoarthritis. This review provides a comprehensive analysis of the current state of the art.

The cartilage extracellular matrix as a transient developmental scaffold for growth plate maturation

The cartilage growth plate is a specialized developmental tissue containing characteristic zonal arrangements of chondrocytes. The proliferative and differentiative states of chondrocytes are tightly regulated at all stages including the initial limb bud and rudiment cartilage stages of development, the establishment of the primary and secondary ossification centers, development of the growth plates and laying down of bone. A multitude of spatio-temporal signals, including transcription factors, growth factors, morphogens and hormones, control chondrocyte maturation and terminal chondrocyte differentiation/hypertrophy, cell death/differentiation, calcification and vascular invasion of the growth plate and bone formation during morphogenetic transition of the growth plate. This involves hierarchical, integrated signaling from growth and factors, transcription factors, mechanosensory cues and proteases in the extracellular matrix to regulate these developmental processes to facilitate progressive changes in the growth plate culminating in bone formation and endochondral ossification. This review provides an overview of selected components which have particularly important roles in growth plate biology including collagens, proteoglycans, glycosaminoglycans, growth factors, proteases and enzymes.

Recurrent Thyrotoxicosis due to Both Graves’ Disease and Hashimoto’s Thyroiditis in the Same Three Patients

Hashimoto's thyroiditis (HT) and Graves' disease (GD) are the 2 most common autoimmune disease processes affecting the thyroid gland. The relationship between the two is complex and not clearly understood. It has been theorized that HT and GD are 2 separate disease processes due to unique genetic differences demonstrated by genome studies. On the other hand, based on occurrence of both HT and GD in monozygotic twins and within the same family, they have been regarded to represent 2 ends of the same spectrum. This case report describes 3 patients who presented with thyrotoxicosis due to both GD and HT. The initial presentation was thyrotoxicosis due to GD treated with antithyroid medication followed by temporary resolution. They all subsequently experienced recurrence of thyrotoxicosis in the form of Hashitoxicosis due to HT, and then eventually all developed thyrotoxicosis due to GD, requiring radioablation therapy.

Hyperthyroidism and Hypothyroidism in Male Mice and Their Effects on Bone Mass, Bone Turnover, and the Wnt Inhibitors Sclerostin and Dickkopf-1

Thyroid hormones are key regulators of bone homeostasis, and Wnt signaling has been implicated in thyroid hormone-associated bone loss. Here we tested whether hyperthyroidism and hypothyroidism interfere with dickkopf-1 (DKK1) and sclerostin, two inhibitors of Wnt signaling. Twelve-week-old male C57BL/6 mice were rendered either hyperthyroid or hypothyroid. Hyperthyroid mice displayed decreased trabecular (-54%, P < .001) and cortical bone density (-5%, P < .05) and reduced cortical thickness (-15%, P < .001), whereas hypothyroid mice showed a higher trabecular bone density (+26%, P < .001) with unchanged cortical bone parameters. Histomorphometry and biochemical markers of bone remodeling indicated high bone turnover in hyperthyroid mice and low bone turnover in hypothyroid mice. In vivo, serum DKK1 concentrations were decreased in hyperthyroid mice (-24%, P < .001) and increased in hypothyroid mice (+18%, P < .01). The increase of the number of DKK1-positive cells in hypothyroid mice was confirmed at the tissue level. Interestingly, sclerostin was increased in both disease models, although to a higher extent in hyperthyroid mice (+50%, P < .001, and +24%, P < .05). Serum sclerostin concentrations adjusted for bone mass were increased by 3.3-fold in hyperthyroid (P < .001) but not in hypothyroid mice. Consistently, sclerostin mRNA expression and the number of sclerostin-positive cells were increased in hyperthyroid but not in hypothyroid mice. Our data show that thyroid hormone-induced changes in bone remodeling are associated with a divergent regulation of DKK1 and sclerostin. Thus, the modulation of Wnt signaling by thyroid hormones may contribute to thyroid hormone-associated bone disease and altered expression of Wnt inhibitors may emerge as potential therapeutic targets.

Anti-thyroid drugs in pediatric Graves' disease

Graves' disease is the most common cause of hyperthyroidism in children. Most children and adolescents are treated with anti-thyroid drugs as the initial modality. Studies have used Methimazole, Carbimazole and Propylthiouracil (PTU) either as titration regimes or as block and replacement regimes. The various studies of anti-thyroid drug (ATD) treatment of Graves' disease in pediatric patients differ in terms of the regimes, remission rate, duration of therapy for adequate remission, follow up and adverse effects of ATD. Various studies show that lower thyroid hormone levels, prolonged duration of treatment, lower levels of TSH receptor antibodies, smaller goiter and increased age of child predicted higher chance of remission after ATD. A variable number of patients experience minor and major adverse effects limiting initial and long term treatment with ATD. The adverse effects of various ATD seem to more in children compared to that of adults. In view of liver injury including hepatocellular failure need of liver transplantation associated with PTU, the use has been restricted in children. The rate of persistent remission with ATD following discontinuation is about 30%. Radioactive iodine therapy is gaining more acceptance in older children with Graves's disease in view of the limitations of ATD. For individual patients, risk-benefit ratio of ATD should be weighed against benefits of radioactive iodine therapy and patient preferences.

Autonomous adenomas caused by somatic mutations of the thyroid-stimulating hormone receptor in children

In adults, autonomous adenomas of the thyroid causing hyperthyroidism are relatively common and are most often due to somatic mutations that increase the constitutive activity of the thyroid-stimulating hormone receptor (TSHR). By contrast, autonomous adenomas in hyperthyroid children are exceptional and reports of their clinical and molecular characteristics are few. We reviewed papers describing 16 autonomous adenomas due to a somatic mutation activating the TSHR and diagnosed in patients younger than 18 years, to which we added two of our own unpublished observations in a 4- and 8-year-old with the same TSHR mutation (c.CAG>CAC; p.Asp633His). This revealed that (a) autonomous adenomas occur more often in the right lobe (11 of 14 with available information) and the associated hyperthyroidism tends to be more severe, possibly reflecting the richer vascular supply of the right thyroid lobe, and (b) mutations found in benign adenomas in children have been associated with cancer in adults, suggesting that malignancy requires a second 'hit' at a later age.

Neonatal Cholestasis Caused by Undiagnosed Maternal Graves' Disease

Neonatal cholestasis results from a variety of etiologies, including anatomic, infectious, and metabolic abnormalities. Hyperthyroidism, in contrast to hypothyroidism, is infrequently associated with neonatal cholestasis. Newborn screening is an important tool to detect newborn metabolic disorders, including thyroid dysfunction. However, one must exercise caution when interpreting these reports; typically only high thyroid stimulating hormone (TSH) levels are flagged as abnormal, while low or undetectable levels may not be. We present a unique case of cholestasis in a hyperthyroid neonate of an untreated, undiagnosed mother with Graves' disease; the infant's metabolic screen was not flagged as abnormal.

A 3-year-old girl with Graves' disease with literature review

Graves' disease, the main cause of hyperthyroidism in the pediatric age group, is very rare in children younger than 4 years old but can seriously interfere with growth and development if not recognized and treated. Here we report a case of a 3-year-old girl with Graves' disease who presented with goiter, exophthalmos, heat intolerance, and hyperactivity. At her first visit, her serum concentrations of triiodothyronine (T3) and free thyroxine (free T4) were normal, whereas that of thyroid-stimulating hormone (TSH) was decreased. Antimicrosomal antibody was 7,053.94 IU/mL, and TSH-binding inhibitory immunoglobulin was 31.62%. A thyroid scan showed diffuse enlargement with markedly increased uptake of both thyroid glands. Although T3 and free T4 levels were initially normal, she developed hyperthyroidism 3 months later. She was finally diagnosed with Graves' disease and treated with methimazole for 6 months. This is the first report of Graves' disease in children younger than 4 years old in Korea.

Clinical relevance of thyroid-stimulating autoantibodies in pediatric graves' disease-a multicenter study

CONTEXT AND OBJECTIVE

The incidence of TSH receptor (TSHR) stimulating autoantibodies (TSAbs) in pediatric Graves' disease (GD) is controversial. This large, multicenter study evaluated the clinical relevance of TSAbs in children with GD both with Graves' orbitopathy (GO) and without orbital disease.

DESIGN

We conducted a cross-sectional retrospective study.

SETTING

Sera were collected in seven American and European academic referral centers and evaluated in a central laboratory. PATIENTS AND SAMPLES: A total of 422 serum samples from 157 children with GD, 101 control individuals with other thyroid and nonthyroid autoimmune diseases, and 50 healthy children were studied.

MAIN OUTCOME MEASURES

TSAbs were measured using a novel, chimeric TSHR bioassay and a cAMP response element-dependent luciferase. TSH binding-inhibitory Ig (TBII) and parameters of thyroid function were also determined.

RESULTS

In 82 untreated children with GD, sensitivity, specificity, and positive and negative predictive values for TSAb and TBII were: 100 and 92.68% (P = .031), 100 and 100%, 100 and 100%, and 100 and 96.15%, respectively. TSAb and TBII were present in 147 (94%) and 138 (87.9%) of the 157 children with GD (P < .039), respectively; and in 247 (94%) and 233 (89%) of the 263 samples from this group (P < .0075), respectively. In children with GD and GO, TSAb and TBII were noted in 100 and 96% (P < .001), respectively. Hyperthyroid children with GD and GO showed markedly higher TSAb levels compared to those with thyroidal GD only (P < .0001). No significant differences were noted for TBII between the two groups. After a 3-year (median) medical treatment, the decrease of TSAb levels was 69% in GD vs 20% in GD and GO (P < .001). All 31 samples of euthyroid children with GO were TSAb positive; in contrast, only 24 were TBII positive (P = .016). All children with Hashimoto's thyroiditis, nonautoimmune hyperthyroidism, type 1 diabetes, and juvenile arthritis and the healthy controls were TSAb and TBII negative.

CONCLUSIONS

Serum TSAb level is a sensitive, specific, and reproducible biomarker for pediatric GD and correlates well with disease severity and extrathyroidal manifestations.

Vitamin E management of oxidative damage-linked dysfunctions of hyperthyroid tissues

INTRODUCTION

Thyroid hormones affect growth, development, and metabolism of vertebrates, and are considered the major regulators of their homeostasis. On the other hand, elevated circulating levels of thyroid hormones are associated with modifications in the whole organism (weight loss and increased metabolism and temperature) and in several body regions. Indeed, tachycardia, atrial arrhythmias, heart failure, muscle weakness and wasting, bone mass loss, and hepatobiliary complications are commonly found in hyperthyroid animals and humans.

RESULTS

Most thyroid hormone actions result from influences on transcription of T3-responsive genes, which are mediated through nuclear receptors. However, there is significant evidence that tissue oxidative stress underlies some dysfunctions produced by hyperthyroidism.

DISCUSSION

During the last decades, increasing interest has been turned to the use of antioxidants as therapeutic agents in various diseases and pathophysiological disorders believed to be mediated by oxidative stress. In particular, because elevated circulating levels of thyroid hormones are associated with tissue oxidative injury, more attention has been paid to explore the application of antioxidants as therapeutic agents in thyroid related disorders.

CONCLUSIONS

At present, vitamin E is among the most commonly consumed dietary supplements due to the belief that it, as an antioxidant, may attenuate morbidity and mortality. This is due to the results of numerous scientific studies, which demonstrate that vitamin E has a primary function to destroy peroxyl radicals, thus protecting polyunsaturated fatty acids biological membranes from oxidative damage. However, results are also available indicating that protective vitamin E effects against oxidative damage can be obtained even through different mechanisms.

Does transient hypothyroxinemia influence metabolic bone disease of prematurity?

OBJECTIVE

To investigate the relationship between Metabolic Bone Disease (MBD) and Transient Hypothyroxinemia of Prematurity (THOP).

METHOD

One hundred twenty-four infants, born in Marmara University Hospital with a gestational age ≤34 weeks, were enrolled. Clinical features were recorded. Serum TSH, free T4, total T4, calcium, phosphorus and total Alkaline Phosphatase (ALP) levels were determined in the first and third postnatal weeks. MBD was defined as a phosphorus level <4.5 mg/dl and/or ALP >900 IU/l. THOP was defined as a serum free and/or total thyroxine level lower than -1 SD for gestational age at the 7th postnatal day.

RESULT

THOP was diagnosed in nineteen (15.3%) patients. MBD was diagnosed in 52 (41.9%) at the 3rd month. Low birth weight, low gestational age and prolonged parenteral nutrition were associated with MBD. Multivariate analysis documented a significant relationship solely between MBD and gestational age.

CONCLUSION

The risk of MBD does not increase significantly in babies with THOP.

Effects of thyroxine exposure on osteogenesis in mouse calvarial pre-osteoblasts

The incidence of craniosynostosis is one in every 1,800–2500 births. The gene-environment model proposes that if a genetic predisposition is coupled with environmental exposures, the effects can be multiplicative resulting in severely abnormal phenotypes. At present, very little is known about the role of gene-environment interactions in modulating craniosynostosis phenotypes, but prior evidence suggests a role for endocrine factors. Here we provide a report of the effects of thyroid hormone exposure on murine calvaria cells. Murine derived calvaria cells were exposed to critical doses of pharmaceutical thyroxine and analyzed after 3 and 7 days of treatment. Endpoint assays were designed to determine the effects of the hormone exposure on markers of osteogenesis and included, proliferation assay, quantitative ALP activity assay, targeted qPCR for mRNA expression of Runx2, Alp, Ocn, and Twist1, genechip array for 28,853 targets, and targeted osteogenic microarray with qPCR confirmations. Exposure to thyroxine stimulated the cells to express ALP in a dose dependent manner. There were no patterns of difference observed for proliferation. Targeted RNA expression data confirmed expression increases for Alp and Ocn at 7 days in culture. The genechip array suggests substantive expression differences for 46 gene targets and the targeted osteogenesis microarray indicated 23 targets with substantive differences. 11 gene targets were chosen for qPCR confirmation because of their known association with bone or craniosynostosis (Col2a1, Dmp1, Fgf1, 2, Igf1, Mmp9, Phex, Tnf, Htra1, Por, and Dcn). We confirmed substantive increases in mRNA for Phex, FGF1, 2, Tnf, Dmp1, Htra1, Por, Igf1 and Mmp9, and substantive decreases for Dcn. It appears thyroid hormone may exert its effects through increasing osteogenesis. Targets isolated suggest a possible interaction for those gene products associated with calvarial suture growth and homeostasis as well as craniosynostosis.

Role and Mechanisms of Actions of Thyroid Hormone on the Skeletal Development

The importance of the thyroid hormone axis in the regulation of skeletal growth and maintenance has been well established from clinical studies involving patients with mutations in proteins that regulate synthesis and/or actions of thyroid hormone. Data from genetic mouse models involving disruption and overexpression of components of the thyroid hormone axis also provide direct support for a key role for thyroid hormone in the regulation of bone metabolism. Thyroid hormone regulates proliferation and/or differentiated actions of multiple cell types in bone including chondrocytes, osteoblasts and osteoclasts. Thyroid hormone effects on the target cells are mediated via ligand-inducible nuclear receptors/transcription factors, thyroid hormone receptor (TR) α and β, of which TRα seems to be critically important in regulating bone cell functions. In terms of mechanisms for thyroid hormone action, studies suggest that thyroid hormone regulates a number of key growth factor signaling pathways including insulin-like growth factor-I, parathyroid hormone related protein, fibroblast growth factor, Indian hedgehog and Wnt to influence skeletal growth. In this review we describe findings from various genetic mouse models and clinical mutations of thyroid hormone signaling related mutations in humans that pertain to the role and mechanism of action of thyroid hormone in the regulation of skeletal growth and maintenance.

Postnatal toxic and acquired disorders

To develop and function optimally, the brain requires a balanced environment of electrolytes, amino acids, neurotransmitters, and metabolic substrates. As a consequence, organ dysfunction has the potential to induce brain disorders and toxic-metabolic encephalopathies, particularly when occurring during early stages of cerebral maturation. Induced toxicity of three different organ systems that are commonly associated with brain complications are discussed. First, thyroid hormone deficiency caused by intrinsic or extrinsic factors (e.g., environmental toxins) may induce severe adverse effects on child neurological development from reversible impairments to permanent mental retardation. Second, inadequate removal of wastes due to chronic renal failure leads to the accumulation of endogenous toxins that are harmful to brain function. In uremic pediatric patients, the brain becomes more vulnerable to exogenous substances such as aluminum, which can induce aluminum encephalopathy. Following surgical procedures, neurological troubles including focal defects and severe epileptic seizures may result from hypertensive encephalopathy combined with toxicity of immunomodulating substances, or from the delayed consequences of cardiovascular defect. Taken together, this illustrates that organ disorders clearly have an impact on child brain function in various ways.

Thyroid hormone actions in cartilage and bone

Thyroid hormones exert widespread and complex actions in almost all tissues during development, throughout childhood and in adults. The skeleton is an important T3-target tissue that exemplifies these processes, and yet understanding of the specific cellular and molecular mechanisms of T3 action in bone and cartilage remains incomplete. Here, the skeleton is considered as a T3-target tissue. The actions of thyroid hormones during skeletal development and in chondrocytes and growth plate cartilage during post-natal linear growth are outlined. The physiological importance of these actions are discussed in relation to patients with autosomal dominant mutations in genes encoding the thyroid hormone receptors TRα1 and TRβ, and in mice harbouring deletions or mutations of the orthologous genes. The role of thyroid hormones and the control of T3 action in bone turnover and maintenance are also outlined, and T3 action in bone-forming osteoblasts and bone-resorbing osteoclasts discussed. The physiological and functional consequences of T3 action in bone are considered in relation to mutant mouse models and to effects on bone mineral density and fracture susceptibility in humans. Finally, new studies identifying a putative role for thyroid hormone metabolism in articular cartilage maintenance and the pathogenesis of osteoarthritis are considered. The pharmacological context of these new findings is discussed, emphasising the importance of this emerging field of study in thyroid hormone pathophysiology.

Speech and language delay in two children: an unusual presentation of hyperthyroidism

BACKGROUND

Hyperthyroidism is rare in pre-school children. Untreated, it can have a profound effect on normal growth and development, particularly in the first 2 years of life. Although neurological manifestations of dysthyroid states are well known, specific expressive speech and language disorder as a presentation of hyperthyroidism is rarely documented.

METHODS

Case reports of two children with hyperthyroidism presenting with speech and language delay.

RESULTS

We report two pre-school children with hyperthyroidism, who presented with expressive speech and language delay, and demonstrated a significant improvement in their language skills following treatment with anti-thyroid medication.

CONCLUSIONS

Hyperthyroidism must be considered in all children presenting with speech and language difficulties, particularly expressive speech delay. Prompt recognition and early treatment are likely to improve outcome.

The skeletal consequences of thyrotoxicosis

Euthyroid status is essential for normal skeletal development and the maintenance of adult bone structure and strength. Established thyrotoxicosis has long been recognised as a cause of high bone turnover osteoporosis and fracture but more recent studies have suggested that subclinical hyperthyroidism and long-term suppressive doses of thyroxine (T4) may also result in decreased bone mineral density (BMD) and an increased risk of fragility fracture, particularly in postmenopausal women. Furthermore, large population studies of euthyroid individuals have demonstrated that a hypothalamic-pituitary-thyroid axis set point at the upper end of the normal reference range is associated with reduced BMD and increased fracture susceptibility. Despite these findings, the cellular and molecular mechanisms of thyroid hormone action in bone remain controversial and incompletely understood. In this review, we discuss the role of thyroid hormones in bone and the skeletal consequences of hyperthyroidism.

Juvenile Graves' disease with opthalmopathy, lymphadenopathy, accelerated growth and congestive cardiac failure

Graves' disease in childhood is a rare clinical entity. The authors report a case of Graves' disease in a 3-year-old child, who had opththalmopathy, accelerated growth, cervical lymphadenopathy, hepatosplenomegaly and congestive cardiac failure; and responded well to treatment.

Thyroid hormone metabolism in skeletal development and adult bone maintenance

Metabolism of thyroid hormones by the type 2 and type 3 iodothyronine deiodinases (D2, D3) in T3-responsive target cells is a sophisticated mechanism that helps to maintain local T3 concentrations and facilitates T3 action in a cell-specific manner that is independent of circulating thyroid hormone concentrations. Recent findings have demonstrated an essential physiological role for the thyroid hormone-activating enzyme D2 in the optimization of bone mineralization and strength. Emerging population studies have also identified the genes encoding D2 and the thyroid hormone-inactivating enzyme D3 as susceptibility loci for osteoarthritis. These new data reveal an essential role for the local control of T3 availability in osteoblasts and chondrocytes during maintenance and repair of bone and cartilage.

Advanced bone formation in mice with a dominant-negative mutation in the thyroid hormone receptor β gene due to activation of Wnt/β-catenin protein signaling

Thyroid hormone (T(3)) acts in chondrocytes and bone-forming osteoblasts to control bone development and maintenance, but the signaling pathways mediating these effects are poorly understood. Thrb(PV/PV) mice have a severely impaired pituitary-thyroid axis and elevated thyroid hormone levels due to a dominant-negative mutant T(3) receptor (TRβ(PV)) that cannot bind T(3) and interferes with the actions of wild-type TR. Thrb(PV/PV) mice have accelerated skeletal development due to unknown mechanisms. We performed microarray studies in primary osteoblasts from wild-type mice and Thrb(PV/PV) mice. Activation of the canonical Wnt signaling in Thrb(PV/PV) mice was confirmed by in situ hybridization analysis of Wnt target gene expression in bone during postnatal growth. By contrast, T(3) treatment inhibited Wnt signaling in osteoblastic cells, suggesting that T(3) inhibits the Wnt pathway by facilitating proteasomal degradation of β-catenin and preventing its accumulation in the nucleus. Activation of the Wnt pathway in Thrb(PV/PV) mice, however, results from a gain of function for TRβ(PV) that stabilizes β-catenin despite the presence of increased thyroid hormone levels. These studies demonstrate novel interactions between T(3) and Wnt signaling pathways in the regulation of skeletal development and bone formation.

Graves disease with exophtalmia in a two-year old child

Thyrotoxicosis is one of the rare disorders diagnosed in childhood and adolescence. The most frequent cause is Graves disease. One of the Graves' disease complications is thyroid-associated orbitopathy. A 2-year-old girl was referred to our hospital for decreased weight gain. Her physical examination was normal except for a palpable thyroid tissue and exophtalmia. After laboratory examination, she was referred to the departments of nuclear medicine and radiology for the diagnosis of hyperthyroidism and Graves ophthalmia. When evaluated with the physical examination, laboratory and imaging results the patient was diagnosed as Graves' disease with orbital involvement. So the patient was taken on methimazole treatment.

CONFLICT OF INTEREST

None declared.

Approach to the pediatric patient with Graves' disease: when is definitive therapy warranted?

Pediatric Graves' disease accounts for 10-15% of thyroid disorders in patients less than 18 yr of age. The onset of symptoms may be insidious and subsequently associated with a delay in diagnosis. Decreased concentration and poor school performance are frequent complaints and can be quite frustrating for the patient and family. Severe ophthalmopathy is uncommon. The diagnosis is established by the findings of an increased heart rate and goiter in the setting of a suppressed TSH and elevated T(3) and/or T(4). The majority of pediatric patients are initially placed on antithyroid medications and maintained on these medications for prolonged periods of time in hopes of achieving remission. Unfortunately, for many children and adolescents remission is unattainable, ultimately occurring in only 15-30% of patients. Several recent studies have suggested that the age of the patient, the degree of thyrotoxicosis at diagnosis, the initial response to therapy, and the level of TSH receptor antibodies serve as reasonable predictors of remission and relapse. However, a consensus on the utility of these markers has not been reached. The present clinical case describes an adolescent with Graves' disease and highlights the negative impact that prolonged medical therapy can have on quality of life and school performance; it reviews pertinent data on the diagnosis, comorbidities, and treatment options; and it identifies gaps in knowledge for when definitive therapy should be pursued. The case serves as a reminder that earlier discussion and decision for definitive therapy should be more commonplace in caring for our pediatric patients with Graves' disease.

Radioiodine treatment in pediatric Graves' disease and thyroid carcinoma

Graves' disease (GD) is the most common cause of hyperthyroidism in pediatric patients. Most pediatric thyroid cancer is differentiated thyroid cancer (DTC). The two diseases can be treated using different methods, such as antithyroid drug, radioactive iodine (RAI), and surgery for GD and surgery and RAI for DTC. RAI can be used to treat pediatric GD and DTC. In the article, we reviewed the RAI application in pediatric GD and DTC.

Autoimmune thyroid diseases in children

The two major autoimmune thyroid diseases (ATDs) include Graves' disease (GD) and autoimmune thyroiditis (AT); both of which are characterized by infiltration of the thyroid by T and B cells reactive to thyroid antigens, by the production of thyroid autoantibodies and by abnormal thyroid function (hyperthyroidism in GD and hypothyroidism in AT). While the exact etiology of thyroid autoimmunity is not known, it is believed to develop when a combination of genetic susceptibility and environmental encounters leads to breakdown of tolerance. It is important to recognize thyroid dysfunction at an early stage by maintaining an appropriate index of suspicion.

Evaluation of diagnosis and treatment results in children with Graves' disease with emphasis on the pubertal status of patients

This retrospective study evaluated the clinical and laboratory characteristics at presentation and treatment results of patients with Graves' disease (GD) with respect to pubertal status. Records of 143 patients (108 F, 35 M) were reviewed in a multicenter study. At diagnosis, 38% of patients were prepubertal. Anti-thyroid drugs (ATD) were used as initial therapy. There was no significant difference in clinical and laboratory characteristics at diagnosis, during treatment and adverse reaction to ATD with respect to pubertal status. Twenty patients (7 prepubertal, 13 pubertal) reached remission on ATD. Surgery was performed in seven and radioiodine (RAI) in four patients. Duration of treatment needed to achieve remission was longer in prepubertal (4.2 +/- 1.0 yr) than in pubertal patients (3.1 +/- 1.3 yr) (p = 0.02). The rate of remission was not different between prepubertal (25.9%) and pubertal patients (33.3%) (p = 0.59). ATD were associated with low remission rate in pediatric GD and required longer duration of therapy in prepubertal patients. For definitive treatment in older children, RAI could be evaluated as the initial therapy.

Iodothyronine deiodinase enzyme activities in bone

Euthyroid status is essential for normal skeletal development and maintenance of the adult skeleton, but the mechanisms which control supply of thyroid hormone to bone cells are poorly understood. Thyroid hormones enter target cells via monocarboxylate transporter-8 (MCT8), which provides a functional link between thyroid hormone uptake and metabolism in the regulation of T3-action but has not been investigated in bone. Most circulating active thyroid hormone (T3) is derived from outer ring deiodination of thyroxine (T4) mediated by the type 1 deiodinase enzyme (D1). The D2 isozyme regulates intra-cellular T3 supply and determines saturation of the nuclear T3-receptor (TR), whereas a third enzyme (D3) inactivates T4 and T3 to prevent hormone availability and reduce TR-saturation. The aim of this study was to determine whether MCT8 is expressed in the skeleton and whether chondrocytes, osteoblasts and osteoclasts express functional deiodinases. Gene expression was analyzed by RT-PCR and D1, D2 and D3 function by sensitive and highly specific determination of enzyme activities. MCT8 mRNA was expressed in chondrocytes, osteoblasts and osteoclasts at all stages of cell differentiation. D1 activity was undetectable in all cell types, D2 activity was only present in mature osteoblasts whereas D3 activity was evident throughout chondrocyte, osteoblast and osteoclast differentiation in primary cell cultures. These data suggest that T3 availability especially during skeletal development may be limited by D3-mediated catabolism rather than by MCT8 mediated cellular uptake or D2-dependent T3 production.