Markers Of Thyroid Hormone Action Research Articles

A Historical Reflection on Scientific Advances in Understanding Thyroid Hormone Action.

Background: Thyroid hormone (TH) has actions in every tissue of the body and is essential for normal development, as well as having important actions in the adult. The earliest markers of TH action that were identified and monitored clinically, even before TH could be measured in serum, included oxygen consumption, basal metabolic rate, serum cholesterol, and deep tendon reflex time. Cellular, rodent, amphibian, zebrafish, and human models have been used to study TH action. Summary: Early studies of the mechanism of TH action focused on saturable-specific triiodothyronine (T3) nuclear binding and direct actions of T3 that altered protein expression. Additional effects of TH were recognized on mitochondria, stimulation of ion transport, especially the sodium potassium ATPase, augmentation of adrenergic signaling, role as a neurotransmitter, and direct plasma membrane effects. The cloning of the thyroid hormone receptor (THR) genes in 1986 and report of the THR crystal structure in 1995 produced rapid progress in understanding the mechanism of TH nuclear action, as well as the development of modified THR ligands. These findings revealed nuances of TH signaling, including the role of nuclear receptor coactivators and corepressors, repression of positively stimulated genes by the unliganded receptor, THR isoform-specific actions of TRα (THRA) and TRβ (THRB), and THR binding DNA as a heterodimer with retinoid-x-receptor (RXR) for genes positively regulated by TH. The identification of genetic disorders of TH transport and signaling, especially Resistance to Thyroid Hormone (RTH) and monocarboxylate transporter 8 (Mct8) defects, has been highly informative with respect to the mechanism of TH action. Conclusions: The impact of THR isoform, post-translational modifications, receptor cofactors, DNA response element, and selective TH tissue uptake, on TH action, have clinical implications for diagnosing and treating thyroid disease. Additionally, these findings have led to the development of novel TH and TH analogue therapies for metabolic, neurological, and cardiovascular diseases.

Long-Term Efficacy of T3 Analogue Triac in Children and Adults With MCT8 Deficiency: A Real-Life Retrospective Cohort Study.

ContextPatients with mutations in thyroid hormone transporter MCT8 have developmental delay and chronic thyrotoxicosis associated with being underweight and having cardiovascular dysfunction.ObjectiveOur previous trial showed improvement of key clinical and biochemical features during 1-year treatment with the T3 analogue Triac, but long-term follow-up data are needed.MethodsIn this real-life retrospective cohort study, we investigated the efficacy of Triac in MCT8-deficient patients in 33 sites. The primary endpoint was change in serum T3 concentrations from baseline to last available measurement. Secondary endpoints were changes in other thyroid parameters, anthropometric parameters, heart rate, and biochemical markers of thyroid hormone action.ResultsFrom October 15, 2014 to January 1, 2021, 67 patients (median baseline age 4.6 years; range, 0.5-66) were treated up to 6 years (median 2.2 years; range, 0.2-6.2). Mean T3 concentrations decreased from 4.58 (SD 1.11) to 1.66 (0.69) nmol/L (mean decrease 2.92 nmol/L; 95% CI, 2.61-3.23; P < 0.0001; target 1.4-2.5 nmol/L). Body-weight-for-age exceeded that of untreated historical controls (mean difference 0.72 SD; 95% CI, 0.36-1.09; P = 0.0002). Heart-rate-for-age decreased (mean difference 0.64 SD; 95% CI, 0.29-0.98; P = 0.0005). SHBG concentrations decreased from 245 (99) to 209 (92) nmol/L (mean decrease 36 nmol/L; 95% CI, 16-57; P = 0.0008). Mean creatinine concentrations increased from 32 (11) to 39 (13) µmol/L (mean increase 7 µmol/L; 95% CI, 6-9; P < 0.0001). Mean creatine kinase concentrations did not significantly change. No drug-related severe adverse events were reported.ConclusionsKey features were sustainably alleviated in patients with MCT8 deficiency across all ages, highlighting the real-life potential of Triac for MCT8 deficiency.

Growth hormone replacement may influence the biological action of thyroid hormone on liver and bone tissue

ObjectiveGrowth hormone (GH) replacement alters the peripheral interconversion of thyroxine (T4) and triiodothyronine (T3). However, little is known about the clinical impact of these alterations. We aimed to compare changes observed in the serum T3:T4 ratio with known biological markers of thyroid hormone action derived from different peripheral tissues. DesignWe prospectively studied twenty GH deficient men before and after GH replacement in a tertiary referral endocrine center. Serum biochemical measurements included insulin like growth factor-1 (IGF-1), thyroid hormones (free & total T3, free & total T4 and reverse T3) and TSH. Changes in thyroid hormone concentration were compared to alterations in hepatic and bone biomarkers of thyroid hormone action. ResultsGH replacement provoked a decline in serum free T4 concentration (−1.09 ± 1.99 pmol/L; p = 0.02) and an increase in free T3 (+0.34 ± 0.15 pmol/L; p = 0.03); therefore, the free T3:free T4 ratio increased from 0.40 ± 0.02 to 0.47 ± 0.02 (p = 0.002). Sex hormone binding globulin (SHBG) level was unchanged. However, a decline in serum ferritin (−26.6 ± 8.5 ng/mL; p = 0.005) correlated with a fall in freeT4. Alterations in lipid profile, including a rise in large HDL sub-fractions and Lp (a) (+2.1 ± 21.1 nmol/L; p = 0.002) did not correlate with thyroid hormone levels. Significant increases were recorded in serum bone turnover markers - procollagen type 1 amino-terminal propeptide +57.4%; p = 0.0009, osteocalcin +48.6%; p = 0.0007; c-terminal telopeptides of type 1 collagen +73.7%; p = 0.002. Changes in bone formation markers occurred in parallel with fluctuations in thyroid hormone. ConclusionGH-induced alterations in the thyroid axis are associated with complex, tissue specific effects on thyroid hormone action. Modulation of bone turnover markers suggests that GH may improve the biological action of thyroid hormone on bone.

OR01-05 Long-Term Efficacy of T3 Analogue Triac in MCT8 Deficiency

Background: MCT8 deficiency is a severe disorder caused by mutations in the thyroid hormone transporter MCT8. MCT8 deficiency is characterized by severe intellectual and motor disability and high serum T3 concentrations that result in thyrotoxic symptoms in peripheral tissues. This predisposes to substantial morbidity and mortality. Preclinical studies showed that the T3 analogue Triac can bypass defective MCT8 at the cellular level. Recently, we reported the results of an international multicenter trial, in which biochemical and clinical outcomes improved in patients with MCT8 deficiency who were treated with Triac for 12 months (1). However, long-term follow-up data of patients with MCT8 deficiency treated with Triac are lacking, particularly in young children. Therefore, we aimed to investigate the long-term efficacy of Triac therapy in a worldwide cohort of patients with MCT8 deficiency. Methods: We investigated the efficacy of oral Triac treatment in pediatric (n=78) and adult (n=5) patients with MCT8 deficiency in 20 countries. Triac dose was titrated according a predefined dose-escalation scheme aiming to normalize serum T3 concentrations (target 1.4-2.5 nmol/L). Thyroid function tests and biochemical markers of thyroid hormone action in peripheral tissues (SHBG, creatine kinase, creatinine) were measured at baseline and during control visits. Findings: In total, 83 patients with a median baseline age of 5 years (range 6 months – 66 years) were treated, including 24 patients aged 0-2.5 years and 17 patients aged 2.5-5 years. They were treated with Triac during 144 patient years, of whom the follow-up time was >5 years in 9 patients and 2-5 years in 22 patients. Mean dose was 45 µg/kg/day (range 11-107 µg/kg/day). Once a stable dose was achieved, no further dose adjustments were needed.Mean serum T3 concentrations decreased from 5.02 to 1.94 nmol/L (normal 1.4 – 2.5 nmol/L). SHBG concentrations improved from 238 to 204 nmol/L (normal 40-140 nmol/L). Mean creatine kinase and creatinine concentrations improved from 113 to 140 U/L (normal <230 U/L) and from 32 to 38 µmol/L (normal 31-68 μmol/L), respectively. No drug-related severe adverse events were reported. Interpretation: Triac is a safe treatment that results in sustainable improvements of the severe thyrotoxic state in pediatric and adult patients with MCT8 deficiency.

Influence of thyroid dysfunction on brain natriuretic peptide level in health examination participants.

The tissue-specific circulating markers of thyroid hormone action on cardiac function have not been established. Although the relationship between thyroid function and plasma brain natriuretic peptide (BNP) levels has been evaluated in patients with thyroid disorders, the relationship between these parameters in the general population has not been yet studied. We conducted retrospective cohort study by health examination with concurrent measurements of TSH, free T4, body mass index, systolic blood pressure, hemoglobin, and estimated glomerular filtration rate from participants who visited the Department of Health Checkup, Enshu Hospital between July 2008 and March 2017. After participants with abnormal electrocardiogram and/or any history of cardiac disease were excluded, 2,807 individuals were subjected. Multivariate analyses demonstrated that, when compared to euthyroidism (n = 2,629), the increase in BNP levels was significant in overt thyrotoxicosis (n = 21) but not in subclinical thyrotoxicosis (n = 53) or subclinical hypothyroidism (n = 97). Interestingly, the standardized partial regression coefficient was the smallest for thyroid function category (overt thyrotoxicosis compared to euthyroidisim; β = 0.048, p = 0.006) among the independent variables including age, body mass index, systolic blood pressure, and hemoglobin. In longitudinal comparison, we identified 986 participants who had sequential data on the measurements and were stable as euthyroidism and subclinical hypothyroidism. Their annual percent change in BNP demonstrated no significant differences. In conclusion, a direct stimulatory effect of thyroid hormone on the secretion (or production) of BNP was confirmed even in a large number of health examination participants.

OR10-5 Reduced Sensitivity to Thyroid Hormone Due to Fetal Exposure to High Thyroid Hormone Levels Is Transmitted along Male Line to Two Subsequent Generations

Exposure of human embryos to high maternal thyroid hormone (TH) concentrations not only affects the newborn but also reduces thyrotroph sensitivity to TH during adult life. Based on results in mice, this latter effect is likely due to induction of the pituitary deiodinase type 3, which accelerates TH inactivation, decreasing therefore the suppression of TSH and resulting in reduced sensitivity to TH (RSTH). As the gene encoding for deiodinase 3 has parent of origin imprinting, we wondered if the effect is transmitted to subsequent generations. We therefore studied the progeny of wild-type (WT) adults (second and third generations), descendent of grandmothers and great grandmothers with high TH levels but euthyroid due to THRB gene mutations, who gave birth to the first WT generation exposed to high TH levels during fetal life. Each subject received 200 µg of thyrotropin-releasing hormone (TRH) iv after 3 days oral treatment with 25 µg liothyronine (L-T3) given twice daily. Blood was collected at intervals over the period of 180 min and analyzed for TSH, prolactin (PRL), sex hormone-binding globulin, ferritin, cholesterol, and creatinine kinase. Post L-T3 treatment, baseline TSH ranged from 0.1-0.5 mU/L and serum T3 levels were elevated but not different among all groups. Following the administration of L-T3, the average TSH peak induced by TRH in the first generation born to mothers with high TH levels was 6.8±1.0 mU/liter compared to 1.6±0.5 mU/liter (P <0.005) of those born to mothers with normal TH levels. There was no overlap in the TSH responses to TRH between the two groups. The second generation consisted of descendants of grandmothers with high TH levels. Their parents were exposed to TH excess but they themselves were not. In this second generation we found that the offspring of fathers, but not of mothers exposed to high TH levels had RSTH as, after L-T3 treatment, they exhibited a peak TSH response to TRH significantly higher, 6.39 ± 0.63 mU/liter compared to 1.58 ± 0.41 mU/liter (P<0.001), respectively. The third generation was comprised of descendants from great grandmothers with high TH levels. Neither their parents nor themselves were exposed to TH excess during their fetal life. Similarly to the second generation, the offspring of fathers, but not of mothers, had RSTH, as evidenced by significantly higher TSH in response to TRH after L-T3 treatment, 4.60 ± 0.61 mU/liter compared to 1.37 ± 0.23 mU/litter (P<001), respectively. No differences were observed in the PRL responses to TRH and in the peripheral markers of TH action listed above. These findings indicate that fetal exposure to high TH levels causes permanent RSTH, which is transmitted by males but not females with RSTH to individuals of both genders in two subsequent generations.

Alterations In Thyroid Hormone Levels Following Growth Hormone Replacement Exert Complex Biological Effects

Alterations in the thyroid axis are frequently observed following growth hormone (GH) replacement, but uncertainty exists regarding their clinical significance. We aimed to compare fluctuations in circulating thyroid hormone levels, induced by GH, to changes in sensitive biological markers of thyroid hormone action. This was a prospective observational clinical study. Twenty hypopituitary men were studied before and after GH replacement. Serum thyroid-stimulating hormone (TSH), thyroid hormones, and insulin-like growth factor 1 were measured. Changes in thyroid hormone concentrations were compared to alterations in resting metabolic rate and cardiac time intervals. Health-related quality of life (QOL) was assessed by disease-sensitive and generic questionnaires. Following GH replacement, free thyroxine concentration declined and free triiodothyronine level increased. Resting energy expenditure increased, particularly in subjects with profound hypopituitarism, including TSH deficiency (16.73 ± 1.75 kcal/kg/min vs. 17.96 ± 2.26 kcal/kg/min; P = .01). Alterations in the thyroid axis were more pronounced in subjects with a low/normal baseline respiratory quotient (RQ) who experienced a paradoxical rise in RQ (0.81 vs. 0.86; P = .01). Subjects with a high baseline RQ experienced a slight but nonsignificant fall in RQ without alteration in thyroid axis. The isovolumetric contraction time was shortened during the study; however, this did not reach statistical significance. Improvements in QOL were observed despite alterations in thyroid axis. Changes in the thyroid axis following GH replacement are associated with complex tissue-specific effects. These fluctuations may induce a hypothyroid phenotype in some tissues while appearing to improve the biological action of thyroid hormone in other organs. AGHDA = Assessment of Growth Hormone Deficiency in Adulthood; CHOox = carbohydrate oxidation; ET = ejection time; fT3 = free triiodothyronine; fT4 = free thyroxine; GH = growth hormone; GHD = growth hormone deficiency; HB-RQ = high baseline respiratory quotient; HPT = hypothalamic-pituitary-thyroid; ICT = isovolumetric contraction time; IGF-1 = insulin-like growth factor 1; IRT = isovolumetric relaxation time; LB-RQ = low baseline respiratory quotient; LV = left ventricular; NHP = Nottingham Health Profile; QOL = quality of life; REE = resting energy expenditure; RQ = respiratory quotient; rT3 = reverse triiodothyronine; SF-36 = Short Form 36; TSH = thyroid-stimulating hormone; T3 = triiodothyronine; T4 = thyroxine; TT3 = total triiodothyronine; TT4 = total thyroxine.

Resistance to thyroid hormone caused by a mutation in thyroid hormone receptor (TR)α1 and TRα2: clinical, biochemical, and genetic analyses of three related patients

The thyroid hormone receptor α gene (THRA) transcript is alternatively spliced to generate either thyroid hormone receptor (TR)α1 or a non-hormone-binding variant protein, TRα2, the function of which is unknown. Here, we describe the first patients identified with a mutation in THRA that affects both TRα1 and TRα2, and compare them with patients who have resistance to thyroid hormone owing to a mutation affecting only TRα1, to delineate the relative roles of TRα1 and TRα2. We did clinical, biochemical, and genetic analyses of an index case and her two sons. We assessed physical and radiological features, thyroid function, physiological and biochemical markers of thyroid hormone action, and THRA sequence. The patients presented in childhood with growth failure, developmental delay, and constipation, which improved after treatment with thyroxine, despite normal concentrations of circulating thyroid hormones. They had similar clinical (macrocephaly, broad faces, skin tags, motor dyspraxia, slow speech), biochemical (subnormal ratio of free thyroxine:free tri-iodothyronine [T3], low concentration of total reverse T3, high concentration of creatine kinase, mild anaemia), and radiological (thickened calvarium) features to patients with TRα1-mediated resistance to thyroid hormone, although our patients had a heterozygous mis-sense mutation (Ala263Val) in both TRα1 and TRα2 proteins. The Ala263Val mutant TRα1 inhibited the transcriptional function of normal receptor in a dominant-negative fashion. By contrast, function of Ala263Val mutant TRα2 matched its normal counterpart. In vitro, high concentrations of T3 restored transcriptional activity of Ala263Val mutant TRα1, and reversed the dominant-negative inhibition of its normal counterpart. High concentrations of T3 restored expression of thyroid hormone-responsive target genes in patient-derived blood cells. TRα1 seems to be the principal functional product of the THRA gene. Thyroxine treatment alleviates hormone resistance in patients with mutations affecting this gene, possibly ameliorating the phenotype. These findings will help the diagnosis and treatment of other patients with resistance to thyroid hormone resulting from mutations in THRA. Wellcome Trust, NIHR Cambridge Biomedical Research Centre, Marie Curie Actions, Foundation for Development of Internal Medicine in Europe.

Increased Sex Hormone-Binding Globulin Levels in Children and Adolescents with Thyrotoxicosis

Background: Thyrotoxicosis is a rare condition in pediatric patients, and optimal treatment can be difficult to achieve in some children. To our knowledge, no studies have evaluated sex hormone-binding globulin (SHBG) levels in hyperthyroid children and adolescents in relation to age- and gender-related normative data. Methods: SHBG serum levels were determined before and after 4 months of antithyroid therapy (ATT) in 10 children and adolescents with Graves' disease. A total of 903 healthy children and adolescents served as controls. Results: Serum SHBG levels were elevated (>2 SD) at diagnosis in all hyperthyroid children but normalized rapidly following ATT. At diagnosis, median SHBG was +2.51 SD (interquartile range 2.20-3.27) compared to healthy children without thyroid illness, and it declined significantly during ATT (-0.16 SD, -0.66 to 1.64; p < 0.05). Conclusion: This is the first study to demonstrate that serum SHBG levels are markedly increased in children with Graves' disease, and we suggest that SHBG may be an additional marker of thyroid hormone action in children, as has been shown in adults.

Diagnosis of subclinical central hypothyroidism in patients with hypothalamic–pituitary disease by Doppler echocardiography

The diagnosis of subclinical central hypothyroidism in hypothalamic-pituitary patients cannot be established by serum markers of thyroid hormone action. Myocardial function by echocardiography has been shown to reflect thyroid hormone action in primary thyroid dysfunction. We evaluated the performance of echocardiography in diagnosing subclinical central hypothyroidism. Cross-sectional and before and after. Echocardiography and serum thyroid hormones were assessed in overt primary (n=20) and central (n=10) hypothyroidism, subclinical primary hypothyroidism (n=10), hypothalamic-pituitary disease with normal free thyroxine (FT(4); n=25), and controls (n=28). Receiver operating characteristic (ROC) curves were generated using overt hypothyroidism patients and selected cut-off values were applied to detect both primary and central subclinical hypothyroidism. After levothyroxine (l-T(4)) intervention, patients were echocardiographically reevaluated at predefined targets: normal thyrotropin (TSH) in primary hypothyroidism, normal FT(4) in overt central hypothyroidism, and higher than pretreatment FT(4) in echo-defined subclinical central hypothyroidism. Parameters with highest areas under the ROC curves (area under the curve (AUC) ≥0.94) were as follows: isovolumic contraction time (ICT), ICT/ejection time (ET), and myocardial performance index. Highest diagnostic accuracy (93%) was obtained when at least one parameter was increased (positive and negative predictive values: 93%). Hypothyroidism was echocardiographically diagnosed in eight of ten patients with subclinical primary hypothyroidism and in 14 of 25 patients (56%) with hypothalamic-pituitary disease and normal serum FT(4). Echocardiographic abnormalities improved significantly after l-T(4) and correlated (0.05<P<0.001) with changes in FT(4) (-0.62<r<-0.55) and TSH (0.63<r<0.68) in primary hypothyroidism and with FT(4) in central hypothyroidism (-0.72<r<-0.50). Echocardiography can be useful in diagnosing subclinical central hypothyroidism in patients with hypothalamic-pituitary disease.

Age modulates effects of thyroid dysfunction on sex hormone binding globulin (SHBG) levels.

Symptoms of thyroid dysfunction are difficult to detect in elderly people and TSH is sometimes unreliable. We therefore tested the value of SHBG as a marker of thyroid hormone action on the liver to determine the thyroid status of elderly people. Aging euthyroid men and women have a significant increase in SHBG (p > 0.0001). In aging women the decrease in SHBG with hypothyroidism and increase with hyperthyroidism are highly significant (p < 0.0001 and p < 0.0005 respectively). No significant variation in SHBG was observed in men with thyroid dysfunction. SHBG can help to determine the thyroid status of aging women.

A Child with Resistance to Thyroid Hormone without Thyroid Hormone Receptor Gene Mutation: A 20-Year Follow-Up

We report here the 20-year follow-up study of a male subject diagnosed at 15 months of age as a sporadic case of pituitary resistance to thyroid hormone on the combination of clinical hyperthyroidism, elevated serum thyroid hormone (TH) levels and inappropriate thyrotropin (TSH). On D-thyroxine (D-T(4)) therapy from 30 months of age to 12.5 years, hyperactivity and hyperthyroid signs and symptoms as well as growth abnormalities improved, serum L-thyroxine (L-T(4)) enantiomer normalized, and basal and stimulated TSH decreased significantly without complete suppression. After 8 years off D-T(4), at 20 years of age, clinical status was normal despite persisting high TH levels and inappropriate TSH. Evolution of serum markers of TH action and echocardiography measurements followed up from 15 months to 20 years of age either in basal condition or on triiodothyronine (T(3)), as well as the sequential determination of bone mineral density suggest differences in the tissue responses to T(3): normal in bone with a high remodelling rate, heterogeneity for various hepatic markers, and decreased at heart level. No mutations were found in the coding sequence of TRbeta1, TRbeta2, TRalpha1, RXRgamma, SMRT, NCoR1, and NCoA1. In this patient the putative long-term effects of the persisting high bone resorption are unknown.

Growth hormone (GH) treatment reduces peripheral thyroid hormone action in girls with Turner syndrome

Turner syndrome (TS) is an indication for GH therapy in spite of the modest growth response. Somatic growth depends not only on GH insulin-like growth factor I (IGF-I) axis but also on thyroid hormone (TH) status. We have previously reported that supraphysiological IGF-I levels diminished TH actions in rat tissues by reducing the nuclear TH receptor (TR). GH treatment to TS patients induces high IGF-I levels and therefore a reduction of TH action in tissues may be expected. We aimed at evaluating the effect of GH therapy in TS girls on peripheral TH action. We set up a reverse transcription-polymerase chain reaction (RT-PCR) for TR mRNA estimation in peripheral blood mononuclear cells (PBMC) and compared TR mRNA levels from 10 normal, 10 TS and 10 TS girls under GH therapy (0.33 mg/kg/week for 0.5-2 years). After RNA extraction from PBMC, TR and beta-actin mRNAs were coamplified by RT-PCR. In addition serum biochemical markers of TH action were measured: thyrotropin (TSH), sex hormone binding globulin (SHBG), osteocalcin (OC), beta-crosslaps (beta-CL), iodothyronines by electrochemiluminescency and IGF-I by immunoradiometric assay (IRMA) with extraction. TR mRNAs from PBMC were reduced in TS patients under GH treatment. In turn, serum TSH, OC, beta-CL and IGF-I were increased while SHBG was reduced by GH treatment in TS patients. GH treatment reduced TR expression in PBMC and biochemical serum markers of TH action. These results suggest that GH treatment in TS patients impair peripheral TH action at tissue level and prompt a role in the reduced growth response to the therapy.

Mosaicism of a Thyroid Hormone Receptor-β Gene Mutation in Resistance to Thyroid Hormone

Heterozygous mutations in thyroid hormone receptor-beta (TRbeta) gene are the cause of resistance to thyroid hormone (RTH) in more than 85% of families having the syndrome. In 23% of the families, TRbeta gene mutations occur de novo. Of the 141 families with RTH investigated by us, 21 (15%) had no TRbeta gene mutations detectable by sequencing from genomic DNA (gDNA) or cDNA (non-TR RTH). The objective of the study was to investigate the genotype of a family with RTH and correlate it to the phenotype. The DNA was isolated from different tissues, and the sequence of the TRbeta gene was determined. Clinical studies involved the administration of incremental doses of T(3). The study was conducted at a referral pediatric endocrinology clinic in Turkey and an academic medical center in the United States. Measurement included markers of thyroid hormone action and sequencing of TRbeta revealing a R338W mutation. Patients and Family: We studied two siblings with short stature, panic disorder, psychosis, and high free iodothyronine concentrations with nonsuppressed TSH and their father with similar thyroid function tests without growth or psychiatric abnormalities. Direct sequencing of gDNA obtained from the father's leukocytes, buccal mucosa cells, and prostate tissue showed less amplification of the mutant allele (R338W) than the normal allele as confirmed by PCR/restriction fragment length polymorphism analysis. No sequence abnormalities were detected in gDNA from fibroblasts. Similar results were found in mRNA from the leukocytes and fibroblasts. The sensitivity of various tissues to thyroid hormone was not uniform. The progeny had equal amounts of mutant and wild-type gDNA in leukocytes and skin. The father has a mosaicism for the R338W mutation as it was present in some cell lineages, including his germline, because it was transferred to his children but not in fibroblasts. This indicates that the mutation occurred de novo in early embryonic life. Here is the first report of mosaicism in RTH. The possibility of mosaicism should be considered in subjects with RTH without apparent mutations in the TRbeta gene.

Small Changes in Thyroxine Dosage Do Not Produce Measurable Changes in Hypothyroid Symptoms, Well-Being, or Quality of Life: Results of a Double-Blind, Randomized Clinical Trial

In patients with primary hypothyroidism, anecdotal evidence suggests that well-being is optimized by fine adjustment of T(4) dosage, aiming for a serum TSH concentration in the lower reference range. This has not been tested in a clinical trial. Our objective was to test whether adjustment of T(4) dosage aiming for a serum TSH concentration less than 2 mU/liter improves well-being compared with a serum TSH concentration in the upper reference range. We conducted a double-blind, randomized clinical trial with a crossover design. Fifty-six subjects (52 females) with primary hypothyroidism taking T(4) (>/=100 microg/d) with baseline serum TSH 0.1-4.8 mU/liter participated. Each subject received three T(4) doses (low, middle, and high in 25-microg increments) in random order. Outcome measures included visual analog scales assessing well-being (the primary endpoint) and hypothyroid symptoms, quality of life instruments (General Health Questionnaire 28, Short Form 36, and Thyroid Symptom Questionnaire), cognitive function tests, and treatment preference. Mean (+/- sem) serum TSH concentrations were 2.8 +/- 0.4, 1.0 +/- 0.2, and 0.3 +/- 0.1 mU/liter for the three treatments. There were no significant treatment effects on any of the instruments assessing well-being, symptoms, quality of life, or cognitive function and no significant treatment preference. Small changes in T(4) dosage do not produce measurable changes in hypothyroid symptoms, well-being, or quality of life, despite the expected changes in serum TSH and markers of thyroid hormone action. These data do not support the suggestion that the target TSH range for the treatment of primary hypothyroidism should differ from the general laboratory range.

Thyroid Hormone Action on Liver, Heart, and Energy Expenditure in Thyroid Hormone Receptor -Deficient Mice

Thyroid hormone (TH) responsive genes can be both positively and negatively regulated by TH through receptors (TR) α and β expressed in most body tissues. However, their relative roles in the regulation of specific gene expression remain unknown. The TRβ knockout mouse, which lacks both TRβ1 and TRβ2 isoforms, provides a model to examine the role of these receptors in mediating TH action. TRβ deficient (TRβ−/−) mice that show no compensatory increase in TRα, and wild-type (TRβ+/+) mice of the same strain were deprived of TH by feeding them a low iodine diet containing propylthiouracil, and were then treated with supraphysiological doses of L-T3 (0.5, 5.5, and 25 μg/day/mouse). TH deprivation alone increased the serum cholesterol concentration by 25% in TRβ+/+ mice and reduced it paradoxically by 23% in TRβ−/− mice. TH deprivation reduced the serum alkaline phosphatase (AP) concentration by 31% in TRβ+/+ mice but showed no change in the TRβ−/− mice. Treatment with L-T3 (0.5 to 25μ g/mouse/day) caused a 57% decrease in serum cholesterol and a 231% increase in serum AP in the TRβ+/+ mice. The TRβ−/− mice were resistant to the L-T3 induced changes in serum cholesterol and showed increase in AP only with the highest L-T3 dose. Basal heart rate (HR) in TRβ−/− mice was higher than that of TRβ+/+ mice by 11%. HR and energy expenditure (EE) in both TRβ+/+ and TRβ−/− mice showed similar decreases (49 and 46%)and increases (49 and 41%) in response to TH deprivation and L-T3 treatment, respectively. The effect of TH on the accumulation of messenger RNA (mRNA) of TH regulated liver genes was also examined. TH deprivation down regulated spot 14 (S14) mRNA and showed no change in malic enzyme (ME) mRNA in both TRβ+/+ and TRβ−/− mice. In contrast treatment with L-T3 produced an increase in S14 and ME but no change in TRβ−/− mice. From these results, it can be concluded that regulation of HR and EE are independent of TRβ. With the exception of serum cholesterol concentration and liver ME mRNA accumulation, all other markers of TH action examined during TH deprivation exhibited the expected responses in the absence of TRβ. Thus, as previously shown for serum TSH, TRβ is not absolutely necessary for some changes typical of hypothyroidism to occur. In contrast, except for HR and EE, the full manifestation of TH-mediated action required the presence of TRβ.

Thyroid hormone action on liver, heart, and energy expenditure in thyroid hormone receptor beta-deficient mice.

Thyroid hormone (TH) responsive genes can be both positively and negatively regulated by TH through receptors (TR) alpha and beta expressed in most body tissues. However, their relative roles in the regulation of specific gene expression remain unknown. The TR beta knockout mouse, which lacks both TR beta1 and TR beta2 isoforms, provides a model to examine the role of these receptors in mediating TH action. TR beta deficient (TR beta-/-) mice that show no compensatory increase in TR alpha, and wild-type (TR beta+/+) mice of the same strain were deprived of TH by feeding them a low iodine diet containing propylthiouracil, and were then treated with supraphysiological doses of L-T3 (0.5, 5.5, and 25 microg/day/mouse). TH deprivation alone increased the serum cholesterol concentration by 25% in TR beta+/+ mice and reduced it paradoxically by 23% in TR beta-/- mice. TH deprivation reduced the serum alkaline phosphatase (AP) concentration by 31% in TR beta+/+ mice but showed no change in the TR beta-/- mice. Treatment with L-T3 (0.5 to 25 microg/mouse/day) caused a 57% decrease in serum cholesterol and a 231% increase in serum AP in the TR beta+/+ mice. The TR beta-/- mice were resistant to the L-T3 induced changes in serum cholesterol and showed increase in AP only with the highest L-T3 dose. Basal heart rate (HR) in TR beta-/- mice was higher than that of TR beta+/+ mice by 11%. HR and energy expenditure (EE) in both TR beta+/+ and TR beta-/- mice showed similar decreases (49 and 46%) and increases (49 and 41%) in response to TH deprivation and L-T3 treatment, respectively. The effect of TH on the accumulation of messenger RNA (mRNA) of TH regulated liver genes was also examined. TH deprivation down regulated spot 14 (S14) mRNA and showed no change in malic enzyme (ME) mRNA in both TR beta+/+ and TR beta-/- mice. In contrast treatment with L-T3 produced an increase in S14 and ME but no change in TR beta-/- mice. From these results, it can be concluded that regulation of HR and EE are independent of TR beta. With the exception of serum cholesterol concentration and liver ME mRNA accumulation, all other markers of TH action examined during TH deprivation exhibited the expected responses in the absence of TR beta. Thus, as previously shown for serum TSH, TR beta is not absolutely necessary for some changes typical of hypothyroidism to occur. In contrast, except for HR and EE, the full manifestation of TH-mediated action required the presence of TR beta.

Circulating telopeptide type I is a peripheral marker of thyroid hormone action in hyperthyroidism and during levothyroxine suppressive therapy.

The aim of the present investigation was to evaluate the clinical performance of serum carboxy-terminal-1-telopeptide (ICTP), a new marker of bone resorption, in identifying peripheral overexposure to thyroid hormones, as compared with serum osteocalcin (OC) and serum sex hormone binding globulin (SHBG). Serum ICTP, SHBG, and OC were assayed by specific radioassays in three study groups. Group 1: 50 perimenopausal women on long-term levothyroxine (LT4) suppressive treatment; group 2: 29 women with untreated hyperthyroidism; group 3: 36 normal euthyroid women matched with group 1 patients for age, alcohol, smoking habits, and lifestyle. Serum concentrations of SHBG, ICTP, and OC were markedly increased in hyperthyroid patients, whereas only serum ICTP was slightly but significantly increased in LT4 treated patients. Serum ICTP had higher diagnostic value for hyperthyroidism when compared with SHBG and to OC (sensitivity: 100%, 71%, 55%; accuracy: 97%, 88%, and 76%, respectively). In group 1, increased serum ICTP was observed in 30 of 50 patients, whereas increased SHBG and OC were found only in 11 of 50 (p < .001). Serum free thyroid hormone concentrations correlated with circulating ICTP and SHBG, and the correlation with serum OC was of lower significance. In conclusion, serum ICTP is a sensitive and reliable marker of peripheral thyroid hormone activity at the bone level; its clinical performance is higher than OC and even better than SHBG. Thus, serum ICTP is better than other peripheral markers in monitoring LT4 suppressive therapy in patients at increased risk for osteoporosis such as perimenopausal women.

Carefully monitored levothyroxine suppressive therapy is not associated with bone loss in premenopausal women.

We measured total body and regional (lumbar spine, femoral neck, Ward's triangle, and trochanter) bone mineral density (BMD) in 47 premenopausal women chronically treated with suppressive doses of levothyroxine (L-T4). Treatment was administered to 7 patients with nontoxic goiter or, after thyroidectomy, to 38 patients with differentiated thyroid cancer and 2 with nontoxic goiter. Patients were followed at our institution and treated with the minimal amount of L-T4 necessary to suppress TSH. At the time of evaluation, free T3 was normal in all cases, whereas free T4 was increased in 17 (36.2%). The mean daily dose of L-T4 was 154.3 +/- 5 micrograms, and the mean duration of treatment was 10.1 yr. We found no significant difference between patients and age- and weight-matched controls in BMD at any site of measurement. BMD was not correlated with duration of therapy, cumulative or mean daily dose of L-T4, serum levels of free T4, free T3, and osteocalcin. There was no difference between patients and controls in serum total calcium, intact PTH, osteocalcin, or carboxy-terminal cross-linked telopeptide of type I collagen or in the concentrations of two markers of thyroid hormone action (sex hormone-binding globulin and amino-terminal propeptide of type III procollagen). Our data suggest that L-T4 suppressive therapy, if carefully carried out and monitored, using the smallest dose necessary to suppress TSH secretion has no significant effect on bone metabolism or bone mass.

Molecular mechanisms of thyroid hormone action.

This brief review deals with the key trans- and cis-elements that are currently believed to participate in TH action at the transcriptional level. The reader is also referred to other recent reviews (53,54). In sum, there are multiple levels of complexity that make an understanding of the fundamental mechanisms of TH action difficult. However, the interactions of multiple TRs and TRAPS along with putative coactivators and ligands provide the cell with a wide range of biological responses. An appreciation of these phenomena will ensure insight into regulated gene expression in general, and the mode of action of TH in particular. While the field has progressed greatly since Jack Oppenheimer first described the nuclear binding of TH, there are "many more miles to go" for all of us, including Jack. We trust the journey will continue to be captivating.