Thyroid hormone action: insight from transgenic mouse models.

Thyroid hormone receptors (TRs) are cellular homologues of the viral erythroblastic leukemia oncogene (v-erbA). TRs (c-crbA isoforms) are derived from two separate gene loci in mammals: a and beta. Through a series of knockout experiments in mice in which one or several of the TR isoforms were deleted, it has been demonstrated that the TR-beta isoforms control central regulation of thyroid-stimulating hormone. Of these isoforms, TR-beta2 is the most important in mediating negative feedback control of the hypothalamic-pituitary-thyroid axis. Further analysis of TR knockout animals revealed, however, that they exhibited a much milder overall phenotype than hypothyroid animals, indicating that receptor loss was not equivalent to ligand loss in vivo. To understand this apparent paradox, we generated animals expressing a non-T3 binding receptor (delta337T) from the TR-beta allele. These mice displayed a complete hypothyroid phenotype, demonstrating that the unliganded TR mediates the effect of hypothyroidism. Because this mutant TR constitutively binds to nuclear coreprssors, it also suggests that this class of proteins is essential for mediating hypothyroidism in vivo.     

Melanin-concentrating hormone overexpression in transgenic mice leads to obesity and insulin resistance

Several lines of investigation suggest that the hypothalamic neuropeptide melanin-concentrating hormone (MCH) regulates body weight in mammals. Obese mice lacking functional leptin overexpress the MCH message in the fed or fasted state. Acute intracerebroventricular injection of MCH increases energy intake in rats. Mice lacking the MCH gene are lean. To test the hypothesis that chronic overexpression of MCH in mice causes obesity, we produced transgenic mice that overexpress MCH (MCH-OE) in the lateral hypothalamus at approximately twofold higher levels than normal mice. On the FVB genetic background, homozygous transgenic animals fed a high-fat diet ate 10% more and were 12% heavier at 13 weeks of age than wild-type animals, and they had higher systemic leptin levels. Blood glucose levels were higher both preprandially and after an intraperitoneal glucose injection. MCH-OE animals were insulin-resistant, as demonstrated by markedly higher plasma insulin levels and a blunted response to insulin; MCH-OE animals had only a 5% decrease in blood glucose after insulin administration, compared with a 31% decrease in wild-type animals. MCH-OE animals also exhibited a twofold increase in islet size. To evaluate the contribution of genetic background to the predisposition to obesity seen in MCH-OE mice, the transgene was bred onto the C57BL/6J background. Heterozygote C57BL/6J mice expressing the transgene showed increased body weight on a standard diet, confirming that MCH overexpression can lead to obesity.     

Age-specific thyroid hormone and thyrotropin reference intervals for a pediatric and adolescent population

Abstract Background: Establishment of reliable reference intervals remains valuable for confirming validity and advancing standardization across methods and populations. Moreover, knowledge of the measurement uncertainty (U) and of the reference change value (RCV) has important applications in clinical chemistry. Methods: Starting from the information available in the laboratory data base (29,901 subjects) an initial selection was carried out by eliminating all subjects with a clinical or laboratory pathological report; data from 7581 0- to 20-year-old subjects (53.87% girls) remained in the study. These subjects, divided into nine age groups, were used to define reference distribution percentiles (2.5th, 50th and 97.5th) of serum thyrotropin (TSH), triiodothyronine (T3), thyroxine (T4), and free T4 (fT4), as well as U and RCV of these assays. Results: In early infancy, T4 and fT4 values were higher than in the older age groups. Serum T4 95th percentile reference value, useful for the diagnosis of hyperthyroidism, was 142.9 in 20-year-old boys and 230.4 nmol/L in early infants and serum T3 95th percentile was 2.6 and 3.5 nmol/L, respectively, while fT4 2.5th percentile reference value, useful for the diagnosis of hypothyroidism, was 9.6 and 13.0 pmol/L, respectively. Serum TSH 97.5th percentile showed less age variation, 4.38-4.88 mIU/L. Performance of the four assays resulted in approximately 20% Us, reflecting simple and complex imprecision, trueness, analytical and functional sensitivity. RCV of serum TSH (58.6%) was larger than for thyroid hormones (28.3%-34.7%), probably due to the high biological variation of this hormone. Conclusions: We have established reference interval for TSH and thyroid hormones, as well as Us for assessing reliability of measurements, and RCVs to alert users on the presence of clinical significant changes.     

甲状腺激素抵抗1例临床报道

病例:女,17岁。因心悸、怕热、多汗6年,拟“Graves病”于2004年8月入院。患者6年前无明显诱因出现心悸、怕热、多汗、消瘦、手颤,当地诊所诊断为“甲状腺肿”,予以甲状腺素片口服,治疗半年后症状无好转而停药。2001年家长发现患者颈部较前增粗,又至外院就诊,诊断为“Graves病”     

DNA-based diagnosis of thyroid hormone resistance syndrome: a novel THRB mutation associated with mild resistance to thyroid hormone

BACKGROUND: Thyroid hormones govern a wide range of metabolic processes in the body via thyroid hormone receptors (TR). We report a patient with mild resistance to thyroid hormone who was initially misdiagnosed and treated as having thyrotoxicosis. METHODS: We used direct DNA sequencing of the THRB gene. RESULTS: We identified a novel missense mutation, I276L, located in exon 8 of the gene. The mutation is located in cluster 3 of the ligand-binding domain, a protein domain associated with resistance to thyroid hormone. CONCLUSION: DNA-based diagnosis of thyroid hormone resistance syndrome is simple, reliable, and economical compared to traditional biochemical tests. Once the mutation is identified, targeted screening for the whole family can be performed and the unnecessary use of anti-thyroid drugs or thyroidectomy can be avoided.     

Inhibition of thyrotropin response to thyrotropin-releasing hormone by small quantities of thyroid hormones

Inhibition of thyrotropin (TSH) release by chronic treatment with small quantities of triiodothyronine (T(3)) and thyroxine (T(4)) was evaluated by determining the serum TSH response to thyrotropin-releasing hormone (TRH) in normal subjects and hypothyroid patients. Response to TRH was determined before treatment and after each dosage of a synthetic combination of T(3) + T(4) had been given for 3-4 wk.Treatment of eight normal subjects with 15 mug T(3) + 60 mug T(4) reduced the maximum increase in serum TSH above baseline (maximum DeltaTSH) by 76% in response to 400 mug TRH and by 87% in response to 25 mug TRH. The average serum T(3) level during a 24 hr period in normal subjects who had been taking 15 mug T(3) + 60 mug T(4) for 3-4 wk was 129+/-10 ng/100 ml (mean +/-SEM), well within the normal range, 70-150 ng/100 ml, although higher than the pretreatment level, 98+/-7 ng/100 ml. The average serum T(4) level was unchanged from the pretreatment level. Treatment of the same subjects with 30 mug T(3) + 120 mug T(4) reduced the maximum DeltaTSH further.Six patients with primary hypothyroidism were treated, sequentially, with 15 + 60, 22.5 + 90, and 30 mug T(3) + 120 mug T(4). For each patient there was one increase in dosage of 7.5 mug T(3) + 30 mug T(4) which abruptly converted a maximum DeltaTSH that was greater than, or at the upper limit of, normal to one that was subnormal. Concurrent with these six abrupt changes in TSH response, the mean serum T(3) level increased only from 105+/-5 to 129+/-9 ng/100 ml, and the mean serum T(4) level increased only from 4.9+/-0.8 to 6.3+/-0.5 mug/100 ml.These data demonstrate the extreme sensitivity of TRH-induced TSH release to inhibition by the chronic administration of quantities of T(3) + T(4) which do not raise serum T(3) and T(4) levels above the normal ranges.     

Critical role for thyroid hormone receptor beta2 in the regulation of paraventricular thyrotropin-releasing hormone neurons

Thyroid hormone thyroxine (T(4)) and tri-iodothyronine (T(3)) production is regulated by feedback inhibition of thyrotropin (TSH) and thyrotropin-releasing hormone (TRH) synthesis in the pituitary and hypothalamus when T(3) binds to thyroid hormone receptors (TRs) interacting with the promoters of the genes for the TSH subunit and TRH. All of the TR isoforms likely participate in the negative regulation of TSH production in vivo, but the identity of the specific TR isoforms that negatively regulate TRH production are less clear. To clarify the role of the TR-beta2 isoform in the regulation of TRH gene expression in the hypothalamic paraventricular nucleus, we examined preprothyrotropin-releasing hormone (prepro-TRH) expression in mice lacking the TR-beta2 isoform under basal conditions, after the induction of hypothyroidism with propylthiouracil, and in response to T(3) administration. Prepro-TRH expression was increased in hypothyroid wild-type mice and markedly suppressed after T(3) administration. In contrast, basal TRH expression was increased in TR-beta2-null mice to levels seen in hypothyroid wild-type mice and did not change significantly in response to induction of hypothyroidism or T(3) treatment. However, the suppression of TRH mRNA expression in response to leptin reduction during fasting was preserved in TR-beta2-null mice. Thus TR-beta2 is the key TR isoform responsible for T(3)-mediated negative-feedback regulation by hypophysiotropic TRH neurons.     

Endothelial dysfunction in a murine model of thyroid hormone resistance

BACKGROUND: The development of a knockin mouse model of resistance to thyroid hormone (RTH) has led to a greater understanding of both the molecular and clinical behaviour of this syndrome. We have investigated the vascular response in RTH using a specific (TRbeta PV) knockin mouse model targeting the PV mutation to the thyroid hormone receptor beta gene locus. MATERIALS AND METHODS: Ring segments of the thoracic aorta were used to assess the response of homozygous, heterozygous and wild-type controls to contractile agents, potassium chloride and phenylephrine. Each genotype after maximal contraction was exposed to increasing concentrations of relaxing agents, acetylcholine (ACh) and sodium nitroprusside (SNP). RESULTS: The response of these aortic ring segments to ACh and SNP demonstrates that endothelium-dependent relaxation to ACh was significantly impaired in both heterozygous and homozygous mice compared to controls (69.8 +/- 2.0%, 59.7 +/- 1.4% and 75.0 +/- 1.7%, respectively; P < 0.001). However, endothelium independent responses to SNP showed no difference between genotypes (114.4 +/- 3.2%, 116.8 +/- 2.6% and 106.9 +/- 4.9%; P = NS). CONCLUSION: These data suggest that endothelial function is impaired in the RTH mouse aorta. The respective roles of elevated thyroid stimulating hormone (TSH), elevated thyroid hormone concentrations and the mutated thyroid hormone beta receptor require further elucidation.     

Recombinant human thyrotropin in the management of thyroid disorders

Background: Recombinant human thyroid stimulating hormone (rhTSH) is a valuable tool in the management of thyroid diseases. It is useful for radioiodine ablation in patients with differentiated thyroid carcinoma (DTC), for their follow-up, and for treatment of selected patients with metastatic or recurrent DTC. More recently, it has been suggested that rhTSH is useful for treatment of multinodular goiter and amiodarone-induced thyrotoxicosis, as well as for diagnosis of congenital hypothyroidism. Objective: To provide an outline of literature regarding the uses of rhTSH in thyroid diseases. Methods: We performed a literature search for relevant articles in the PubMed database. Conclusion: rhTSH has important roles in management of thyroid diseases, and some are still controversial. For patients with DTC, it avoids the need for thyroid hormone withdrawal, without being detrimental to short-term outcomes. Further studies are warranted to assess its effects on long-term outcomes.     

Tissue responses to thyroid hormone in a kindred with resistance to thyroid hormone harboring a commonly occurring mutation in the thyroid hormone receptor beta gene (P453T)

Resistance to thyroid hormone (RTH) is a dominantly inherited syndrome of reduced tissue responsiveness to thyroid hormone (TH) usually due to mutations in the TH receptor beta gene (TRbeta). We studied pituitary and peripheral tissue responses to graded doses of liothyronine (L-T3) in 5 affected members (2 children and 3 adults) of a family with RTH due to the common TRbeta mutation P453T. Overall, the 5 subjects studied exhibited suppressed thyrotropin response to thyrotropin-releasing hormone of 51% +/- 8%, 12.1% +/- 1.5%, and 6.3% +/- 3% of the 100% baseline on 50, 100, and 200 microg/dL L-T3, respectively. This degree of suppression was greater than that observed in subjects with RTH due to other TRbeta mutations, indicating less resistance. Compared with normal subjects, however, the family described here demonstrated less suppression by L-T3, compatible with their RTH, although of a mild magnitude. The 2 children with RTH demonstrated less L-T3-mediated suppression of prolactin and cholesterol than the adults. Patients often receive thyroid ablative therapy before the diagnosis of RTH and are left with variable degrees of hypothyroidism. Our results demonstrate that graded doses of L-T3 can be used to evaluate RTH patients, even under the condition of limited thyroid reserve, when results are compared with their baseline. We demonstrate that RTH patients can be evaluated either on or off thyroid hormone and still be distinguished from hypothyroid subjects without RTH.     

甲状腺功能正常的非糖尿病人群甲状腺激素水平与胰岛素抵抗的关系

目的 对甲状腺功能正常的非糖尿病人群中甲状腺激素水平与胰岛素抵抗关系进行横断面研究。方法 依托2011年由中华医学会发起的REACTION研究,选取大连地区符合标准的研究对象共5 428例。以促甲状腺激素（TSH）2.5 mU/L为分界,分为低TSH组及高TSH组,测量两组身高、体重、血压,进行口服葡萄糖耐量(OGTT),检验血脂、血肌酐（SCr）、血尿酸（UA）、血糖、糖化血红蛋白（HbA1c）、空腹胰岛素（FINS）、甲状腺功能,稳态模型评估胰岛素抵抗指数（HOMA IR）。研究甲状腺激素水平与胰岛素抵抗的相关性。结果 与低TSH组比较,高TSH组SCr、UA及游离甲状腺素（FT4）水平降低,低密度脂蛋白胆固醇（LDL C）、胆固醇（TC）、HbA1c、FINS及HOMA IR增高（P＜0.05）。TSH 、游离三碘甲状腺原氨酸（FT3）与HOMA IR呈正相关（r=0.145、0.040,P＜0.05）。而FT4水平与HOMA IR指数呈负相关（r=-0.071,P=0.000）。回归分析显示性别、舒张压（DBP）、BMI、高密度脂蛋白胆固醇（HDL C）、LDL C、TC、HbA1c、UA、TSH、FT4为影响HOMA IR的独立危险因素（P＜0.05）。其中HDL C及FT4为保护性因素（P＜0.05）。结论 在正常范围内的TSH水平与非糖尿病人群的胰岛素抵抗呈正相关,FT4与胰岛素抵抗呈负相关,且TSH、FT4为影响胰岛素抵抗的独立危险因素。     

A novel 1297-1304delGCCTGCCA mutation in the exon 10 of the thyroid hormone receptor beta gene causes resistance to thyroid hormone.

INTRODUCTION: Resistance to the thyroid hormone (RTH) is an inherited syndrome of reduced tissue responsiveness to hormonal action caused by mutations located in the ligand-binding domain and adjacent hinge region of the thyroid hormone receptor beta (TRbeta) gene. PATIENT: The patient in this study, a 42-year-old Caucasian male, came to medical attention because he experienced atrial fibrillation. Clinical evaluation showed a small and diffuse goiter and biochemical tests revealed markedly elevated concentrations of total T4, total T3, and free T4, normal thyroid-stimulating hormone (TSH) values and slightly increased I131 thyroid uptake at 24 hours. The thyroperoxidase, thyroglobulin, and TSH receptor antibodies were positive. He was treated with cabergoline plus methimazole. This treatment was stopped because of the inconsistent response, monotherapy with tri-iodothyroacetic acid (TRIAC) was then prescribed after molecular diagnosis confirmed RTH syndrome. METHODS: The exons 9 and 10 of the TRbeta gene, including splicing signals and the flanking intronic regions of each intron, were amplified with PCR. DNA sequences from each amplified fragment were performed with the Taq polymerase-based chain terminator method and using the specific TRbeta forward and reverse primers. RESULTS: Direct sequence analysis of the exons 9 and 10 of the TRbeta gene revealed an eight basepair deletion, 1297-1304delGCCTGCCA in exon 10. The mutation produces a frameshift at amino acid 433 and introduces a stop codon TGA at position 461, 85 nucleotides downstream from deletion. This alteration was not detected in either the father or mother of the patient, suggesting a de novo mutation that was confirmed by DNA fingerprint analysis. CONCLUSIONS: In the present study we have identified a novel sporadic mutation corresponding to 1297-1304delGCCTGCCA deletion in the activating function 2 (AF-2) region of TRbeta. To our knowledge, this is the first time that the presence of a partial deletion of eight nucleotides in the TRbeta has been reported.     

Complex regulation of thyroid hormone action: multiple opportunities for pharmacological intervention

The thyroid hormone (TH; 3,3',5,5'-tetra-iodothyronine and 3,3',5'-triiodothyronine) regulates growth, development, and critical metabolic functions. Thyroid diseases are among the most prevalent group of metabolic disorders in the Western world. TH exerts effects through complex biological pathways, which offer a wealth of opportunities to pharmacologically intervene in TH signalling at numerous steps. These include biosynthesis, cell-specific uptake or export (involving L-type amino acid transporter, organic anion transporter, organic cation transporter, or multidrug resistance transporter), as well as nuclear targeting and actions (the latter including TH receptor binding and histone acetylation/deacetylation). Such processes represent potentially important pharmacological targets for the design of novel or improved therapies for TH disorders, obesity, and cardiovascular diseases.     

New insights on the mechanism(s) of the dominant negative effect of mutant thyroid hormone receptor in generalized resistance to thyroid hormone.

Generalized resistance to thyroid hormone (GRTH) is a syndrome of hyposensitivity to triiodothyronine (T3) that displays autosomal dominant inheritance. The genetic defect commonly lies in the ligand-binding domain of one of the TR beta alleles. Since there are two major thyroid hormone receptor (TR) isoforms, TR alpha and TR beta, it is not known how the mutant receptor mediates a dominant negative effect. Previously, we showed that T3 caused dissociation of TR homodimers and TR alpha/TR beta dimers from several thyroid hormone response elements (TREs). Hence, we used the electrophoretic mobility shift assay to compare the effect of T3 on the DNA binding of mutant TR beta-1 (Mf-1) from a kindred with GRTH with normal TR beta. Mf-1 bound better as a homodimer than TR beta, but dissociated from DNA only at high T3 concentrations. Both receptors heterodimerized with nuclear auxiliary proteins. They also dimerized with TR alpha and with each other. Surprisingly, T3 disrupted the DNA binding of the Mf-1/TR isoform dimers. Thus, mechanisms for the dominant negative effect by mutant TRs likely involve either increased binding to TREs by mutant homodimers that cannot bind T3 (hence cannot dissociate from DNA) and/or the formation of inactive mutant TR/nuclear protein heterodimers.