Direct Effect Of Thyroid Hormone Research Articles

Distribution of subpopulations of dendritic cells in peripheral blood of patients treated with exogenous thyrotropin

BackgroundDendritic cells (DCs) play a major role as regulators of inflammatory events associated with thyroid pathology. The immunoregulatory function of DCs depends strongly on their subtype, as well as maturation and activation status. Numerous hormonal factors modulate the immune properties of DCs, however, little is known about effects exerted by the hypothalamus-pituitary-thyroid-axis. Recently, we have shown a direct regulatory influence of thyroid hormones (TH) on human DCs function. The aim of the present study was to analyze the effect of systemically administered thyrotropin (TSH) on human blood DCs ex vivo.MethodsBlood samples for the cytometric analysis of peripheral blood plasmacytoid and myeloid DCs subtypes were collected from patients subjected to total thyroidectomy because of differentiated thyroid carcinoma at 2 time points: (i) directly before the commencement of TSH administration and (ii) 5 days after first TSH injection. The whole blood quantitative and phenotypic analysis of plasmacytoid and myeloid DCs subtypes was performed by flow cytometry.ResultsAdministration of TSH did not influence the percentage of plasmacytoid DCs in peripheral blood of study participants. Also the percentage of the two main myeloid DCs subpopulations – CD1c/BDCA1+ DCs and CD141/BDCA3+ DCs did not change significantly. TSH administration had no effect on the surface expression of CD86 – one of the major costimulatory molecules – neither in the whole peripheral blood mononuclear cell (PBMC) fraction nor in particular DCs subtypes.ConclusionsIn the present study, we demonstrated no influence of systemic TSH administration on human peripheral blood DCs subtypes. These results are in accordance with our previous work suggesting the direct effect of TH on human DCs ex vivo.

Thyroid Disorders and Hypocoagulability

This review focuses attention on some practical aspects of the relationship between thyroid disorders and hypocoagulability, as related to an impairment of hemostasis and fibrinolysis. An understanding of this topic in daily clinical practice is important given that the interaction between hemostatic abnormalities and thyroid disorders is still poorly recognized by the medical community. Even if the bleeding tendency is in general mild and may be reversed by restoration of an euthyroid state, severe hemorrhagic events may complicate the course of both hyper- and hypothyroidism, as precipitated by such conditions as thrombocytopenia, acquired von Willebrand syndrome, and acquired hemophilia. The pathogenesis of the hemostatic abnormalities resides in either the direct effect of thyroid hormones or some conditions in an autoimmune mechanism. Physicians and endocrinologists should pay close attention to both clinical hemorrhagic events in their patients as well as to any laboratory abnormalities identified by blood coagulation testing.

Thyroid Disorders and Hypercoagulability

Various abnormalities of coagulation and fibrinolysis, ranging from subclinical laboratory abnormalities to clinically significant disorders of hemostasis, but rarely major hemorrhage or thromboembolism, may occur in patients with thyroid diseases. This review discusses the relationships between thyroid dysfunction and the coagulation/fibrinolytic system. According to the recent literature, most of the coagulation/fibrinolytic abnormalities associated with thyroid dysfunction are the consequences of direct effects of thyroid hormones on the synthesis of various hemostatic parameters. Thyroid autoimmunity may also modify the processes of secondary hemostasis. Hyperthyroidism is generally associated with hypercoagulability and hypofibrinolysis, whereas the hemostatic profile in hypothyroidism depends on the severity of the disease. Both hypercoagulable and hypocoagulable states including increased fibrinolytic activity have been reported in hypothyroidism. Few data are available on hemostasis in subclinical thyroid diseases. In conclusion, adequate further prospective clinical studies of high quality including a larges series of patients are needed to explain the degree and type of coagulation/fibrinolytic abnormalities in patients with thyroid diseases.

Brugada-type electrocardiogram in a patient with hypothyroidism

We encountered a case of hypothyroidism showing Brugada-type electrocardiogram (ECG). A 52-year-old man was referred to our hospital in August 2009. Past medical history showed that liver dysfunction and face edema of unknown origin had been pointed out 1 year earlier. He was diagnosed with primary hypothyroidism at this admission. ECG exhibited first-degree atrio-ventricular block (0.24s) and showed Brugada-type ST-segment elevation ≥2mm followed by a negative T wave (coved type) in the V1, V2 leads. On genetic analysis, the patient demonstrated three common variants in the SCN5A gene, L1988R (c.5963 T>G), H558R (c.1673 A>G), and R1193Q (c.3578 G>A). Brugada-type ECG disappeared when the thyroid function normalized. We hypothesize that Brugada-type ECG in hypothyroidism is modified not only by a direct effect of thyroid hormone, but also due to SCN5A variants. Some SCN5A gene polymorphisms or mutations will induce changes on ECG when ion channels are affected by hypothyroidism.

Impact of Hyperthyroidism and Its correlation on regional blood flow in rats

Thyroid hormones (TH) exert multiple effects on the heart and vascular system. In the hyperthyroidism, besides the direct effects of TH on cardiac tissue inducing hypertrophy in rats, TH also lead to an important increase in heat rate and reduction in systemic vascular resistance. In the present study we evaluated the effect of hyperthyroidism (T3, 7ug/100g body wt/2 wk) on regional blood flow (coronary, lung and kidney) and blood pressure parameters in Wistar rats. Hyperthyroidism promoted significant left ventricular (+27.56%) and right ventricular (+33.56%) hypertrophy. Although T3 has promoted a marked increase on blood flow in the myocardium (+98,7%), an important decrease on lung blood flow (−35 %) was observed, which was followed by discrete but significant decrease on cardiac output (−14%). Regard to hemodynamic variables, the hyperthyroidism promoted a significant increase in mean, systolic and diastolic arterial pressure as well as HR compared to control group. These findings show that hyperthyroidism is accompanied by intense changes on the blood flow to lung and myocardium and hypertension. Considering that the reduction in cardiac output and pulmonary blood flow are associated with right ventricular hypertrophy, these findings may have clinical implications by serving as an explanation for the development of pulmonary hypertension and heart failure observed in hyperthyroid patients and point out the importance of adequate control of TH levels.

Thyroid hormones regulate phosphate homoeostasis through transcriptional control of the renal type IIa sodium-dependent phosphate co-transporter (Npt2a) gene

The type IIa renal sodium-dependent phosphate (Na/Pi) co-transporter Npt2a is implicated in the control of serum phosphate levels. It has been demonstrated previously that renal Npt2a protein and its mRNA expression are both up-regulated by the thyroid hormone T3 (3,3',5-tri-iodothyronine) in rats. However, it has never been established whether the induction was mediated by a direct effect of thyroid hormones on the Npt2a promoter. To address the role of Npt2a in T3-dependent regulation of phosphate homoeostasis and to identify the molecular mechanisms by which thyroid hormones modulate Npt2a gene expression, mice were rendered pharmacologically hypo- and hyper-thyroid. Hypothyroid mice showed low levels of serum phosphate and a marked decrease in renal Npt2a protein abundance. Importantly, we also showed that Npt2a-deficient mice had impaired serum phosphate responsiveness to T3 compared with wild-type mice. Promoter analysis with a luciferase assay revealed that the transcriptional activity of a reporter gene containing the Npt2a promoter and intron 1 was dependent upon TRs (thyroid hormone receptors) and specifically increased by T3 in renal cells. Deletion analysis and EMSAs (electrophoretic mobility-shift assays) determined that there were unique TREs (thyroid-hormone-responsive elements) within intron 1 of the Npt2a gene. These results suggest that Npt2a plays a critical role as a T3-target gene, to control phosphate homoeostasis, and that T3 transcriptionally activates the Npt2a gene via TRs in a renal cell-specific manner.

Coagulation and fibrinolysis in thyroid dysfunction

Various abnormalities of hemostasis, ranging from subclinical laboratory abnormalities to clinically significant disorders of hemostasis, and rarely major hemorrhage or thromboembolism, may occur in patients with thyroid diseases. The objective of this review is to discuss the relationships between thyroid dysfunction and hemostasis (primary hemostasis and coagulation/fibrinolytic system). According to the recent literature, most of the hemostatic abnormalities associated with thyroid dysfunction are the consequences of direct effects of thyroid hormones on the synthesis of various hemostatic parameters. Thyroid autoimmunity may also modify the processes of primary and secondary hemostasis. We have concluded that hyperthyroidism is generally associated with hypercoagulability and hypofibrinolysis, whereas the hemostatic profile in hypothyroidism depends on the severity of the disease. As few data are available on hemostasis in subclinical thyroid disease, further studies on this subject are needed.

Influence of hyperthyroid conditions on gene expression in extraocular muscles of rats

Extraocular muscles (EOMs) are a highly specialized type of tissue with a wide range of unique properties, including characteristic innervation, development, and structural proteins. Even though EOMs are frequently and prominently affected by thyroid-associated diseases, little is known about the direct effects of thyroid hormone on these muscles. To create a comprehensive profile of changes in gene expression levels in EOMs induced by thyroid hormone, hyperthyroid conditions were simulated by treating adult Sprague-Dawley rats with intraperitoneal injections of the thyroid hormone 3,3',5-triiodo-L-thyronine (T(3)); subsequently, microarray analysis was used to determine changes in mRNA levels in EOMs from T(3)-treated animals relative to untreated control animals. The expression of 468 transcripts was found to be significantly altered, with 466 of these transcripts downregulated in EOMs from T(3)-treated animals. The biological processes into which the affected genes could be grouped included cellular metabolism, transport, biosynthesis, protein localization, and cell homeostasis. Moreover, 15 distinct biochemical canonical pathways were represented among the genes with altered transcription levels. Strikingly, myostatin (Gdf8), a potent negative regulator of muscle growth, was found to be strongly downregulated in EOMs from T(3)-treated animals. Together, these findings suggest that pathological concentrations of thyroid hormone have a unique effect on gene expression in EOMs, which is likely to play a hitherto neglected role in thyroid-associated ophthalmopathies.

Thyroid Function and Alzheimer's Disease

Thyroid dysfunction has been implicated as a cause of reversible cognitive impairment and as such, the thyroid stimulating hormone has long been part of the screening laboratory test for dementia. Recently, several population-based studies demonstrated an association between hypo- or hyperthyroidism and Alzheimer's disease. This review discusses the role of thyroid hormone in the normal development and regulation of central nervous system functions and summarizes the studies that have linked thyroid function and dementia risk. Finally, it explores possible biological mechanisms to explain this association, including the direct effects of thyroid hormone on cerebral amyloid processing, neurodegeneration and thyrotropin-mediated mechanisms and vascular mediated enhancement of Alzheimer's disease risk.

Thyroid hormone effect in human hepatocytes

We have already demonstrated that a combined treatment of methimazole and an antioxidant mixture improved the condition of hyperthyroid patients both biochemically and clinically. Elevated thyroid hormone levels might trigger signs and symptoms of hyperthyroidism through the increase of free radicals. To study the direct effect of thyroid hormone on cellular markers of oxidative stress, we carried out in vitro assays in which 0.1–20.0 nM T3 (6.5–1300.0 ng/dl) doses were added to culture media of the human hepatocyte cell line Hep G2 for 1–24 h. T3 increased malondialdehyde (MDA) and intracellular oxidized glutathione (GSSG) levels; SOD activity was also higher with hormone treatment, whereas catalase and glutathione peroxidase activities showed no variation at different T3 doses and during all experimental times. When ascorbic acid was added to the culture, the MDA level decreased and SOD activity was increased. With higher doses of T3 (e.g. 200 nM), cell death occurred (69% of apoptotic cells). The increase in SOD activity was not enough to overcome the effect of T3 since MDA and GSSG remained high during a 24-h experiment. We showed a beneficial effect of ascorbic acid when cells were exposed to a T3 dose of 20 nM, a higher level of hormone than that achieved in hyperthyroidism.

Mitochondrial gene expression is regulated at multiple levels and differentially in the heart and liver by thyroid hormones

Biogenesis of the oxidative phosphorylation system (OXPHOS) requires the coordinated expression of the nuclear and the mitochondrial genomes. Thyroid hormones play an important role in cell growth and differentiation and are one of the main effectors in mitochondrial biogenesis. To determine how mtDNA expression is regulated, we have investigated the response of two different tissues, the heart and liver, to the thyroid hormone status in vivo and in vitro. We show here that mtDNA expression is a tightly regulated process and that several levels of control can take place simultaneously. In addition, we show that the mechanisms operating in the control of mtDNA expression and their relevance differ between the two tissues, being gene dosage important only in heart while transcription rate and translation efficiency have more weight in liver cells. Another interesting difference is the lack of a direct effect of thyroid hormones on heart mitochondrial transcription.

Treatment of severe hypothyroidism in a patient with progressive renal failure leads to significant improvement of renal function

The case of a 41-year-old patient with end-stage renal failure and diabetes mellitus Type 1 who was being prepared for renal replacement therapy is described. After severe hypothyroidism was diagnosed, thyroid hormone substitution therapy was started. Subsequently, a substantial decline in serum creatinine was observed. Creatinine clearance rose from 19 to 40 ml/min and renal replacement therapy was no longer imminent. Several studies have described the pathophysiology of diminished renal function in hypothyroidism. Few studies or case reports have shown amelioration of end-stage renal failure as seen in our patient. The etiology is presumed to be multifactorial, in which hemodynamic effects and a direct effect of thyroid hormone on the kidney play an important role. Diagnosing signs of hypothyroidism and therapy with thyroid hormone in progressive renal failure could be very important in delaying the need for renal replacement therapy.

Two uniquely arranged thyroid hormone response elements in the far upstream 5′ flanking region confer direct thyroid hormone regulation to the murine cholesterol 7α hydroxylase gene

Cholesterol 7α hydroxlyase (CYP7A1) is a key enzyme in cholesterol catabolism to bile acids and its activity is important for maintaining appropriate cholesterol levels. The murine CYP7A1 gene is highly inducible by thyroid hormone in vivo and there is an inverse relationship between thyroid hormone and serum cholesterol. Eventhough gene expression has been shown to be upregulated, whether the induction was mediated through a direct effect of thyroid hormone on the CYP7A1 promoter has never been established. Using gene targeted mice, we show that either of the two TR isoforms are sufficient to maintain normal hepatic CYP7A1 expression but a loss of both results in a significant decrease in expression. We also identified two new functional thyroid hormone receptor-binding sites in the CYP7A1 5′ flanking sequence located 3 kb upstream from the transcription start site. One site is a DR-0, which is an unusual type of TR response element, and the other consists of only a single recognizable half site that is required for TR/retinoid X receptor (RXR) binding. These two independent TR-binding sites are closely spaced and both are required for full induction of the CYP7A1 promoter by thyroid hormone, although the DR-0 site was more crucial.

Relationship between Subclinical Thyroid Dysfunction and Femoral Neck Bone Mineral Density in Women

Osteoporosis associated with thyroid dysfunction has been traditionally viewed as a secondary consequence of altered thyroid function, but there was recently a report about the direct effects of thyroid-stimulating hormone (TSH) on bone remodeling, which was mediated via the TSH receptor found on osteoblast and osteoclast precursor cells. Endogenous subclinical thyroid dysfunction seems to be an appropriate model to examine the direct effect of TSH on bone metabolism while ruling out the direct effect of thyroid hormone on bone metabolism. Thus, we aimed to investigate the relationship between subclinical thyroid dysfunction and bone mineral metabolism in women. We enrolled 413 women (mean age: 52.2 +/- 6.6 years) in our study. Serum levels of TSH, free T4 and the biochemical markers of bone turnover were measured by the standard methods. BMD at the lumbar spine and femoral neck were measured by dual energy X-ray absorptiometry. Femoral neck BMD was significantly reduced both in the subclinical hyperthyroid group and in the subclinical hypothyroid group as compared with the euthyroid group (one-way ANOVA, p <0.001; post-hoc analysis, p = 0.041, p = 0.033). In contrast to the femoral neck BMD, the lumbar spine BMD showed no difference between the two groups. Additionally, serum calcium and alkaline phosphatase levels, urine deoxypyridinoline levels, and urine calcium to creatinine ratio showed no differences between the two groups. Women having subclinical hyperthyroidism and women with subclinical hypothyroidsm have reduced femoral neck BMD. Additional studies are required to elucidate the mechanism for this finding.

Blockade of Angiotensin II Type 1 Receptor Diminishes Cardiac Hypertrophy, but Does Not Abolish Thyroxin-induced Preconditioning

Growth and stress seem to share common intracellular pathways and activation of growth signaling can increase resistance to stress. Thyroid hormone induces cardiac hypertrophy and preconditions the myocardium against ischemia reperfusion injury. The present study investigated whether this response is mediated by renin-angiotensin system (RAS). RAS is shown to be activated in hyperthyroidism and is involved in the development of cardiac hypertrophy. Male Wistar rats were treated with L-thyroxin (25 microg/100 g, sc, od) for fourteen days, while normal rats served as controls. In addition, irbesartan (150 mg/kg po), a potent blocker of angiotensin II type 1 receptor (AT1), was given with L-thyroxin for fourteen days. Isolated hearts were perfused in Langendorff mode; after stabilization, they were subjected to 20 min zero-flow global ischemia and 45 min of reperfusion. Thyroxin induced cardiac hypertrophy, which was diminished with irbesartan administration. Post-ischemic recovery of function was increased in thyroxin-treated hearts as compared to controls while ischemic contracture was accelerated and intensified. Irbesartan did not abolish this response. In conclusion, blockade of angiotensin II type 1 receptor with irbesartan preserves thyroxin-induced cardioprotection while diminishing cardiac hypertrophy. It is likely that thyroxin-induced cardioprotection is due to a direct effect of thyroid hormone.

Myocardial ultrastructure in cardiac hypertrophy induced by thyroid hormone?an acute study in rats

The early responses of the myocardium ultrastructure under thyroid dysfunction conditions, hemodynamic parameters, cardiac hypertrophy and ultrastructural evaluations were performed in hypothyroid and hyperthyroid rats submitted to different doses [T4-25 and T4-100; 0.025 mg and 0.1 mg kg(-1) body weight (BW).per day, respectively)]. All groups were treated for 7 days. The animals were sacrificed, the hearts were excised and weighed and the left ventricle tissue samples were processed for transmission election microscopy. Systolic blood pressure (SBP) was not altered by administration of T4. An increased heart rate and ratio of heart weight to body weight (HW/BW) were found in the hyperthyroid rats. However, the SBP and HW/BW decreased significantly in hypothyroid rats. No significant ultrastructural alterations were detected when the hypothyroid and T4-25 groups were compared with the control group. Alterations of cardiomyocytes nuclei of these groups were also not detected. Notably, disorganization of intercellular junctions was observed in many cardiomyocytes of T4-100 group. The present results indicate that in the early stages of hyperthyroidism, the cardiac hypertrophy development was mainly due to direct effects of thyroid hormone. Despite cardiac hypertrophy development, there is no ultrastructural evidence of myocardial degeneration.

Inhibition of the activity of the native γ-aminobutyric acid A receptor by metabolites of thyroid hormones: correlations with molecular modeling studies

To characterize the direct effects of thyroid hormones on native γ-aminobutyric acid A (GABA A) receptors, rapid (5 s) actions of a series of iodothyronines on muscimol-stimulated uptake of 36Cl − were investigated in synaptoneurosomes prepared from rat brain. The results were correlated with molecular modeling of the active compounds. Dose–response curves for muscimol in the presence of 3,3′, 5- l-triiodothyronine ( l-T3) indicated a noncompetitive inhibition of muscimol-stimulated 36Cl − uptake by the thyroid hormone. Synaptoneurosomes prepared from cerebellum were less sensitive to l-T3 than those from cerebral cortex, in terms of the potency of the hormone. The overall efficacy approached complete inhibition for both brain regions. Muscimol-stimulated 36Cl − uptake was inhibited differentially by iodothyronine derivatives. One group of compounds with IC 50 values of 18–30 μM included l-thyroxine ( l-T4), d-thyroxine ( d-T4), 3,3′, 5,5′-tetraiodothyroacetic acid (Tetrac), and 3,3′, 5-triiodothyroacetic acid (Triac). A second group with values of 75–100 μM included 3,3′, 5′- l-triiodothyronine (reverse T3; r-T3), 3,3′-diiodo- l-thyronine (3,3′- l-T2) and 3,5-diiodo- l-thyronine (3,5- d-T2). A final group of inactive compounds with IC 50 values greater than 100 μM included 3′,5′-diiodo- l-thyronine (3′,5′- l-T2), 3-iodo- l-thyronine ( l-T1), 3′-iodo- l-thyronine (3′- l-T1), and l-thyronine ( l-T0). Molecular modeling of the active iodothyronines using the Gaussian03 series of programs indicated close correspondences with models of the GABA-inhibitory neurosteroid pregnenolone sulfate (PREGS), suggesting common mechanisms of action at the GABA A receptor.

Hormonal and nutritional regulation of lipogenic enzyme mRNA levels in rat primary white and brown adipocytes.

Hyroid hormone stimulates hepatic lipogenesis in the rat by increasing the expression of relevant genes, including acetyl-CoA carboxylase and fatty acid synthase. S14 mRNA, which encodes a protein thought to be involved in lipogenesis, responds in parallel. The effects of thyroid hormone on lipogenesis in white and brown adipose tissue are less clear, and may be complicated by indirect effects of the hormone. Rat white and brown preadipocytes were therefore isolated, grown to confluence, and used to test direct effects of thyroid hormone, insulin, and glucose. Lipogenesis was assessed by tritiated water incorporation, and acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), and S14 mRNAs were measured by Northern analysis. Insulin (1 nM) increased lipogenesis about 9-fold in both white and brown adipocytes. Similar increases were seen in the levels of the three mRNAs. Thyroid hormone (1 microM) stimulated lipogenesis and acetyl-CoA carboxylase, fatty acid synthase, and S14 mRNA levels up to 2-fold in both types of adipocyte in the presence or absence of insulin. A high carbohydrate level (25 mM glucose) had no effect on lipogenesis compared to a low carbohydrate level (5 mM glucose) in white and brown adipocytes. There was no synergistic effect on lipogenesis by the combination of thyroid hormone and high carbohydrate level in both types of adipocytes. These experiments have shown that T3 has small, direct stimulatory effects on lipogenesis in adipocytes. These effects are seen at a pre-translational level, through the coordinate induction of ACC, FAS, and S14 mRNAs. Although lipogenic rates were usually higher in brown adipocytes than white adipocytes, very similar patterns of regulation were seen in the two cell types. These data support the idea that the divergent results seen concerning T3 regulation of the lipogenic pathway in both brown and white adipose tissue in vivo arise from secondary effects of the alteration of thyroid status.

Ovarian Ultrasound and Ovarian and Adrenal Hormones before and after Treatment for Hyperthyroidism

Objective: To relate thyroid, steroid and pituitary hormones to ovarian ultrasonographic findings in hyperthyroid patients before and during treatment. Study Design: Ultrasonography of the ovaries and serum hormone determination by immunoassay were performed before and during thiamazole therapy in 18 women of fertile age treated for hyperthyroidism at the Danderyd Hospital from 1996 to 1998. Results: When hyperthyreotic, the patients had elevated serum levels of sex hormone-binding globulin (SHBG) and subnormal values of cortisol, free testosterone (fT) and dehydroepiandrosterone (DHEA). In the euthyreotic state following treatment, endocrine variables were normalized. Patients with a short duration of the disease had higher pretreatment levels of free thyroxine (fT4), SHBG and testosterone and lower corticosteroid binding globulin (CBG) and cortisol levels compared to patients with a long duration of the disease. The pretreatment ultrasonographic picture was abnormal in 16 of 18 patients. Of the 8 patients who were examined by ultrasonography after 3 months of treatment, all but 1 showed a normal picture. Samples from patients showing an abnormal ultrasonographic picture had significantly higher fT4 and lower free testosterone (fT) values than samples from patients with a normal ultrasonographic picture. Conclusion: Ultrasonographic findings showing a multicystic/multifollicular picture, resembling polycystic ovaries (PCO), in hyperthyroidism may be related to direct effects of thyroid hormones on the ovaries and/or altered intraovarian androgen environment due to elevated SHBG levels. It is highly recommended to assess the thyroid status in patients with multicystic/multifollicular ovaries/PCO.

TR expression and function in human bone marrow stromal and osteoblast-like cells.

Thyroid hormones influence both bone formation and bone resorption. In vitro studies demonstrate direct effects of thyroid hormones on cells of the osteoblast lineage. Transcriptional regulation by thyroid hormones is mediated by ligand-dependent transcription factors called TRs. The three main T(3)-binding TR isoforms are TRalpha1, TRbeta1, and TRbeta2. TRs have been identified in cells of the osteoblast lineage, but it is still not known whether TR isoform expression differs in primary cultures of human osteoblasts. We used immunocytochemistry, Western blotting, nuclear binding assays, and transient transfection studies to examine the expression of functional TR isoforms in primary cultures of osteoblasts (hOb) derived from explants of trabecular bone, in human bone marrow stromal cells (hBMS), which are believed to be the source of osteoblast progenitor cells, and for comparison in the transformed human osteosarcoma cell lines MG63 and SaOs-2. TRalpha1, TRbeta1, and TRbeta2 proteins were expressed in all cells, although expression was greatest in MG63 > hBMS > SaOs-2 > hOb. Differences between isoforms were also apparent, with TRalpha1> TRbeta1 > TRbeta2 in all cell types. Incubation with [(125)I]T(3) confirmed reversible T(3) binding to cell nuclei. Specific binding was greatest in MG63 > hBMS > SaOs-2 > hOb. Finally, endogenous TR activity was determined in transfections using a thyroid hormone response element derived from the rat GH gene linked to the luciferase reporter gene. In MG63 and hBMS cells T(3) treatment increased luciferase activity 5.5 +/- 0.7-fold (P < 0.05), confirming the presence of endogenous receptors. In SaOs-2 and hOb cells, T(3) treatment had no effect on thyroid hormone response element-thymidine kinase-luciferase expression, suggesting that in these cells TR expression was too low to be detected. These results indicate that three main TR isoforms are expressed in cells of the human osteoblast lineage, but that expression and endogenous TR activity are predominantly present in hBMS cells. Whether there are distinct mechanisms of thyroid hormone action mediated by TRalpha1, TRbeta1, and TRbeta2 in hOb and hBMS cells remains to be shown.