RESUMEN

OBJECTIVE: Thyroid hormone influences key metabolic pathways, and reduced sensitivity to thyroid hormone is considered a new risk factor for adverse metabolic outcomes. However, the association between thyroid hormone resistance and obesity in euthyroid individuals is still unknown. METHODS: We enrolled 8021 euthyroid individuals, calculated thyroid hormone resistance indices, and analyzed the association between thyroid hormone resistance and obesity by regression analysis. Furthermore, we conducted the thyrotropin-releasing hormone stimulation test in both control and obese mice (n = 5) to demonstrate the association. RESULTS: The euthyroid adults with overweight and obesity had increased thyroid hormone resistance indices (all p < 0.05). BMI and prevalence of overweight and obesity increased (odds ratio of thyroid feedback quantile-based index [ORTFQI] = 1.164, p = 0.036; OR of free triiodothyronine/free thyroxine [ORFT3/FT4] = 1.508, p < 0.001) following the elevation of thyroid hormone resistance indices. Mediation analysis indicated a complete mediation effect (beta coefficient of indirect effect [ßInd]= 6.838, p < 0.001) of metabolic disorders in the relationship. Furthermore, in the mice with obesity, the thyrotropin response to thyrotropin-releasing hormone stimulation (68.33-90.89 pg/mL) was comparatively blunted (p = 0.029). CONCLUSIONS: Euthyroid individuals with obesity exhibit both central and peripheral thyroid hormone resistance, a phenomenon that is more pronounced in individuals with metabolic abnormalities. Thyroid hormone resistance is associated with an increased prevalence of overweight and obesity mediated by metabolic disorders.

Asunto(s)

Obesidad , Animales , Ratones , Estudios Transversales , Masculino , Humanos , Femenino , Adulto , Persona de Mediana Edad , Índice de Masa Corporal , Triyodotironina/sangre , Hormonas Tiroideas/sangre , Síndrome de Resistencia a Hormonas Tiroideas/epidemiología , Tiroxina/sangre , Hormona Liberadora de Tirotropina , Ratones Obesos , Tirotropina/sangre , Ratones Endogámicos C57BL , Sobrepeso

RESUMEN

CONTEXT: Recently, reduced sensitivity to thyroid hormone as a more common finding in the general population and its possible association with metabolic parameters has been the focus of attention. OBJECTIVE: The objective was to evaluate the cross-sectional association of thyroid hormone sensitivity with diabetes, metabolic syndrome (MetS), and its components. METHODS: The study included a Tehranian representative sample of 5124 subjects aged ≥20 years participating in the Tehran Thyroid Study (2008-2011). Body weight, waist circumference, and blood pressure (BP) were measured, and serum concentrations of lipids and lipoproteins, fasting blood glucose, insulin, free thyroxine (fT4), and thyrotropin (TSH) were assayed. Thyroid hormone resistance was calculated by the Thyroid Feedback Quantile-based Index (TFQI) and Iranian-referenced Parametric TFQI (PTFQI) and compared with 2 other indices: Thyrotroph T4 Resistance Index (TT4RI) and TSH Index. RESULTS: TFQI was significantly associated with high BP MetS criterion (OR = 1.14, 95% CI: 1.06, 1.23) and diabetes mellitus (OR = 1.16, 95% CI: 1.04, 1. 30, P = .009) in euthyroid subjects after adjusting for age, sex, smoking, physical activity, body mass index, and Homeostasis Model Assessment Index for Insulin Resistance. TFQI was not associated with new-onset diabetes contrary to known diabetes in subgroup analysis. The results were similar for PTFQI. TSHI (OR = 1.22, 95% CI: 1.08, 1.38, P = .001) and TT4RI (OR = 1.08, 95% CI: 1.01, 1.16, P < .001) were associated only with high BP in euthyroid subjects. CONCLUSION: The new TFQI index seems to be the indicator of reduced sensitivity to thyroid hormone most suitable to associate its population variations with diabetes and hypertension in euthyroid subjects; however, interpretation for diabetes should be concerned with cautions, necessitating future studies.

Asunto(s)

Diabetes Mellitus/epidemiología , Hipertensión/epidemiología , Síndrome Metabólico/epidemiología , Síndrome de Resistencia a Hormonas Tiroideas/epidemiología , Hormonas Tiroideas/metabolismo , Adolescente , Adulto , Anciano , Niño , Preescolar , Estudios Transversales , Diabetes Mellitus/metabolismo , Femenino , Humanos , Hipertensión/metabolismo , Irán/epidemiología , Masculino , Síndrome Metabólico/metabolismo , Persona de Mediana Edad , Factores de Riesgo , Pruebas de Función de la Tiroides , Glándula Tiroides , Síndrome de Resistencia a Hormonas Tiroideas/diagnóstico , Síndrome de Resistencia a Hormonas Tiroideas/metabolismo , Adulto Joven

RESUMEN

OBJECTIVE: It has been proposed that a mild form of acquired resistance to thyroid hormone may occur in the general population. Its clinical significance remains largely unknown. The objective of the study was to explore whether a newly described thyroid hormone resistance index is associated with the risk of mortality in a sample of community-dwelling euthyroid subjects representative of the adult population of Spain. DESIGN: Longitudinal observational study including 3750 individuals, free of thyroid disease, TPO antibodies-negative (<50 IU/mL) and with TSH levels within the euthyroid range (≥0.5 and ≤5.0 mUI/mL) participating in the nationwide study Di@bet.es (2008-2010). METHODS: We used the Thyroid Feedback Quantile-based Index (TFQI) as a marker of resistance to thyroid hormone. The study population was grouped into categories according to their TFQI values at baseline. Fatal events were ascertained from the national death registry (end of follow-up December 2016). RESULTS: A total of 231 deaths were recorded during an average follow-up of 7.3 years. Compared with the category with the highest sensitivity to free thyroxine (TFQI ≤ p5) (reference), the relative risk of mortality in the categories with TFQI > p5 and ≤p25; >p25 and ≤p50; >p50 and ≤p75; >p75 and ≤p95 and >p95 were 1.01, (0.47-2.19), 1.42 (0.68-2.97), 1.54 (0.74-3.22), 1.47 (0.70-3.11) and 2.61 (1.16-5.89), respectively (P for trend 0.003). The association remained significant after multivariate adjustment of the data (P for trend 0.017). CONCLUSIONS: A thyroid hormone resistance index focused on deviations of the average pituitary response to thyroid hormones may be associated with all-cause mortality independently of other conventional risk factors and comorbidities.

Asunto(s)

Enfermedades Asintomáticas/epidemiología , Síndrome de Resistencia a Hormonas Tiroideas/epidemiología , Síndrome de Resistencia a Hormonas Tiroideas/mortalidad , Adolescente , Adulto , Anciano , Anciano de 80 o más Años , Comorbilidad , Femenino , Indicadores de Salud , Humanos , Vida Independiente/estadística & datos numéricos , Estudios Longitudinales , Masculino , Persona de Mediana Edad , Gravedad del Paciente , Factores de Riesgo , España/epidemiología , Pruebas de Función de la Tiroides , Síndrome de Resistencia a Hormonas Tiroideas/patología , Adulto Joven

RESUMEN

OBJECTIVE: Thyroid hormone resistance (RTH ß) is a rare genetic disorder characterized by an altered response of target tissue to the action of thyroid hormone. Few studies on RTH ß have been carried out in southern European populations. We aimed to describe the clinical and genetic characteristics at the time of diagnosis in a Spanish cohort of patients with genetically confirmed RTH ß, with ages ranging from newborns to adults. METHODS: Retrospective multicenter study of 28 patients who were genetically confirmed as RTH ß. Clinical and biochemical data were collected from the reference centers, and the studied variables included age, sex, anthropometric data, clinical characteristics and biochemical results. In the Basque country, a simultaneous analysis of TSH and T4 is carried out in the program for the screening of inborn errors of metabolism. A molecular analysis of the thyroid hormone beta (THRB) gene was performed. RESULTS: The total cohort included 20 adults and eight pediatric patients (six newborns). Of the total, 5 (17.8%) were diagnosed by clinical characteristics (goiter, hypertension or tachycardia), 13 (46.4%) were analyzed in the context of a family study and 10 (35.7%) were diagnosed after obtaining an altered fT4 and/or TSH level in a biochemical analysis performed due to clinical symptoms unrelated to RTH ß. Four of the newborns included in the series were diagnosed by the result of neonatal screening, which allows us to estimate a minimum local incidence of RTH ß of 1/18,750 live newborns. The genetic analysis showed the presence of 12 different heterozygous mutations in the THRB gene. CONCLUSIONS: We report the clinical and genetic characteristics of a Spanish RTH ß cohort, from neonates to adults. We also describe one novel mutation in the THRB gene as the cause of the disease. The simultaneous analysis of TSH and T4 carried out in the program for the screening of inborn errors of metabolism facilitates the early diagnosis of RTH ß in newborns and has allowed us to estimate a minimum local incidence of RTH of 1/18,750 live newborns.

Asunto(s)

Biomarcadores/análisis , Resistencia a Medicamentos/genética , Mutación , Receptores beta de Hormona Tiroidea/genética , Síndrome de Resistencia a Hormonas Tiroideas/diagnóstico , Hormonas Tiroideas/efectos adversos , Adolescente , Adulto , Niño , Preescolar , Femenino , Estudios de Seguimiento , Humanos , Lactante , Recién Nacido , Masculino , Persona de Mediana Edad , Pronóstico , Estudios Retrospectivos , España/epidemiología , Síndrome de Resistencia a Hormonas Tiroideas/inducido químicamente , Síndrome de Resistencia a Hormonas Tiroideas/epidemiología , Síndrome de Resistencia a Hormonas Tiroideas/genética , Adulto Joven

RESUMEN

OBJECTIVE: Diabetes prevalence and incidence increase among individuals with hypothyroidism but also among those with hyperthyroxinemia, which seems contradictory. Both high free thyroxine (fT4) and high thyroid-stimulating hormone (TSH) are present in the resistance to thyroid hormone syndrome. A mild acquired resistance to thyroid hormone might occur in the general population and be associated with diabetes. We aimed to analyze the association of resistance to thyroid hormone indices (the Thyroid Feedback Quantile-based Index [TFQI], proposed in this work, and the previously used Thyrotroph T4 Resistance Index and TSH Index) with diabetes. RESEARCH DESIGN AND METHODS: We calculated the aforementioned resistance to thyroid hormone indices based on a U.S. representative sample of 5,129 individuals ≥20 years of age participating in the 2007-2008 National Health and Nutrition Examination Survey (NHANES). Also, to approximate TFQI, a U.S.-referenced Parametric TFQI (PTFQI) can be calculated with the spreadsheet formula =NORM.DIST(fT4_cell_in_pmol_per_L,10.075,2.155,TRUE)+NORM.DIST(LN(TSH_cell_in_mIU_per_L),0.4654,0.7744,TRUE)-1. Outcomes of interest were glycohemoglobin ≥6.5%, diabetes medication, diabetes-related deaths (diabetes as contributing cause of death), and additionally, in a fasting subsample, diabetes and metabolic syndrome. Logistic and Poisson regressions were adjusted for sex, age, and race/ethnicity. RESULTS: Odd ratios for the fourth versus the first quartile of TFQI were 1.73 (95% CI 1.32, 2.27) (P trend = 0.002) for positive glycohemoglobin and 1.66 (95% CI 1.31, 2.10) (P trend = 0.001) for medication. Diabetes-related death rate ratio for TFQI being above versus below the median was 4.81 (95% CI 1.01, 22.94) (P trend = 0.015). Further adjustment for BMI and restriction to normothyroid individuals yielded similar results. Per 1 SD in TFQI, odds increased 1.13 (95% CI 1.02, 1.25) for diabetes and 1.16 (95% CI 1.02, 1.31) for metabolic syndrome. The other resistance to thyroid hormone indices showed similar associations for diabetes-related deaths and metabolic syndrome. CONCLUSIONS: Higher values in resistance to thyroid hormone indices are associated with obesity, metabolic syndrome, diabetes, and diabetes-related mortality. Resistance to thyroid hormone may reflect energy balance problems driving type 2 diabetes. These indices may facilitate monitoring treatments focused on energy balance.

Asunto(s)

Diabetes Mellitus Tipo 2/epidemiología , Síndrome Metabólico/epidemiología , Síndrome de Resistencia a Hormonas Tiroideas/epidemiología , Adulto , Anciano , Anciano de 80 o más Años , Diabetes Mellitus Tipo 2/sangre , Diabetes Mellitus Tipo 2/complicaciones , Femenino , Humanos , Hipertiroidismo/complicaciones , Hipertiroidismo/epidemiología , Hipotiroidismo/sangre , Hipotiroidismo/complicaciones , Hipotiroidismo/epidemiología , Incidencia , Masculino , Síndrome Metabólico/sangre , Síndrome Metabólico/complicaciones , Persona de Mediana Edad , Mortalidad , Encuestas Nutricionales , Obesidad/sangre , Obesidad/complicaciones , Obesidad/epidemiología , Prevalencia , Síndrome de Resistencia a Hormonas Tiroideas/sangre , Síndrome de Resistencia a Hormonas Tiroideas/complicaciones , Síndrome de Resistencia a Hormonas Tiroideas/diagnóstico , Hormonas Tiroideas/sangre , Tirotropina/sangre , Estados Unidos/epidemiología , Adulto Joven

RESUMEN

Context: Resistance to thyroid hormone-ß (RTH-ß) is an autosomal dominant disorder characterized by reduced sensitivity of target tissues to thyroid hormones (THs). Individuals with RTH-ß have high TH levels usually due to mutations in the TH receptor-ß (THRB) gene. The management of RTH-ß during pregnancy is challenging, as wild-type (WT) fetuses born to RTH-ß mothers have low birth weight and suppressed postnatal thyroid-stimulating hormone (TSH), due to intrauterine exposure to excess TH. Objective: To determine birth weight and postnatal TSH of WT fetuses carried by mothers with RTH-ß whose fT4 levels were maintained below 20% of the upper limit of normal (ULN). Design: Retrospective chart review. Setting: Academic institution in collaboration with off-site hospitals and private practices. Patients: Thirteen women harboring THRB gene mutations were evaluated during 18 pregnancies. Intervention: Prenatal genetic diagnosis by amniocentesis. Women carrying WT fetuses were given the option of treatment with antithyroid medication by their treating physicians with the aim to avoid serum fT4 levels above 20% of the ULN. Results: No significant difference was found in birth weight corrected for gestational age and in serum TSH levels at birth between WT and RTH-ß infants born to RTH-ß mothers. Conclusions: Prenatal diagnosis may play an important role in the management of RTH-ß during pregnancy. Aiming for maternal fT4 levels not above 50% of the ULN in RTH-ß mothers carrying WT fetuses seems to be a prudent approach that prevents the otherwise expected low birth weight and postnatal TSH suppression.

Asunto(s)

Complicaciones del Embarazo/diagnóstico , Complicaciones del Embarazo/epidemiología , Resultado del Embarazo/epidemiología , Diagnóstico Prenatal , Síndrome de Resistencia a Hormonas Tiroideas/diagnóstico , Síndrome de Resistencia a Hormonas Tiroideas/epidemiología , Adulto , Sustitución de Aminoácidos , Peso al Nacer , Femenino , Genotipo , Humanos , Masculino , Madres , Mutación Missense , Embarazo , Complicaciones del Embarazo/genética , Diagnóstico Prenatal/estadística & datos numéricos , Pronóstico , Estudios Retrospectivos , Receptores beta de Hormona Tiroidea/genética , Síndrome de Resistencia a Hormonas Tiroideas/genética , Hormonas Tiroideas/sangre , Tirotropina/sangre

RESUMEN

Context: Patients with resistance to thyroid hormone (TH) α (RTHα) are characterized by growth retardation, macrocephaly, constipation, and abnormal thyroid function tests. In addition, almost all RTHα patients have mild anemia, the pathogenesis of which is unknown. Animal studies suggest an important role for TH and TH receptor (TR)α in erythropoiesis. Objective: To investigate whether a defect in TRα affects the maturation of red blood cells in RTHα patients. Design, Setting, and Patients: Cultures of primary human erythroid progenitor cells (HEPs), from peripheral blood of RTHα patients (n = 11) harboring different inactivating mutations in TRα (P398R, F397fs406X, C392X, R384H, A382fs388X, A263V, A263S), were compared with healthy controls (n = 11). During differentiation, erythroid cells become smaller, accumulate hemoglobin, and express different cell surface markers. We assessed cell number and cell size, and used cell staining and fluorescence-activated cell sorter analysis to monitor maturation at different time points. Results: After ∼14 days of ex vivo expansion, both control and patient-derived progenitors differentiated spontaneously. However, RTHα-derived cells differentiated more slowly. During spontaneous differentiation, RTHα-derived HEPs were larger, more positive for c-Kit (a proliferation marker), and less positive for glycophorin A (a differentiation marker). The degree of abnormal spontaneous maturation of RTHα-derived progenitors did not correlate with severity of underlying TRα defect. Both control and RTHα-derived progenitors responded similarly when differentiation was induced. T3 exposure accelerated differentiation of both control- and RTHα patient-derived HEPs. Conclusions: Inactivating mutations in human TRα affect the balance between proliferation and differentiation of progenitor cells during erythropoiesis, which may contribute to the mild anemia seen in most RTHα patients.

Asunto(s)

Anemia/genética , Eritropoyesis/genética , Regulación de la Expresión Génica , Receptores alfa de Hormona Tiroidea/genética , Síndrome de Resistencia a Hormonas Tiroideas/genética , Adolescente , Adulto , Anemia/epidemiología , Anemia/fisiopatología , Estudios de Casos y Controles , Células Cultivadas , Niño , Preescolar , Eritrocitos/metabolismo , Femenino , Humanos , Incidencia , Masculino , Persona de Mediana Edad , Mutación , Pronóstico , Valores de Referencia , Rol , Células Madre/citología , Células Madre/fisiología , Síndrome de Resistencia a Hormonas Tiroideas/epidemiología , Síndrome de Resistencia a Hormonas Tiroideas/fisiopatología , Adulto Joven

RESUMEN

Hypothyroidism may occur in association with congenital parathyroid disorders determining parathyroid hormone insufficiency, which is characterized by hypocalcemia and concomitant inappropriately low secretion of parathormone (PTH). The association is often due to loss of function of genes common to thyroid and parathyroid glands embryonic development. Hypothyroidism associated with hypoparathyroidism is generally mild and not associated with goiter; moreover, it is usually part of a multisystemic involvement not restricted to endocrine function as occurs in patients with 22q11 microdeletion/DiGeorge syndrome, the most frequent disorders. Hypothyroidism and hypoparathyroidism may also follow endocrine glands' damages due to autoimmunity or chronic iron overload in thalassemic disorders, both genetically determined conditions. Finally, besides PTH deficiency, hypocalcemia can be due to PTH resistance in pseudohypoparathyroidism; when hormone resistance is generalized, patients can suffer from hypothyroidism due to TSH resistance. In evaluating patients with hypothyroidism and hypocalcemia, physical examination and clinical history are essential to drive the diagnostic process, while routine genetic screening is not recommended.

Asunto(s)

Hipotiroidismo/complicaciones , Enfermedades de las Paratiroides/complicaciones , Autoinmunidad/fisiología , Calcio/metabolismo , Femenino , Bocio/complicaciones , Bocio/metabolismo , Bocio/fisiopatología , Humanos , Hipocalcemia/complicaciones , Hipocalcemia/epidemiología , Hipoparatiroidismo/etiología , Hipoparatiroidismo/metabolismo , Hipotiroidismo/epidemiología , Hipotiroidismo/fisiopatología , Masculino , Glándulas Paratiroides/metabolismo , Glándulas Paratiroides/fisiopatología , Hormona Paratiroidea/metabolismo , Síndrome de Resistencia a Hormonas Tiroideas/complicaciones , Síndrome de Resistencia a Hormonas Tiroideas/epidemiología , Síndrome de Resistencia a Hormonas Tiroideas/metabolismo

RESUMEN

BACKGROUND: Levothyroxine (l-T4) is commonly employed to correct hormone deficiency in children with congenital hypothyroidism (CH) and in adult patients with iatrogenic hypothyroidism. OBJECTIVE: To compare the daily weight-based dosage of the replacement therapy with l-T4 in athyreotic adult patients affected by CH and adult patients with thyroid nodular or cancer diseases treated by total thyroidectomy. DESIGN AND METHODS: A total of 36 adult patients (27 females and nine males) aged 18-29 years were studied; 13 patients (age: 21.5±2.1, group CH) had athyreotic CH treated with l-T4 since the first days of life. The remaining 23 patients (age: 24±2.7, group AH) had hypothyroidism after total thyroidectomy (14 patients previously affected by nodular disease and nine by thyroid carcinoma with clinical and biochemical remission). Patient weight, serum free thyroid hormones, TSH, thyroglobulin (Tg), anti-Tg, and anti-thyroperoxidase antibodies were measured. Required l-T4 dosage was evaluated. At the time of the observations, all patients presented free thyroid hormones within the normal range and TSH between 0.8 and 2 µIU/ml. RESULTS: Patients had undetectable Tg and anti-thyroid antibodies. The daily weight-based dosage of the replacement therapy with l-T4 to reach euthyroidism in patients of group CH was significantly higher than that in those of group AH (2.16±0.36 vs 1.73±0.24 µg/kg, P<0.005). Patients of group CH treated with l-T4 had significantly higher serum TSH levels than patients of group AH (P=0.05) as well as higher FT4 concentrations. CONCLUSIONS: To correct hypothyroidism, patients of group CH required a daily l-T4 dose/kg higher than group AH patients, despite higher levels of TSH. The different requirement of replacement therapy between adult patients with congenital and those with surgical athyroidism could be explained by a lack of thyroid hormones since fetal life in CH, which could determine a different set point of the hypothalamus-pituitary-thyroid axis.

Asunto(s)

Hipotiroidismo Congénito/tratamiento farmacológico , Terapia de Reemplazo de Hormonas/métodos , Hipotiroidismo/tratamiento farmacológico , Disgenesias Tiroideas/tratamiento farmacológico , Síndrome de Resistencia a Hormonas Tiroideas/tratamiento farmacológico , Tiroxina/uso terapéutico , Adolescente , Adulto , Hipotiroidismo Congénito/sangre , Hipotiroidismo Congénito/epidemiología , Femenino , Terapia de Reemplazo de Hormonas/estadística & datos numéricos , Humanos , Sistema Hipotálamo-Hipofisario/patología , Hipotiroidismo/sangre , Hipotiroidismo/epidemiología , Masculino , Disgenesias Tiroideas/sangre , Disgenesias Tiroideas/epidemiología , Síndrome de Resistencia a Hormonas Tiroideas/sangre , Síndrome de Resistencia a Hormonas Tiroideas/epidemiología , Hormonas Tiroideas/sangre , Tiroidectomía , Tiroxina/administración & dosificación , Tiroxina/sangre , Adulto Joven

RESUMEN

Resistance to thyroid hormone (RTH) is characterized by elevated serum levels of thyroid hormones and normal or slightly increased serum thyrotropin (TSH) levels. Recently it has been suggested that chronic TSH stimulation in RTH activates intrathyroidal lymphocytes, leading to thyroid damage and autoimmune thyroid disease (AITD). Therefore, individuals with RTH have an increased likelihood of AITD compared to unaffected relatives. We here report a 33-year-old woman in whom we diagnosed Graves' disease and treated her with thiamazole (MMI). For two years, her TSH levels were suppressed when thyroid hormones were elevated and conversely they were increased when thyroid hormones levels were decreased. These findings were common for a clinical course during treatment for Graves' disease with anti-thyroid drug. However, three years after the initiation of MMI therapy, she had a normal or gradually elevated serum TSH level even though the level of thyroid hormones never decreased, indicating inappropriate secretion of TSH. We concluded she had RTH clinically, and we demonstrated by direct sequence analysis a mutation of the TRß gene, causing replacement of a glycine (G) with arginine (R) at codon 251. The finding of an elevated TSH level without decreased thyroid hormones should suggest the presence of RTH during therapy of Graves' disease.

Asunto(s)

Enfermedad de Graves/diagnóstico , Enfermedad de Graves/epidemiología , Síndrome de Resistencia a Hormonas Tiroideas/diagnóstico , Síndrome de Resistencia a Hormonas Tiroideas/epidemiología , Adulto , Antitiroideos/uso terapéutico , Comorbilidad , Femenino , Genes erbA/genética , Enfermedad de Graves/tratamiento farmacológico , Humanos , Metimazol/uso terapéutico , Mutación/genética , Síndrome de Resistencia a Hormonas Tiroideas/genética , Tirotropina/sangre

RESUMEN

At least six major steps are required for secreted thyroid hormone (TH) to exert its action on target tissues. Mutations interfering with three of these steps have been so far identified. The first recognized defect, which causes resistance to TH, involves the TH receptor beta gene and has been given the acronym RTH. Occurring in approximately 1 per 40,000 newborns, more than 1000 affected subjects, from 339 families, have been identified. The gene defect remains unknown in 15% of subjects with RTH. Two novel syndromes causing reduced sensitivity to TH were recently identified. One, producing severe psychomotor defects in > 100 males from 26 families, is caused by mutations in the cell-membrane transporter of TH, MCT8; the second, affecting the intracellular metabolism of TH in four individuals from two families, is caused by mutations in the SECISBP2 gene, which is required for the synthesis of selenoproteins, including TH deiodinases.

Asunto(s)

Yoduro Peroxidasa/genética , Proteínas de Transporte de Membrana/genética , Receptores de Hormona Tiroidea/genética , Síndrome de Resistencia a Hormonas Tiroideas/genética , Hormonas Tiroideas/metabolismo , Animales , Diagnóstico Diferencial , Humanos , Proteínas de Transporte de Membrana/metabolismo , Proteínas de Transporte de Membrana/fisiología , Modelos Biológicos , Transportadores de Ácidos Monocarboxílicos , Mutación , Linaje , Simportadores , Síndrome de Resistencia a Hormonas Tiroideas/epidemiología , Síndrome de Resistencia a Hormonas Tiroideas/patología , Síndrome de Resistencia a Hormonas Tiroideas/terapia , Hormonas Tiroideas/deficiencia , Hormonas Tiroideas/fisiología

RESUMEN

Reduced sensitivity to thyroid hormone (TH) in peripheral tissues can occur as defects in TH transport into the cell, intracellular TH metabolism, cytosolic mechanisms, TH entry into the nucleus, thyroxin receptors (TRs) and receptor binding, transcription and post-transcriptional mechanisms. Current literature reveals an extensive list of mutations, drugs, toxins, metabolites and autoimmune antibodies that may impair TH action in the cell, but such impairment may not be picked up by assays of TH and TSH in blood plasma. Substances may induce tissue specific resistance to thyroid hormone (RTH), e.g. by affecting numbers of different TR isoforms. Recent literature also indicates mechanisms by which different conditions, for example, chronic fatigue syndrome (CFS), chronic renal failure (CRF) and nonthyroidal illness, can be accompanied by acquired RTH caused by inhibition of TH metabolism, cell uptake, TR binding and transcription. This prompts us to reassess commonness and rarity of congenital vs. acquired RTH. We hypothesise that observed clinical symptoms of hypothyroidism in chemically euthyroid patients are typically caused by changes in hormonal systems, autoimmune antibodies, metabolites or other substances in the body, leading to reduced sensitivity to TH in peripheral tissues. These changes may be a by-product of other processes and a reversible biological response in the body, and may also result in chronic acquired RTH. Antibodies may prove to be the most common cause of chronic reduction in TH sensitivity. It is argued that the acquired form of RTH, caused by endogenous and exogenous sources, may indeed be more common than the congenital, as in insulin resistance. If acquired RTH exists, then it may not be picked up by blood assays of TH and TSH. An appropriate test to assess TH action in peripheral tissues is therefore greatly desired.

Asunto(s)

Inmunidad Innata , Síndrome de Resistencia a Hormonas Tiroideas/fisiopatología , Hormonas Tiroideas/fisiología , Transporte Biológico , Núcleo Celular/metabolismo , Citosol/metabolismo , Humanos , Hipotiroidismo/genética , Hipotiroidismo/fisiopatología , Incidencia , Modelos Biológicos , Síndrome de Resistencia a Hormonas Tiroideas/epidemiología , Hormonas Tiroideas/genética , Hormonas Tiroideas/metabolismo , Transcripción Genética