Increased fat and skeletal muscle beta-adrenergic receptors but unaltered metabolic and hemodynamic sensitivity to epinephrine in vivo in experimental human thyrotoxicosis

Effect of Hyperthyroidism Treatments on Heart Rate Variability: A Systematic Review and Meta-Analysis

The reversibility of HRV abnormalities in hyperthyroidism remains contradictory. The design of this study involves conducting a systematic review and meta-analysis on the effect of antithyroid treatments on HRV in hyperthyroidism. PubMed, Cochrane, Embase, and Google Scholar were searched until 4 April 2022. Multiple reviewers selected articles reporting HRV parameters in treated and untreated hyperthyroidism. Independent data extraction by multiple observers was stratified by degree of hyperthyroidism for each HRV parameter: RR intervals, SDNN (standard deviation of RR intervals), RMSSD (square root of the mean difference of successive RR intervals), pNN50 (percentage of RR intervals with >50 ms of variation), total power (TP), LFnu (low-frequency normalized unit) and HFnu (high-frequency), VLF (very low-frequency), and LF/HF ratio. We included 11 studies for a total of 471 treated hyperthyroid patients, 495 untreated hyperthyroid patients, and 781 healthy controls. After treatment, there was an increase in RR, SDNN, RMSSD, pNN50, TP, HFnu, and VLF and a decrease in LFnu and LF/HF ratio (p < 0.01). Overt hyperthyroidism showed similar results, in contrast to subclinical hyperthyroidism. Compared with controls, some HRV parameter abnormalities persist in treated hyperthyroid patients (p < 0.05) with lower SDNN, LFnu, and higher HFnu, without significant difference in other parameters. We showed a partial reversibility of HRV abnormalities following treatment of overt hyperthyroidism. The improvement in HRV may translate the clinical cardiovascular benefits of treatments in hyperthyroidism and may help to follow the evolution of the cardiovascular morbidity.

Heart Rate Variability in Hyperthyroidism: A Systematic Review and Meta-Analysis

Objective: Cardiovascular effects of thyroid hormones may be measured through heart rate variability (HRV). We sought to determine the impact of hyperthyroidism on HRV. Design: A systematic review and meta-analysis on the impact of hyperthyroidism on HRV. Methods: PubMed, Cochrane, Embase and Google Scholar were searched until 20 August 2021 for articles reporting HRV parameters in untreated hyperthyroidism and healthy controls. Random-effects meta-analysis was stratified by degree of hyperthyroidism for each HRV parameter: RR intervals (or Normal-to-Normal intervals—NN), SDNN (standard deviation of RR intervals), RMSSD (square root of the mean difference of successive RR intervals), pNN50 (percentage of RR intervals with >50 ms of variation), total power (TP), LFnu (low-frequency normalized unit) and HFnu (high-frequency), VLF (very low-frequency), and LF/HF ratio. Results: We included 22 studies with 10,811 patients: 1002 with hyperthyroidism and 9809 healthy controls. There was a decrease in RR (effect size = −4.63, 95% CI −5.7 to −3.56), SDNN (−6.07, −7.42 to −4.71), RMSSD (−1.52, −2.18 to −0.87), pNN50 (−1.36, −1.83 to −0.88), TP (−2.05, −2.87 to −1.24), HFnu (−3.51, −4.76 to −2.26), and VLF power (−2.65, −3.74 to −1.55), and an increase in LFnu (2.66, 1.55 to 3.78) and LF/HF ratio (1.75, 1.02 to 2.48) (p < 0.01). Most parameters had ES that was twice as high in overt compared to subclinical hyperthyroidism. Increased peripheral thyroid hormones and decreased TSH levels were associated with lower RR intervals. Conclusions: Hyperthyroidism is associated with a decreased HRV, which may be explained by the deleterious effect of thyroid hormones and TSH. The increased sympathetic and decreased parasympathetic activity may have clinical implications.

p43, a Truncated Form of Thyroid Hormone Receptor α, Regulates Maturation of Pancreatic β Cells

P43 is a truncated form of thyroid hormone receptor α localized in mitochondria, which stimulates mitochondrial respiratory chain activity. Previously, we showed that deletion of p43 led to reduction of pancreatic islet density and a loss of glucose-stimulated insulin secretion in adult mice. The present study was designed to determine whether p43 was involved in the processes of β cell development and maturation. We used neonatal, juvenile, and adult p43-/- mice, and we analyzed the development of β cells in the pancreas. Here, we show that p43 deletion affected only slightly β cell proliferation during the postnatal period. However, we found a dramatic fall in p43-/- mice of MafA expression (V-Maf Avian Musculoaponeurotic Fibrosarcoma Oncogene Homolog A), a key transcription factor of beta-cell maturation. Analysis of the expression of antioxidant enzymes in pancreatic islet and 4-hydroxynonenal (4-HNE) (a specific marker of lipid peroxidation) staining revealed that oxidative stress occurred in mice lacking p43. Lastly, administration of antioxidants cocktail to p43-/- pregnant mice restored a normal islet density but failed to ensure an insulin secretion in response to glucose. Our findings demonstrated that p43 drives the maturation of β cells via its induction of transcription factor MafA during the critical postnatal window.

Increased Incidence of Tinnitus Following a Hyperthyroidism Diagnosis: A Population-Based Longitudinal Study

BACKGROUND

An association between thyroid disease and tinnitus has been described previously but further longitudinal, population-based studies are limited.

OBJECTIVE

To investigate the incidence of tinnitus in patients with hyperthyroidism in a national sample, and to identify risk level and associated factors for tinnitus in hyperthyroidism patients.

DESIGN

Retrospective cohort study. Patient data were collected from the Longitudinal Health Insurance Database (LHID 2000), which includes national claims data of patient expenditures for admissions or ambulatory care from 1996 to 2011.

SETTING

Taiwan hospitals and clinics providing healthcare nationwide.

PARTICIPANTS

Patients aged 20 years and older with newly diagnosed hyperthyroidism (ICD-9-CM code 242) between 2000-2010 were selected as the study cohort. Hyperthyroidism patient cohort were identified from the LHID2000. Those with tinnitus history (ICD-9-CM code 388.3) before the index date (first hyperthyroidism diagnosis), younger than 20 years, and with incomplete demographic data were excluded. The non-hyperthyroidism cohort included patients with no history of hyperthyroidism and no documented tinnitus.

MAIN OUTCOMES AND MEASURES

Incidence of tinnitus was the primary outcome. Baseline demographic factors and comorbidities possibly associated with tinnitus, including age, sex, and comorbidities of hearing loss, vertigo, insomnia and anxiety, were retrieved from the LHID 2000. Patients were followed until end of 2011.

RESULTS

During the study period, 780 (4.9%) hyperthyroidism patients and 2007 (3.2%) non-hyperthyroidism controls developed tinnitus. Incidence rate of tinnitus in the hyperthyroidism cohort was significantly higher in hyperthyroidism cohort (7.86 vs. 5.05 per 1000 person-years) than that in non-hyperthyroidism cohort. A higher proportion of patients with hyperthyroidism had comorbid insomnia (45.1% vs. 30.9%) and anxiety (14.0% vs. 5.73%) than those without hyperthyroidism. After adjusting for age, gender and comorbidities (vertigo, insomnia, anxiety, hearing loss), hyperthyroidism patients had 1.38-fold higher risk of tinnitus (95% CI = 1.27-1.50) than those without hyperthyroidism.

CONCLUSIONS

This large population-based study suggests patients with diagnosed hyperthyroidism was more prone to develop tinnitus. Our findings suggest evaluation for comorbid vertigo, insomnia, anxiety and/or hearing loss may identify patients who are at high risk of developing tinnitus in patients with hyperthyroidism.

Heart rate variability in hyperthyroidism on sub Saharan African patients: a case-control study

Objective

We aimed to determine heart rate variability in freshly diagnosed untreated hyperthyroidism patients. We enrolled 10 patients (9 females) and 10 matched controls for sex and age. Each eligible patient underwent five different tests according to Ewing battery tests for cardiac autonomic dysfunction assessment. HRV was assessed during each maneuver and on 24 h using a continuous electrocardiogram with automatic estimation of SDNN, RMSSD, LF HF and HF/LH ratio. Results of tests were compared between hyperthyroidism patients and matched controls using the non-parametric test of Mann–Whitney.

Results

Heart rate was significantly higher in patients with thyrotoxicosis (82.91 ± 10.99 vs 67.04 ± 6.80; 0.006) compared to their controls. On time-domain analysis, there was a trend towards reduction in SDNN (39.52 vs. 63.75; p = 0.2) as well as the RMSSD (30.44 vs 64.03; p = 0.09) in patients with hyperthyroidism. The frequency-domain analysis showed non-significant higher values for the LF (43.87 vs 38.85 ± 12.85; p = 0.8) and lower for the HF (32.54 vs 43.39; p = 0.3). Test’s results were mostly impaired in hyperthyroid patients and all patients presented abnormal results for parasympathetic activity. Untreated and recently diagnosed hyperthyroidism is associated to an altered parasympathetic activity in sub Saharan African patients.

Aerobic Exercise and Pharmacological Therapies for Skeletal Myopathy in Heart Failure: Similarities and Differences

Skeletal myopathy has been identified as a major comorbidity of heart failure (HF) affecting up to 20% of ambulatory patients leading to shortness of breath, early fatigue, and exercise intolerance. Neurohumoral blockade, through the inhibition of renin angiotensin aldosterone system (RAS) and β-adrenergic receptor blockade (β-blockers), is a mandatory pharmacological therapy of HF since it reduces symptoms, mortality, and sudden death. However, the effect of these drugs on skeletal myopathy needs to be clarified, since exercise intolerance remains in HF patients optimized with β-blockers and inhibitors of RAS. Aerobic exercise training (AET) is efficient in counteracting skeletal myopathy and in improving functional capacity and quality of life. Indeed, AET has beneficial effects on failing heart itself despite being of less magnitude compared with neurohumoral blockade. In this way, AET should be implemented in the care standards, together with pharmacological therapies. Since both neurohumoral inhibition and AET have a direct and/or indirect impact on skeletal muscle, this review aims to provide an overview of the isolated effects of these therapeutic approaches in counteracting skeletal myopathy in HF. The similarities and dissimilarities of neurohumoral inhibition and AET therapies are also discussed to identify potential advantageous effects of these combined therapies for treating HF.

Takotsubo cardiomyopathy associated with thyrotoxicosis: a case report and review of the literature

BACKGROUND

Takotsubo or stress-induced cardiomyopathy is a form of reversible cardiomyopathy commonly associated with emotional or physical stress. Thyrotoxicosis has been identified as a rare cause of Takotsubo cardiomyopathy, with only 12 cases reported in the literature. Here, we report a case of thyroid storm presenting with Takotsubo cardiomyopathy in the setting of Graves' disease.

PATIENT FINDINGS

A 71-year-old woman presented with abdominal pain, vomiting, confusion, and history of weight loss. She was initially diagnosed and treated for diabetic ketoacidosis at another hospital and was transferred to our hospital one day after initial presentation because of concern for acute coronary syndrome. A diagnosis of Takotsubo cardiomyopathy was made on the basis of cardiac catheterization. At that time, she was diagnosed and treated for thyroid storm. Follow-up 7 weeks later revealed improvement of her cardiac function and near-normalization of thyroid hormone levels.

SUMMARY

In this patient, who presented with symptoms of heart failure, acute coronary syndrome was initially considered, but the diagnosis of Takotsubo cardiomyopathy associated with thyroid storm was ultimately made based on cardiac catheterization and laboratory investigation.

CONCLUSIONS

Thyrotoxicosis is associated with adverse disturbances in the cardiovascular system. Takotsubo cardiomyopathy could be a presenting manifestation of thyroid storm, perhaps related to excess catecholamine levels or sensitivity.

Mice lacking the p43 mitochondrial T3 receptor become glucose intolerant and insulin resistant during aging

Thyroid hormones (TH) play an important regulatory role in energy expenditure regulation and are key regulators of mitochondrial activity. We have previously identified a mitochondrial triiodothyronine (T3) receptor (p43) which acts as a mitochondrial transcription factor of the organelle genome, which leads in vitro and in vivo, to a stimulation of mitochondrial biogenesis. Recently, we generated mice carrying a specific p43 invalidation. At 2 months of age, we reported that p43 depletion in mice induced a major defect in insulin secretion both in vivo and in isolated pancreatic islets, and a loss of glucose-stimulated insulin secretion. The present study was designed to determine whether p43 invalidation influences life expectancy and modulates blood glucose and insulin levels as well as glucose tolerance or insulin sensitivity during aging. We report that from 4 months old onwards, mice lacking p43 are leaner than wild-type mice. p43−/− mice also have a moderate reduction of life expectancy compared to wild type. We found no difference in blood glucose levels, excepted at 24 months old where p43−/− mice showed a strong hyperglycemia in fasting conditions compared to controls animals. However, the loss of glucose-stimulated insulin secretion was maintained whatever the age of mice lacking p43. If up to 12 months old, glucose tolerance remained unchanged, beyond this age p43−/− mice became increasingly glucose intolerant. In addition, if up to 12 months old p43 deficient animals were more sensitive to insulin, after this age we observed a loss of this capacity, culminating in 24 months old mice with a decreased sensitivity to the hormone. In conclusion, we demonstrated that during aging the depletion of the mitochondrial T3 receptor p43 in mice progressively induced an increased glycemia in the fasted state, glucose intolerance and an insulin-resistance several features of type-2 diabetes.

Skeletal muscle insulin resistance in endocrine disease

We summarize the existing literature data concerning the involvement of skeletal muscle (SM) in whole body glucose homeostasis and the contribution of SM insulin resistance (IR) to the metabolic derangements observed in several endocrine disorders, including polycystic ovary syndrome (PCOS), adrenal disorders and thyroid function abnormalities. IR in PCOS is associated with a unique postbinding defect in insulin receptor signaling in general and in SM in particular, due to a complex interaction between genetic and environmental factors. Adrenal hormone excess is also associated with disrupted insulin action in peripheral tissues, such as SM. Furthermore, both hyper- and hypothyroidism are thought to be insulin resistant states, due to insulin receptor and postreceptor defects. Further studies are definitely needed in order to unravel the underlying pathogenetic mechanisms. In summary, the principal mechanisms involved in muscle IR in the endocrine diseases reviewed herein include abnormal phosphorylation of insulin signaling proteins, altered muscle fiber composition, reduced transcapillary insulin delivery, decreased glycogen synthesis, and impaired mitochondrial oxidative metabolism.

Hyperthyroidism-associated coronary vasospasm with myocardial infarction and subsequent euthyroid angina

A 40-year-old African-American woman presented with atypical chest pain, an acute non-ST segment elevation myocardial infarction, and angiographic evidence for severe ostial vasospasm of the left main and right coronary arteries. Subsequently, she was diagnosed with hyperthyroidism and treated with antithyroid therapy and oral nitrates. Repeat angiography revealed resolution of the vasospasm; however, the chest pain recurred in the euthyroid state. Hyperthyroidism-associated coronary vasospasm is a rare disorder that characteristically causes angina in young Asian women and resolves with correction of hyperthyroidism. We present an atypical case of an African-American woman presenting with a myocardial infarction who developed recurrent angina while euthyroid.

Hyperthyroidism is characterized by both increased sympathetic and decreased vagal modulation of heart rate: evidence from spectral analysis of heart rate variability

OBJECTIVE

The clinical manifestations of hyperthyroidism resemble those of the hyperadrenergic state. This study was designed to evaluate the impact of hyperthyroidism on the autonomic nervous system (ANS) and to investigate the relationship between serum thyroid hormone concentrations and parameters of spectral heart rate variability (HRV) analysis in hyperthyroidism.

DESIGN AND PATIENTS

Thirty-two hyperthyroid Graves' disease patients (mean age 31 years) and 32 sex-, age-, and body mass index (BMI)-matched normal control subjects were recruited to receive one-channel electrocardiogram (ECG) recording.

MEASUREMENTS

The cardiac autonomic nervous function was evaluated by the spectral analysis of HRV, which indicates the autonomic modulation of the sinus node. The correlation coefficients between serum thyroid hormone concentrations and parameters of the spectral HRV analysis were also computed.

RESULTS

The hyperthyroid patients revealed significant differences (P < 0.001) compared with the controls in the following HRV parameters: a decrease in total power (TP), very low frequency power (VLF), low frequency power (LF), high frequency power (HF), and HF in normalized units (HF%); and an increase in LF in normalized units (LF%) and in the ratio of LF to HF (LF/HF). After correction of hyperthyroidism in 28 patients, all of the above parameters were restored to levels comparable to those of the controls. In addition, serum thyroid hormone concentrations showed significant correlations with spectral HRV parameters.

CONCLUSIONS

Hyperthyroidism is in a sympathovagal imbalanced state, characterized by both increased sympathetic and decreased vagal modulation of the heart rate. These autonomic dysfunctions can be detected simultaneously by spectral analysis of HRV, and the spectral HRV parameters could reflect the disease severity in hyperthyroid patients.

Metabolism of lipids in human white adipocyte

Adipose tissue is considered as the body's largest storage organ for energy in the form of triacylglycerols, which are mobilized through lipolysis process, to provide fuel to other organs and to deliver substrates to liver for gluconeogenesis (glycerol) and lipoprotein synthesis (free fatty acids). The release of glycerol and free fatty acids from human adipose tissue is mainly dependent on hormone-sensitive lipase which is intensively regulated by hormones and agents, such as insulin (inhibition of lipolysis) and catecholamines (stimulation of lipolysis). A special attention is paid to the recently discovered perilipins which could regulate the activity of the lipase hormono-sensible. Most of the plasma triacylglycerols are provided by dietary lipids, secreted from the intestine in the form of chylomicron or from the liver in the form of VLDL. Released into circulation as non-esterified fatty acids by lipoprotein lipase, those are taken up by adipose tissue via specific plasma fatty acid transporters (CD36, FATP, FABPpm) and used for triacylglycerol synthesis. A small part of triacylglycerols is synthesized into adipocytes from carbohydrates (lipogenesis) but its regulation is still debated in human. Physiological factors such as dieting/fasting regulate all these metabolic pathways, which are also modified in pathological conditions e.g. obesity.

Hemodynamic and tissue oxygenation responses to exercise and beta-adrenergic blockade in patients with hyperthyroidism

BACKGROUND

Exercise-induced dyspnea is a frequent feature in patients with hyperthyroidism.

HYPOTHESIS

Data from clinical studies to elucidate the origin of this symptom are lacking. In the current study, we examined the hemodynamic and oxygenation responses to exercise and beta-adrenergic blockade in patients with hyperthyroidism and their relationship with dyspnea.

METHODS

Hemodynamic studies were performed under resting conditions and after isotonic exercise in 15 patients with hyperthyroidism and 11 control subjects. Exercise was applied using a bicycle ergometer, with progressive loads. In the hyperthyroid group, measurements were repeated at rest and during supine exercise after administering 15 mg of intravenous metoprolol.

RESULTS

End-diastolic pulmonary artery pressure and cardiac index were higher in the hyperthyroid group than in controls (18.6 +/- 5.3 vs. 11.2 +/- 4.9 mmHg; p = 0.02, and 6.0 +/- 1.7 vs. 2.8 +/- 0.5 l/min/m2; p = 0.0001, respectively). After exercise, there was an increase in end-diastolic pulmonary artery pressure in the hyperthyroid group (18.6 +/- 5.3 to 25.5 +/- 9.9 mmHg; p = 0.02), revealing impaired cardiocirculatory reserve. Pulmonary arteriolar resistance increased significantly in parallel with end-diastolic pulmonary artery pressure after drug administration, suggesting an inadequate cardiovascular response after beta blockade in patients with hyperthyroidism.

CONCLUSION

We observed that functional left ventricular reserve is impaired in patients with hyperthyroidism, suggesting an explanation for the frequent symptom of dyspnea and impaired exercise tolerance. Moreover, we also suggest that beta-adrenergic blockade may adversely affect cardiovascular function in patients with hyperthyroidism.

Thyroid hormone regulation of cardiac bioenergetics: role of intracellular creatine

The effect of thyroid hormone (T(3)) on the content of myocardial creatine (Cr), Cr phosphate (CrP), and high-energy adenine nucleotides and on cardiac function was examined. In the hearts of control and T(3)-treated rats perfused in vitro, while "low" and "high" contractile work was performed, T(3) treatment resulted in a approximately 50% reduction in CrP, Cr, total Cr content (Cr + CrP), and in the CrP-to-Cr ratio. In addition, there was a slight fall in myocardial content of ATP and a large rise in calculated free ADP (fADP), resulting in a significant decrease in the ATP-to-fADP ratio in the hearts of hyperthyroid compared with euthyroid rats. Moreover, there was a substantial decrease in the level of ATP in hearts of T(3)-treated rats under high work conditions. Importantly, the ratio of cardiac work to oxygen consumption was not altered by thyroid status. Treatment with T(3) also resulted in an almost threefold reduction in the content of Na(+)/Cr transporter mRNA in the ventricular myocardium and skeletal muscle but not in the brain. We conclude with the following: 1) changes in the expression of the Na(+)/Cr transporter mRNA correlate with Cr + CrP in the myocardium; 2) hearts of hyperthyroid rats contain lower levels of ATP and higher levels of fADP under both low and high work conditions but no reduction in efficiency of work output; and 3) the reduction in Cr and ATP in hearts of hyperthyroid rats may be the basis for the reduced maximal work capacity of the hyperthyroid heart.

Impact of hyperthyroidism and its correction on vascular reactivity in humans

BACKGROUND

Although thyroid hormone (TH) exerts relevant effects on the cardiovascular system, it is unknown whether TH also regulates vascular reactivity in humans. Methods and Results- We studied 8 patients with hyperthyroidism, basally (H) and 6 months after euthyroidism was restored by methimazole (EU). Thirteen healthy subjects served as control subjects (C). We measured forearm blood flow (FBF) by strain-gauge plethysmography during intrabrachial graded infusion of acetylcholine, sodium nitroprusside (SNP), norepinephrine, and L-NMMA (inhibitor of NO synthesis). Basal FBF (in mL. dL(-1). min(-1)) was markedly higher in H than in C (5.8+/-1.2 and 1.9+/-0.1, respectively; P<0.001) and was close to normal in EU (2.6+/-0.3, P<0.01 versus H). During acetylcholine infusion, FBF increased much more in H (+33+/-5) than in C (+14+/-3, P<0.01 versus H) and in EU (+20+/-5, P=0.01 versus H and P=NS versus C). In contrast, the response to SNP infusion was comparable in the patients and control subjects. During norepinephrine infusion, the fall in FBF was much more pronounced in H (-6+/-1) than in C (-0.7+/-0.3, P<0.005 versus H) and in EU (-1.5+/-0.3, P<0.01 versus H). Finally, inhibition of NO synthesis by L-NMMA decreased FBF by 2.8+/-0.6, 0.61+/-0.7, and 1.4+/-0.3 in H, C, and EU, respectively (H versus C and EU, P<0.05).

CONCLUSIONS

In hyperthyroidism, (1) the marked basal vasodilation is largely accounted for by excessive endothelial NO production, (2) vascular reactivity is exaggerated because of enhanced sensitivity of the endothelial component, (3) the vasoconstrictory response to norepinephrine is potentiated, and (4) this abnormal vascular profile is corrected when euthyroidism is restored by medical therapy. The data demonstrate that vascular endothelium is a specific target of TH.

Reversible autonomic dysfunction in hyperthyroid patients affects gastric myoelectrical activity and emptying

The autonomic nervous system (ANS) function in hyperthyroidism has been so far investigated mainly from the cardiovascular point of view. The aim of this study is to show that the ANS dysfunction in hyperthyroidism is also expressed in gastric myoelectrical activity disturbances and gastric emptying disorders and to search for a correlation between the severity of clinical manifestation and free thyroid hormone levels and the degree of the ANS dysfunction. The analyzed group included 50 recently diagnosed patients with hyperthyroidism who were examined twice: before and after 3 months of thyrostatic treatment. Results were compared with those of a sex-, age- and BMI-matched control group of 50 healthy volunteers. The study included: heart rate variability analysis in time and frequency domain, at rest and during a deep-breathing test, surface electrogastrography in preprandial and postprandial periods measured simultaneously with the ultrasound assessment of gastric emptying time by Bolondi method. In patients with hyperthyroidism in comparison with the control group, the following significant differences were observed: a sharp reduction of the high-frequency component and a decrease of heart rate variability, a high incidence of dysrhythmia with dominant bradyarrhythmia, and a delay of gastric emptying. The degree of disorders related to the degree of clinical manifestation of hyperthyroidism's symptoms and free triiodothyronine serum concentration both. All the disorders were functional and disappeared in a stable euthyroidism. To conclude, the ANS dysfunction in hyperthyroidism results not only in withdrawal of vagal inhibitory effect on sinoatrial node, but in impaired mutual neuro-hormonal regulation (decrease of vagal influence) of gastric myoelectrical activity followed by delay of gastric emptying.

Sympathovagal imbalance in hyperthyroidism

We assessed sympathovagal balance in thyrotoxicosis. Fourteen patients with Graves' hyperthyroidism were studied before and after 7 days of treatment with propranolol (40 mg 3 times a day) and in the euthyroid state. Data were compared with those obtained in a group of age-, sex-, and weight-matched controls. Autonomic inputs to the heart were assessed by power spectral analysis of heart rate variability. Systemic exposure to sympathetic neurohormones was estimated on the basis of 24-h urinary catecholamine excretion. The spectral power in the high-frequency domain was considerably reduced in hyperthyroid patients, indicating diminished vagal inputs to the heart. Increased heart rate and mid-frequency/high-frequency power ratio in the presence of reduced total spectral power and increased urinary catecholamine excretion strongly suggest enhanced sympathetic inputs in thyrotoxicosis. All abnormal features of autonomic balance were completely restored to normal in the euthyroid state. beta-Adrenoceptor antagonism reduced heart rate in hyperthyroid patients but did not significantly affect heart rate variability or catecholamine excretion. This is in keeping with the concept of a joint disruption of sympathetic and vagal inputs to the heart underlying changes in heart rate variability. Thus thyrotoxicosis is characterized by profound sympathovagal imbalance, brought about by increased sympathetic activity in the presence of diminished vagal tone.

Negative functional effects of cGMP mediated by cGMP protein kinase are reduced in T4 cardiac myocytes

We tested the hypothesis that in isolated rabbit cardiac myocytes, the negative functional effects of cyclic GMP are partly mediated by cyclic GMP-dependent protein kinase activity, and that these effects are altered in thyroxine (T4, 0.5 mg/kg/day for 16 days)-induced hypertrophic myocytes. Using isolated ventricular myocytes from control (N=8) and T4 (N=8) hypertrophic hearts, data for percent cell shortening (%) and maximum rate of contraction (microm/s) were collected using a video edge detector at baseline, after the addition of 10(-6) M 8-bromo-cyclic GMP (8-Br-cGMP), 10(-5) M 8-Br-cGMP, and 10(-6) M KT5823 (10-methoxy-10-methoxycarbonyl-9, 10, 11, 12-tetrahydro-9, 12-epoxy-(1H)-diinidolo [1, 2, 3, f-g: 3', 2', 1'-k-j]-pyrrolidino-[3,4-i] [1,6]-benzodiazocin-2-methyl-1-one, cyclic GMP protein kinase inhibitor). Protein phosphorylation was determined autoradiographically after gel electrophoresis. In both control and T(4) myocytes, 8-Br-cGMP caused a significant decrease in percent shortening (5.56+/-0.49% to 3.02+/-0.47% in control and 4.34+/-0.33% to 3.13+/-0.17% in T4 myocytes) and maximal rate of contraction 57.35+/-6.05 to 36.82+/-3.17 microm/s in control and 58.49+/-3.28 to 42.88+/-2.29 microm/s in T4 myocytes). KT5823 significantly increased percent shortening to 3.77+/-0.28% and rate to 48.68+/-4.71 microm/s after 8-Br-cGMP only in control myocytes. In T4 myocytes, the changes in percent shortening and rate after KT5823 were not significant. Protein phosphorylation was increased by 8-Br-cGMP in control and to a lesser extent in T4 myocytes, but the increment was reduced by KT-5823 in control only. These data demonstrated that cyclic GMP had negative functional effects partially mediated by cyclic GMP protein kinase in control myocytes. Cyclic GMP also exerted negative functional effects in thyroxine-induced hypertrophic myocytes, but cyclic GMP protein kinase activity was not an important regulator of these effects in T4 ventricular myocytes.

Changes in adenylyl cyclase isoforms as a mechanism for thyroid hormone modulation of cardiac beta-adrenergic receptor responsiveness

Although thyroid hormones are known to modulate cardiac beta-adrenergic receptor expression, the physiologic implications of these changes in the cardiac manifestations of altered thyroid hormone metabolism have been disputed. This study examined whether thyroid hormone modulates signaling via the cyclic adenosine monophosphate (cAMP) pathway by regulating cardiac adenylyl cyclase (AC) isoform expression. Northern blot analyses and AC enzyme assays were performed on preparations from hypothyroid, euthyroid, and hyperthyroid rat ventricles. Steady-state levels of cardiac AC mRNA types V and VI in hypothyroid ventricles were 173% +/- 8% and 149% +/- 12%, respectively, of the values in euthyroid ventricles (P < .01). This increase in AC mRNA isoforms was accompanied by a 1.5-fold increase (P < .05) in the activation of catalytic AC by forskolin and Mn. In contrast, the relative abundance of transcripts for types V and VI AC was similar in hyperthyroid and euthyroid ventricles, but catalytic AC activation by forskolin and Mn was significantly reduced by 35% in membranes obtained from hyperthyroid ventricles. AC activation through beta-adrenergic receptor stimulation by isoproterenol was not altered by thyroid hormone status. Thus, the effect of thyroid hormone to repress AC catalytic activity would be anticipated to offset the increase in beta-adrenergic receptor expression in hyperthyroidism. These studies identify cardiac AC enzymes as important targets for thyroid hormone-dependent regulation of signaling via the cAMP pathway, and support the finding that cardiac adrenergic responsiveness is unaltered in thyroid disease states.

Reduced epinephrine clearance and glycemic sensitivity to epinephrine in older individuals

To test the hypothesis that glycemic sensitivity to epinephrine is reduced in older individuals and to assess the impact of a sedentary lifestyle on responses to the hormone, we performed 30-min sequential intravenous infusions of epinephrine (0, 41, 82, 164, 246, and 328 pmol. kg-1. min-1) in young (n = 10) and older (n = 23) healthy subjects. We performed these again after 12 mo of physical training, which raised peak O2 consumption from 24.4 +/- 1.0 to 30.4 +/- 1.4 ml. kg-1. min-1 (P < 0.01) in most of the older subjects (n = 21). During epinephrine infusions, plasma epinephrine concentrations were higher (P = 0.0001) in older than in young subjects (e.g., final values of 7,280 +/- 500 vs. 4,560 +/- 380 pmol/l, respectively), indicating that the clearance of epinephrine from the circulation was reduced in the older individuals. Plasma epinephrine concentration-response curves disclosed reduced glycemic sensitivity to the hormone in the older subjects (P = 0.0001), a finding plausibly attributed to increased sympathetic neural activity, as evidenced here by higher plasma norepinephrine concentrations (P = 0.0001) in the older subjects and consequent desensitization of cellular responsiveness to catecholamines. Training did not correct reduced epinephrine clearance, reduced glycemic sensitivity to epinephrine, or raised norepinephrine levels. We conclude that aging is associated with reduced clearance of epinephrine from the circulation and reduced glycemic sensitivity to epinephrine, the latter plausibly attributed to an age-associated increase in sympathetic neural norepinephrine release. These age-associated changes are not the result of a sedentary lifestyle.

Myocardial effects of cyclic AMP phosphodiesterase inhibition are dampened in thyroxine-induced cardiac hypertrophy

We tested the hypothesis that the increase in myocardial O2 consumption (MVO2) and myocardial wall thickening in response to milrinone would not be limited by thyroxine (T4)-induced (0.5 mg/kg for 16 days) cardiac hypertrophy. Anesthetized open-chest New Zealand white rabbits were divided into four groups: control vehicle (CV, n = 5), control milrinone (CM, n = 8), T4 vehicle (T4V, n = 7), and T4 milrinone (T4M, n = 9). Vehicle or milrinone (10(-3) M) were topically applied to the left ventricular epicardium for 15 min. Coronary blood flow (radioactive microspheres) and O2 extraction (microspectrophotometry) were used to determine O2 consumption. Cyclic AMP levels were determined by radioimmunoassay. T4 increased the heart weight to body weight ratio from 2.6 +/- 0.1 to 3.1 +/- 0.1 (g/kg). T4 rabbits had significantly higher baseline heart rates, blood pressures, and dP/dtmax and both subepicardial (EPI) and subendocardial (ENDO) blood flows. Topical application of milrinone did not have significant hemodynamic effects in either group. Baseline cyclic AMP levels (pmol/g) in the EPI and ENDO myocytes were comparable between control and T4 rabbits (CVEPI = 599 +/- 34, CVENDO = 532 +/- 26, T4VEPI = 656 +/- 42, T4VENDO = 657 +/- 17). Milrinone increased cyclic AMP in all groups although the increases were less in the T4 rabbits (CMEPI = 742 +/- 115, CMENDO = 698 +/- 101, T4MEPI = 742 +/- 103, T4MENDO = 690 +/- 55). Baseline MVO2 (ml O2/min/100 g) was significantly higher in T4 rabbits than controls (T4VEPI = 17.7 +/- 3.5 vs CVEPI = 8.5 +/- 1.5, T4VENDO = 17.2 +/- 3.2 vs CVENDO = 9.2 +/- 1.5). Significant increases in MVO2 were noted with the addition of milrinone in control (CMEPI = 14.8 +/- 3.0, CMENDO = 13.5 +/- 1.6) and T4 (T4MEPI = 25.5 +/- 3.4, T4MENDO = 22.0 +/- 3.3) rabbits; however, the percentage increase in MVO2 was significantly greater in controls (CEPI = 73%, CENDO = 47%) than T4 (T4,EPI = 44%, T4,ENDO = 28%). Thus, although the cyclic AMP phosphodiesterase activity was comparable between T4 rabbit hearts and controls, the metabolic effects and cyclic AMP effects of milrinone were dampened in this form of hypertrophy.

Mechanisms of thyroid hormone action. Implications for the clinical manifestation of thyrotoxicosis

Serum thyroid hormone concentrations alone do not explain the variability and severity of the range of symptoms observed in thyrotoxic patients. Despite gaps in our understanding of the links between the clinical manifestations of thyrotoxicosis and the underlying mechanisms, much has been learned. A limited number of markers directly reflect T3 action. The future elucidation of T3 targets that mediate these effects should ultimately lead to additional clinical markers of tissue-specific T3 action. The availability of such tests should allow for more specific treatment of individual patients.

Thyrotoxicosis and the heart

This review examines the molecular mechanisms by which thyroid hormone affects the cardiovascular system in naturally occurring thyroid disease states. The potential utility of thyroid hormone therapy in the management of patients with various forms of cardiovascular disease is also discussed.

Thyroid hormone and cardiovascular disease

Thyroid hormone directly affects the heart and peripheral vascular system. The hormone can increase myocardial inotropy and heart rate and dilate peripheral arteries to increase cardiac output. An excessive deficiency of thyroid hormone can cause cardiovascular disease and aggravate many preexisting conditions. In severe systemic illness and after major surgical procedures changes in thyroid function can occur, leading to the "euthyroid sick syndrome." Patients will have normal or decreased levels of T4, decreased free and total T3, and usually normal levels of thyroid stimulating hormone. This syndrome may be an adaptive response to systemic illness that usually will revert to normal without hormone supplementation as the illness subsides. Recently, however, many investigators have explored the benefits of thyroid hormone supplementation in those diseases associated with euthyroid sick syndrome. Thyroid hormone's effects on the cardiovascular system make it an attractive therapy for those patients with impaired hemodynamics and low T3. Thyroid hormone has also been considered a treatment for patients with congestive heart failure, for patients undergoing cardiopulmonary bypass and heart transplantation, and for patients with hyperlipidemia. At present there is no evidence suggesting a favorable treatment outcome using thyroid hormone supplementation for any systemic condition except in those patients with documented hypothyroidism.

Basal muscarinic activity does not impede beta-adrenergic activation in rabbit hearts in controls or thyroxine-induced cardiac hypertrophy

We tested the hypothesis that basal myocardial muscarinic receptor activity acts as a "brake" on beta-adrenergic activation and that this effect would be greater in hearts subjected to thyroxine (T4)-induced (0.5 mg/kg for 16 days) hypertrophy due to an increase in muscarinic receptor density. Twenty control and 20 T4-treated open-chest anesthetized New Zealand white rabbits were given isoproterenol (0.5 microg/kg/min, 10 min i.v.) and/or atropine (3 mg/kg bolus). Coronary blood flow (radioactive microspheres), aortic and left ventricular (LV) pressure, and wall thickening of the LV free wall were recorded. Hearts were quickly excised and stored in liquid nitrogen. Cyclic guanosine monophosphate (GMP) and cyclic adenosine monophosphate (AMP) were determined by radioimmunoassay. T4 increased heart weight/body weight ratio, blood pressures, and the first derivative of the maximal rate of increase of LV systolic pressure (dP/dt[max]). Isoproterenol increased heart rate in both groups. Atropine had no effects on hemodynamic parameters either alone or after stimulation with isoproterenol. At this dose, atropine completely blocked the depressant effects of acetylcholine (10 microg/kg). Isoproterenol increased the maximal time derivative of wall thickening (dWT/dt[max]) in control (from 11.0 +/- 1.0 to 16.4 +/- 1.5 mm/s) but not in T4 animals. T4 increased subepicardial (EPI) and subendocardial (ENDO) coronary blood flow. Isoproterenol increased coronary flow (control: EPI, from 173 +/- 11 to 346 +/- 28 ml/min/100 g; ENDO, from 197 +/- 15 to 364 +/- 30 ml/min/100 g; T4: EPI, from 314 +/- 45 to 459 +/- 43 ml/min/100 g; ENDO, from 339 +/- 48 to 458 +/- 43 ml/min/100 g). Cyclic AMP levels were higher in T4 animals. Isoproterenol increased cyclic AMP (control: EPI, from 540 +/- 82 pmol/g to 1,096 +/- 110; ENDO, 596 +/- 58 to 1,050 +/- 145 pmol/g; T4: EPI, from 882 +/- 107 pmol/g to 1,319 +/- 222; ENDO, from 954 +/- 134 to 1 ,409 +/- 261 pmol/g). Atropine, alone or after stimulation with isoproterenol, had no effect on coronary flow or cyclic AMP in either group. Cyclic GMP levels were unaffected by T4-induced hypertrophy or by any of the treatments in either group. Thus it appears that basal muscarinic activity does not significantly influence function or signal transduction either at baseline or during beta-adrenergic stimulation in controls or in T4-induced hypertrophy.

Effects of thyroid hormone on cardiac beta-adrenergic responsiveness in conscious baboons

BACKGROUND

Many of the cardiovascular manifestations of thyroid hormone excess resemble those produced by sympathoadrenal stimulation. The objective of this study was to determine the effects of thyroid hormone excess on myocardial beta-adrenergic expression and responsiveness to infused agonists in the primate heart.

METHODS AND RESULTS

The responses of left ventricular isovolumic contraction (dP/dt(max)) and relaxation (tau) during graded dobutamine infusion were studied both before and after 4 weeks of thyroid hormone administration in 8 chronically instrumented baboons. At matched (atrially paced) heart rates, thyroid hormone significantly increased resting dP/dt(max) (3073+/-1034 versus 2318+/-829 mm Hg/s, P<.05) and decreased tau (24.0+/-5.5 versus 28.2+/-5.4 ms, P<.05). The change from baseline for dP/dt(max) and tau in response to beta1-adrenergic stimulation was significant at each dobutamine dose (2.5 to 10 microg x kg(-1) x min(-1)), but when expressed as a percent change, it was similar before versus after thyroid hormone. Similar changes were found when beta2-adrenergic stimulation was produced by terbutaline infusion in three additional baboons. beta-Adrenergic receptor (betaAR) expression was higher in five thyroxine-treated than in five control baboons (37.4+/-1.2 versus 15.7+/-3.2 fmol/mg, P<.001), and this was due to a greater increase in the beta2AR (5.9+/-1.5 to 20.6+/-1.2 fmol/mg, P<.001) than the beta1AR (9.7+/-1.7 to 16.8+/-0.1 fmol/mg, P<.01) subtype.

CONCLUSIONS

In the primate heart, thyroid hormone produces positive inotropic and lusitropic effects in the resting state and upregulates both beta1AR and beta2AR, with the beta2AR increase predominating. At equivalent rates, however, thyroid hormone excess does not appear to enhance the sensitivity of left ventricular contractility and relaxation to either beta1- or beta2-adrenergic stimulation.

Significant reversal of thyrotoxicosis-associated dilated cardiomyopathy with induction of the euthyroid state

Thyrotoxicosis rarely precipitates heart failure. Older patients and those with underlying cardiac disease are at greater risk of experiencing this complication of thyrotoxicosis. A 43 year old male who presented with subclinical thyrotoxicosis, atrial fibrillation, and a dilated cardiomyopathy is discussed. There was no evidence of clinically significant underlying cardiac disease. At admission, the patient had an ejection fraction of 25%. Thyrotoxicosis was treated with propylthiouracil. At 14 weeks after hospitalization, the patient had an ejection fraction of 50% with significant reduction in cardiac chamber sizes and left ventricular mass index. He was biochemically euthyroid at that time. After ten months of propylthiouracil therapy, he had progressive improvement in cardiac function and decrease in left ventricular mass index. Thyrotoxicosis-associated cardiomyopathy may reverse significantly with treatment of thyrotoxicosis alone; this complication of thyrotoxicosis should be considered in any young individual with a dilated cardiomyopathy of unknown etiology.

Thyroid hormone therapy in cardiovascular disease

The relationship between thyroid disease states and cardiovascular hemodynamics is well recognized. Although the long-term effects of thyroid hormone are thought to result from changes in myocardial gene expression, attention has recently focused on acute, non-nuclear-mediated actions of L-triidothyronine (T3), the biologically active form of the hormone. Various lines of evidence have documented that T3 can act as a vasodilator and inotrope. With this recognition have come novel treatment strategies targeted at specific clinical conditions including heart failure and cardiac surgery that are associated with impaired cardiovascular performance and low serum T3 levels. An understanding of the mechanisms of action of thyroid hormone on the heart and peripheral vasculature is essential for the rational implementation of thyroid hormone as a therapeutic agent. As outlined in this review, initial clinical experience suggests that the ability of thyroid hormone to increase cardiac output and to lower systemic vascular resistance may provide a novel treatment option for physicians caring for patients with cardiovascular illness.

Cardiovascular and metabolic responses to adrenaline infusion in patients with short-term hypothyroidism

OBJECTIVE

The relation between the clinical manifestations of thyroid disease (both hypo and hyper-thyroidism) and tissue sensitivity to catecholamines remains uncertain. It has been suggested that tissue adrenergic responsiveness is decreased in hypothyroidism, but the reports have been conflicting and have invariably focused on a single physiological response. Therefore the aim of the present study was to determine in patients with moderate, short-term, symptomatic hypothyroidism the responses of heart rate, systolic and diastolic blood pressure, forearm blood flow and metabolic rate to adrenaline infused at a rate known to achieve plasma concentrations in the middle of the physiological range.

PATIENTS

Ten subjects (5M, age 43 +/- 3 years, mean +/- SEM) were studied. All were on thyroxine replacement for hypothyroidism following either thyroidectomy or radioactive iodine and had been biochemically euthyroid for at least 6 months.

DESIGN

Studies were performed in random order. One study was undertaken on full replacement therapy and the other after 50 micrograms thyroxine daily for 2 weeks. After basal, supine measurements adrenaline was infused at 25 ng/kg/min for 30 minutes.

MEASUREMENTS

Heart rate, blood pressure, blood glucose, metabolic rate and forearm blood flow were measured at rest and at 10-minute intervals throughout the adrenaline infusion.

RESULTS

Free T4 (10.6 +/- 1.3 vs 17.6 +/- 2.0 pmol/l, P < 0.001) and free T3 (3.6 +/- 0.2 vs 4.6 +/- 0.3 pmol/l, P < 0.01) concentrations were significantly lower on 50 micrograms thyroxine than full replacement therapy. Fasting blood glucose concentrations (4.7 +/- 0.2 vs 4.7 +/- 0.1 mmol/l) were similar. The resting adrenaline concentrations were comparable, 0.29 +/- 0.18 and 0.24 +/- 0.14 nmol/l on 50 micrograms thyroxine and full replacement therapy respectively, and increased to a similar level (2.36 +/- 0.39 and 2.36 +/- 0.35 nmol/l) throughout the adrenaline infusion. The resting heart rate and metabolic rate were significantly lower on 50 micrograms thyroxine than full replacement therapy (68 +/- 2 vs 72 +/- 3 beats/min, P < 0.01; and 4.48 +/- 0.35 vs 4.88 +/- 0.39 kJ/min, P < 0.01) respectively, but the increase in heart rate (7 +/- 2 vs 8 +/- 2 beats/min) and metabolic rate (0.43 +/- 0.09 vs 0.43 +/- 0.06 kJ/min) did not differ on the two study days. Resting systolic blood pressure, diastolic blood pressure and forearm blood flow were comparable on 50 micrograms thyroxine and full replacement therapy as were the changes in systolic blood pressure (1 +/- 1 vs 1 +/- 1 mmHg), diastolic blood pressure (-7 +/- 2 vs -7 +/- 1 mmHg), forearm blood flow (1.4 +/- 0.1 vs 1.7 +/- 0.2 ml/min/100ml forearm) and blood glucose concentration (0.7 +/- 0.1 vs 0.7 +/- 0.1 mmol/l).

CONCLUSIONS

Patients with short-term hypothyroidism appear to have a normal response to adrenaline infusion despite reduced baseline heart rate and metabolic rate. Thus, under physiological and mild pathophysiological conditions there appears to be no evidence of any synergy between thyroid status and sensitivity to catecholamines.

Thyroid status and exercise tolerance. Cardiovascular and metabolic considerations

Both hypo- and hyperthyroidism are characterised by exercise intolerance. In hypothyroidism, inadequate cardiovascular support appears to be the principal factor involved. Insufficient skeletal muscle blood flow compromises exercise capacity via reduced oxygen delivery, and endurance through decreased delivery of blood-borne substrates. The latter effect results in increased dependence on intramuscular glycogen. Additionally, decreased mobilisation of free fatty acids from adipose tissue and, consequently, lower plasma free fatty acid levels compound the problem of reduced lipid delivery to active skeletal muscle in the hypothyroid state. In contrast, cardiovascular support is enhanced in hyperthyroidism, implicating other factors in exercise tolerance. Greater reliance on muscle glycogen appears to be the primary reason for decreased endurance. Biochemical changes with hyperthyroidism that would favour enhanced flux through glycolysis may account for this dependence on glycogen. Deviations from normal thyroid function, and the ensuing exercise tolerance, require appropriate medical therapy to attain euthyroid status.

Palpebral fissure height and downgaze in patients with Graves upper eyelid retraction and congenital blepharoptosis

PURPOSE

To compare the relation between vertical eyelid fissure height and downgaze in healthy subjects and in patients with Graves upper eyelid retraction and congenital blepharoptosis.

METHODS

With a caliper rule, the authors measured the vertical eyelid fissure height of 25 patients with no previous history of eye disease, 34 patients with Graves disease, and 16 patients with congenital blepharoptosis. All measurements were done at the primary position and at four levels of downgaze (10 degrees, 20 degrees, 30 degrees, and 40 degrees). In 13 patients with Graves disease, vertical phoria was quantified with a Maddox rod and prism.

RESULTS

The relation between vertical eyelid fissure height and downgaze is linear. The slope was identical for the control subjects and for the patients with Graves disease (b = -1.36) but it was almost zero for the patients with congenital blepharoptosis (b = -0.037). When vertical eyelid fissure height was corrected to arc values, the slope of the relation between vertical eyelid fissure height and downgaze was significantly greater for the patients with Graves disease. Vertical phoria was not related to the amount of Graves upper eyelid retraction.

CONCLUSIONS

The data support the hypotheses that upper eyelid retraction secondary to Graves eye disease results from hyperactivity of the levator palpebrae superioris muscle.

Thyrotoxicosis-induced congestive heart failure in an urban hospital

Thyrotoxicosis has been associated with several cardiac complications, including atrial fibrillation (AF), functional cardiomyopathy, and congestive heart failure (CHF). Thyrotoxicosis-induced CHF has been considered an unusual complication of thyrotoxicosis, occurring generally in older patients. To determine the nature and frequency of this complication of thyrotoxicosis in an adult, hospitalized population, the authors consecutively evaluated all admissions to a 535-bed urban hospital during a 14-month period. Admitted during the study period were 177 individuals with a principal diagnosis of AF and 21 individuals with a principal diagnosis of thyrotoxicosis. Eleven of these individuals had concomitant AF and thyrotoxicosis, which was 6% of the AF group. Of these 11 patients, 5 had clinically apparent CHF without evidence of other organic heart disease (Group A) and 6 had only AF without clinical CHF (Group B). The 5 patients with CHF had a significantly lower mean age (36) than the 6 with thyrotoxicosis and AF only (P < 0.03). Echocardiography revealed Group A to have a mean of 2.8 cardiac chambers enlarged compared with a mean of 1 in Group B (P < 0.008). Four of the five patients with CHF had normal left ventricular function. Mean levels of serum thyroxine, albumin, hematocrit, left ventricular percent shortening, and left atrial diameter were similar in groups A and B. Based on the authors' data, thyrotoxicosis-induced CHF is not as rare a complication of thyrotoxicosis in young individuals as reported previously. Thyrotoxicosis-induced CHF most commonly has a dilated cardiomyopathy and high output failure.

Alpha 2-adrenergic activity is normal in patients with thyroid disease

OBJECTIVE

Several studies indicate an inverse relationship between the sympathetic nervous system activity and thyroid function. Altered adrenoceptor sensitivity, particularly alpha 1 and beta, have been described in hypothyroid and hyperthyroid patients. No information in patients with thyroid disease is available on the main mechanism regulating sympathetic nervous system outflow, i.e. the alpha 2-adrenoceptor pathway. In our study we evaluated alpha 2-adrenergic activity in patients with thyroid disease by the assessment of cardiovascular and catecholamine response to clonidine, a central alpha 2 adrenergic agonist.

PATIENTS

Ten patients with hypothyroidism, six patients with hyperthyroidism before and during adequate therapy, and ten healthy subjects.

MEASUREMENTS

After three blood samples for the basal determination of noradrenaline and adrenaline, the subjects swallowed 4 micrograms/kg body weight of clonidine. Blood pressure and pulse rate were measured 30, 60, 90, 120, 130 and 140 minutes after clonidine administration; blood samples for determination of catecholamines were drawn at 120, 130 and 140 minutes.

RESULTS

At presentation the decrease in plasma noradrenaline after clonidine in the patients was similar to that of the control group (hypothyroids: 1.07 +/- 0.23 nmol/l mean +/- SEM; hyperthyroids: 0.54 +/- 0.06 nmol/l; controls; 0.36 +/- 0.10 nmol/l; F = 1.2, P = NS). No differences were detected in the fall in adrenaline and mean arterial pressure (MAP) after clonidine. The adequate therapy induced in hypothyroid patients a decrease in the basal levels of noradrenaline (1.88 +/- 0.28 vs 0.67 +/- 0.10 nmol/l; P < 0.05) and a lesser fall in mean arterial pressure after clonidine (delta MAP 20.4 +/- 2.0 vs 9.7 +/- 2.8 mmHg; P < 0.05). No variations were detected in hyperthyroid patients after therapy either in basal hormones levels or in the magnitude of decrement in MAP and noradrenaline induced by clonidine.

CONCLUSIONS

We conclude that in spite of the previously reported abnormalities in alpha 1 and beta-adrenergic receptor activity, the inhibitory alpha 2-receptor pathway is normal in patients with altered thyroid function.

Sympathomimetic drug abuse masking an endogenous hyperadrenergic state, Graves' disease

Sympathomimetic drug use or abuse in a thyrotoxic individual can be clinically devastating as a result of enhanced cellular sensitivity to adrenergic stimulation. Thus, it is critical to recognize thyrotoxicosis in a patient with coexistent sympathomimetic drug abuse. This case report describes a patient with Graves' disease and severe hyperadrenergic symptomatology that was thought to result from sympathomimetic drug abuse.

Case report: pseudoephedrine-associated thyroid storm: thyroid hormone-catecholamine interactions

Thyroid storm is an increasingly rare, life-threatening manifestation of thyrotoxicosis that can be precipitated by many physiologic stressors. The exact mechanism by which thyrotoxicosis decompensates into thyroid storm in certain individuals is unknown. These individuals may have an enhanced cellular sensitivity to either catecholamines or thyroid hormone. Serum levels of catecholamines are usually low or normal in thyrotoxic individuals. Ingestion of sympathomimetic drugs in thyrotoxic individuals may precipitate thyroid storm. This article presents an unusual case of Graves' disease that decompensated into thyroid storm after the ingestion of an over-the-counter cold medication containing pseudoephedrine.

Combination treatment with captopril and the thyroid hormone analogue 3,5-diiodothyropropionic acid. A new approach to improving left ventricular performance in heart failure

BACKGROUND

An agent that improves left ventricular (LV) performance by non-cAMP-mediated mechanisms would be valuable in the treatment of chronic heart failure. We have shown earlier that the thyroid hormone analogue 3,5-diiodothyropropionic acid (DITPA) binds to nuclear receptors, alters transcription of T3-responsive genes, and increases +dP/dtmax in hypothyroid rats with substantially less effect on heart rate and metabolism than thyroid hormone, which makes it a selective cardiotonic agent.

METHODS AND RESULTS

To determine whether DITPA might be useful in treating heart failure, we compared chronic treatment with normal saline, captopril (2 g/L), or the combination of DITPA (375 micrograms/100 g) and captopril (2 g/L) in Sprague-Dawley rats beginning 3 weeks after coronary artery ligation. Both DITPA/captopril and captopril treatment decreased LV end-diastolic pressure compared with controls (21 +/- 2 and 26 +/- 2 mm Hg, respectively, vs 34 +/- 3 mm Hg, P < .05 for each). The addition of DITPA to captopril produced a 36% increase in resting cardiac index (P < .05) and shifted the cardiac function curve upward and to the left, indicative of enhanced myocardial performance. Also, DITPA/captopril compared with captopril treatment or control produced an increase in the rate of LV relaxation, as manifested by a decrease in tau, the time constant of LV pressure decline (17.5 +/- 1.0 vs 22.2 +/- 1.7 milliseconds, P < .05) and a larger absolute value for -dP/dtmax (-4561 +/- 361 vs -3346 +/- 232 mm Hg/s, P < .05). These changes occurred without changes in heart rate, LV mass, LV systolic pressure, or peripheral resistance relative to captopril treatment (P > .05).

CONCLUSIONS

The combination of DITPA and captopril improved cardiac output, increased -dP/dtmax, and increased the rate of LV relaxation to a greater extent than captopril treatment in the rat postinfarction model of heart failure. Use of a cardiotonic analogue of thyroid hormone represents a new approach to improving LV performance and may be a useful adjunct to afterload reduction for the treatment of heart failure.

Cardiac function in thyroid disease: clinical features and management considerations

Thyroid disease is often manifested by cardiac abnormalities. The site of the cardiac actions of thyroid hormone, whether from a direct, nuclear effect or an extranuclear effect, remains to be established. Nuclear effects are delayed 1/2 to 1 hour after administration of thyroid hormone, require ongoing protein synthesis, and are thought to result from the binding of thyroid hormone to two separate isoforms of the nuclear thyroid hormone receptor. This binding, which is specific to thyroid hormone response elements, stimulates transcription and results in translation of specific enzymes or contractile proteins. Extranuclear effects may influence plasma membrane transport of calcium, sugar, and amino acids in addition to directly influencing mitochondria and are very rapid, occurring within minutes. It is possible that there exists an interaction between the adrenergic system and the thyroid hormone system, which may also contribute to the cardiac actions of thyroid hormone. This review highlights the clinical manifestations of thyroid disease and the mechanisms of thyroid hormone involved in the cardiac abnormalities.

Triiodothyronine, beta-adrenergic receptors, agonist responses, and exercise capacity

Although thyroid hormone excess results in increased beta-adrenergic receptor density or agonist responses in some cells of experimental animals, the role of these effects in contributing to clinical manifestations of hyperthyroidism in human subjects is unclear. To shed further light on this issue, we characterized the effect of 2 weeks of excess triiodothyronine administration on cardiac and metabolic responses to graded-dose isoproterenol infusion, skeletal muscle beta-adrenergic receptor density, and physiologic determinants of exercise capacity in young healthy subjects. The slope of the heart rate response to isoproterenol was 36% greater (p < 0.05) after triiodothyronine administration. In addition, beta-adrenergic receptor density was increased (p < 0.01) in all types of skeletal muscle fibers. Maximal oxygen uptake during treadmill exercise declined 5% (p < 0.001) after triiodothyronine administration because of a decrease in the arteriovenous oxygen difference (p < 0.05). The plasma lactate response to submaximal exercise was 25% greater (p < 0.01) in the hyperthyroid state. These effects were paralleled by a decrement in skeletal muscle oxidative capacity and a decrease in cross-sectional area of type 2A skeletal myocytes. Thus, thyroid hormone excess enhances cardiac beta-adrenergic sensitivity under in vivo conditions in human subjects. Nevertheless, exercise capacity is diminished in the hyperthyroid state, an effect that may be related to reduced skeletal muscle oxidative capacity and type 2A fiber atrophy.

The thyroid and the heart

Cardiovascular manifestations are a frequent finding in hyperthyroid and hypothyroid states. In this review, potential mechanisms by which thyroid hormones may exert their cardiovascular effects and pathophysiological consequences of such effects are briefly discussed. Two major concepts have emerged about how thyroid hormones exert their cardiovascular effects. First, there is increasing evidence that thyroid hormones exert direct effects on the myocardium, which are mediated by stimulation of specific nuclear receptors, which in turn leads to specific mRNAs production. Furthermore, there is some evidence that thyroid hormones may also activate extranuclear sites and may directly alter plasma membrane function. Second, thyroid hormones interact with the sympathetic nervous system by altering responsiveness to sympathetic stimulation presumably by modulating adrenergic receptor function and/or density. Pathophysiological consequences of such direct and indirect thyroid hormone effects include increased myocardial contractility and relaxation that may be related to stimulation by T3 of specific myocardial enzymes. However, when left ventricular hypertrophy occurs in association with hyperthyroidism, it may be related to either direct thyroid hormone-induced stimulation of myocardial protein synthesis or to thyrotoxicosis-induced increases in cardiac work load. Although hyperthyroidism generally has little or no effect on mean arterial blood pressure, hypothyroidism is often associated with increases in diastolic blood pressure that are reversible after hormone substitution and may be mediated in part by sympathetic activation. Moreover, there is increasing evidence that thyroid hormones have direct chronotropic effect on the heart that are independent of the sympathetic nervous system.(ABSTRACT TRUNCATED AT 250 WORDS)

Rat cardiac calcium channels and their relationships with beta-adrenergic and muscarinic receptors in hypothyroidism

Thyroid hormone deficiency is associated with changes in the cardiovascular system. No one has reported the measures of both atrial and ventricular calcium channel density and function, in association with measures of negative and positive channel modulators, in hypothyroid hearts. Hormonally-induced modulation of calcium channels has clinical significance in the development and application of therapeutic agents in dysthyroid states. We thyroidectomized male rats and sham-operated euthyroid controls, in order to measure radioligand binding to ventricular and atrial membrane Ca2+ channels ([3H]-isradipine), beta-adrenoceptors ([125I]-iodocyanopindolol) and atrial muscarinic receptors ([3H]-quinuclidinyl benzilate) and related these data to contraction and heart rate responses to isoproterenol, carbachol and calcium. When data from hypothyroid tissues were compared to those of controls, the densities of calcium channels increased 50% in ventricles, but no differences were seen in atrial homogenates. In both atria and ventricles, beta-adrenoceptors decreased modestly with no change in affinity. Atrial muscarinic receptor density was also unchanged. Dose response curves of left atrial contractions showed: decreased sensitivity (increased EC50 value) but equal maximal responsiveness to extracellular calcium; an increased carbachol sensitivity (decreased EC50 value); and no significant difference in isoproterenol response. Comparisons of within-individual preparation ratioed EC50 values confirm the changed calcium and carbachol sensitivities. Heart-rate dose response curves displayed: increased maximal heart rate responsiveness to calcium associated with increased EC50 values; isoproterenol sensitivity was decreased nearly 3-fold. There was no significant difference in heart rate response to carbachol; however, ratioed values of carbachol and calcium EC50s were significantly different. These data are consistent with a tissue-level state of enhanced negative chronotropism and inotropism occurring in hypothyroid myocardia. We also confirm an earlier controversial finding of increased calcium channel density in ventricles from hypothyroid rats.

Cardiac beta-adrenoceptor changes in experimental hyperthyroidism in dogs

1. Triiodothyronine (T3; 1.0 mg/kg per day subcutaneously) was administered to 10 dogs for 14 days; 10 saline-treated dogs served as controls. T3-treated dogs showed the expected physiological responses of hyperthyroidism; further, chronotropic responses to isoprenaline in vivo were significantly increased in T3-treated dogs. 2. Beta-adrenoceptor subtype density was measured in membrane preparations by displacement of 125I-iodocyanopindolol binding by the selective beta 2-adrenoceptor antagonist, ICI 118, 551. T3 treatment led to a 93% increase in right atrial beta 1-adrenoceptor density and a 141% increase in left ventricular beta 1-adrenoceptor density; beta 2-adrenoceptor densities in right atrial, left ventricular and lung membranes were unchanged. 3. T3-treatment did not change basal or maximally stimulated adenylate cyclase activities in left ventricular membranes. 4. Thus, the cardiovascular changes in experimental hyperthyroidism in dogs were accompanied by an increased chronotropic response in vivo to isoprenaline and an increased beta 1-adrenoceptor density in atrial and ventricular membranes. However, there was no corresponding change in basal or maximal responsiveness of adenylate cyclase in ventricular membranes.

The alpha 1-adrenergic receptor in human erythrocyte membranes mediates interaction in vitro of epinephrine and thyroid hormone at the membrane Ca(2+)-ATPase

Membrane Ca(2+)-ATPase activity was stimulated in vitro separately by T4 (10(-10) M) and by epinephrine (10(-6) M). In the presence of a fixed concentration of T4, additions of 10(-8) and 10(-6) M epinephrine reduced the T4 effect on the enzyme. beta-Adrenergic blockade with propranolol (10(-6) M) prevented stimulation by epinephrine of Ca(2+)-ATPase activity, but did not prevent the suppressive action of epinephrine on T4-stimulable Ca(2+)-ATPase. In contrast, alpha 1-adrenergic blockade with unlabelled prazosin restored the effect of T4 on Ca(2+)-ATPase activity in the presence of epinephrine. Like propranolol, prazosin prevented enhancement of enzyme activity by epinephrine in the absence of thyroid hormone. Neither prazosin nor propranolol had any effect on the stimulation by T4 of red cell Ca(2+)-ATPase in the absence of epinephrine. Analysis of radiolabelled prazosin binding to human red cell membranes revealed the presence of a single class of high-affinity binding sites (Kd, 1.2 x 10(-8) M; Bmax, 847 fmol/mg membrane protein). Thus, the human erythrocyte membrane contains alpha 1-adrenergic receptor sites that are capable of regulating Ca(2+)-ATPase activity.