Effect of hyperthyroidism of short duration on cardiac sensitivity to beta-adrenergic stimulation

Severe reversible dilated cardiomyopathy and hyperthyroidism: case report and review of the literature

OBJECTIVE

To describe a case of a 46-year-old woman with Graves' disease and reversible low-output congestive heart failure and present a comparative analysis of 23 similar cases reported in the literature.

METHODS

A detailed case report is presented. In addition, a review of the pertinent literature published between 1960 and 2002 was performed to identify similar cases of dilated cardiomyopathy and thyrotoxicosis and to assess the findings in these patients.

RESULTS

A 46-year-old woman without primary heart disease was admitted to the hospital with Graves' thyrotoxicosis and severe low-output congestive heart failure. Her left ventricular ejection fraction (LVEF) at the time of initial assessment was less than 20%, and her condition was categorized as New York Heart Association (NYHA) functional class III. Nineteen months after she was treated for hyperthyroidism, her LVEF was 49% and her status was NYHA class I. A severe hypotensive episode occurred when b-adrenergic blockade therapy was initiated. The group of 23 similar cases from the literature plus our currently described patient had a mean age of 45 years, a male-to-female ratio of 1:1.2, Graves' disease as the principal cause, and LVEF improvement from 29% to 58%.

CONCLUSION

Dilated cardiomyopathy is an unusual manifestation of hyperthyroidism with unclear cause. Clinicians should be aware of this entity because it is treatable and hypotension can occur if b-adrenergic blockade treatment is initiated.

Hyperthyroidism: a "curable" cause of congestive heart failure--three case reports and a review of the literature

With the increasing incidence of coronary artery disease and the aging population, the prevalence of congestive heart failure (CHF) is increasing. In the majority of these cases the etiology is underlying coronary artery disease. Other less common causes of CHF include valvular heart disease, hypertension, alcoholic cardiomyopathy, and dilated cardiomyopathy. In addition, there are rare causes, one of which is hyperthyroidism. Hyperthyroidism can affect the cardiovascular system in a variety of ways. The cardiovascular manifestations range from sinus tachycardia to atrial fibrillation and from a high cardiac output state to CHF due to systolic left ventricular dysfunction. If the underlying hyperthyroidism is recognized and treated early the CHF in such cases can be cured. The authors present three cases of CHF due to systolic left ventricular dysfunction secondary to hyperthyroidism, which showed considerable improvement in the left ventricular function once the hyperthyroidism was treated.

Cardiovascular hemodynamics and exercise tolerance in thyroid disease

The heart is an organ sensitive to the action of thyroid hormone, and measurable changes in cardiovascular performance are detected with small variations in thyroid hormone serum concentrations. Most patients with thyroid disease experience cardiovascular manifestations, and the most serious complications of thyroid dysfunction occur as a result of cardiac involvement. The increased metabolic state and oxygen consumption that occur in hyperthyroid patients require an increased supply of oxygen and removal of metabolic products from the periphery. This is accomplished by increasing the cardiac output to meet the needs of the periphery. Circulation time is decreased in hyperthyroid patients, and a lowered arterial resistance and increased venous resistance promote the return of blood to the heart. Thyroid hormones may significantly decrease the strength of respiratory and skeletal muscles and affect regulatory mechanisms of adaptation to incremental effort. In hyperthyroidism, cardiovascular exercise testing and analysis of respiratory gas exchange demonstrate low efficiency of cardiopulmonary function as well as impaired chronotropic, contractile, and vasodilatatory reserves, which are reversible when euthyroidism is restored. During exercise, the increment (delta) of minute ventilation (respiratory rate x tidal volume), and oxygen pulse (oxygen uptake per heart beat) are significantly lower in dysthyroidism versus euthyroidism. Especially in older patients with thyroid dysfunction, markedly reduced workload, delta ejection fraction, and delta heart rate, both at the anaerobic threshold as well as at maximal exercise, are observed. In thyrotoxicosis, mitochondria oxidative dysfunction during exercise mostly causes intracellular acidosis, whereas in hypothyroidism, inadequate cardiovascular support appears to be one of the principal factors involved. These abnormalities partly explain why subjects with dysthyroidism are intolerant to exertion. Thus, in thyroid disease, both cardiac structures and function may remain normal at rest, however impaired cardiovascular and respiratory adaptation to effort becomes unmasked during exercise.

Nongenomic cardiovascular effects of triiodothyronine in euthyroid male volunteers

T(3) has been shown to exert cardiovascular effects. These effects have not yet been defined with regard to the mode of action (nongenomic vs. genomic) and with regard to an interaction with the adrenergic system in humans. To address these issues we conducted a randomized, double blind, 6-fold cross-over trial in 18 healthy male volunteers. After pretreatment with the beta-agonist dobutamine, the beta-blocking agent esmolol, or placebo (0.9% NaCl), 100 microg T(3) or placebo were injected. Primary target variables were systemic vascular resistance (SVR) and cardiac output (CO) within 45 min after injection of T(3) vs. placebo after placebo pretreatment. Sympatho-vagal balance was assessed by measurement of heart rate variability. T(3) caused a lower SVR and a higher CO than placebo (P < 0.001) after pretreatment with placebo. An increased low frequency (LF)/high frequency (HF) ratio (power in LF/power in HF band) after T(3) compared with placebo (P = 0.004) suggests an increase in sympathetic tone. After pretreatment with dobutamine, the effects of T(3) on SVR and CO were abolished, and the effect on LF/HF ratio was reversed. After pretreatment with esmolol, the effects on SVR and LF/HF ratio were reversed. Our data show, for the first time, nongenomic cardiovascular effects of T(3) in humans.

Age-related changes in thyroid hormone effects on glucose transporter isoforms of rat heart

To determine the age-related changes in thyroid hormone (TH) effects on cardiac glucose transporter one (GLUT-1) and four (GLUT-4) isoforms, male Fischer 344 rats at 4, 12, and 25 months of age were studied at euthyroid, hyperthyroid and hypothyroid conditions. Hyperthyroidism was induced with daily intraperitoneal injections of triiodothyronine (15 microg/100 gm) for 10 days. Hypothyroidism was achieved with 0.025% methimazole in the drinking water for 4 weeks. Immunoblot analysis indicated that at euthyroid basal conditions GLUT-1 protein was not significantly altered with age while GLUT-4 protein was significantly reduced in 25 month old rats (82.0 +/- 28.8% of a 4 month old rat p <0.01). In 4 months old rats, GLUT-1 was increased in both hypothyroidism (432.5 +/- 208.7% of age-matched euthyroid control) and to a lesser extent in hyperthyroidism (242.0 +/- 93.3% of control) p<0.01. In 25 month old rats, hyperthyroidism was also associated with increased GLUT-1 mass (190.8 +/- 117.6% of age-matched euthyroid control) p<0.01. Hypothyroidism in this age group was not associated with significant change in GLUT-1 protein. The cardiac GLUT-4 protein was increased during both hypothyroidism and hyperthyroidism. The changes of GLUT-4 in aged rats were similar to those found in young rats. It is concluded that TH effect on GLUT-1 expression in the heart is altered with age while TH effects on GLUT-4 are age independent.

Stress echocardiography in hyperthyroidism

Exertion symptoms occur frequently in subjects with hyperthyroidism. Using stress echocardiography, exercise capacity and global left ventricular function can be assessed noninvasively. To evaluate stress-induced changes in cardiovascular function, 42 patients with untreated thyrotoxicosis were examined using exercise echocardiography. Studies were performed during hyperthyroidism, after treatment with propranolol, and after restoration of euthyroidism. Twenty-two healthy subjects served as controls. Ergometry was performed with patients in a semisupine position using a continuous ramp protocol starting at 20 watts/min. In contrast to control and euthyroidism, the change in end-systolic volume index from rest to maximal exercise was lower in hyperthyroidism. At rest, the stroke volume index, ejection fraction, and cardiac index were significantly increased in hyperthyroidism, but exhibited a blunted response to exercise, which normalized after restoration of euthyroidism. Propranolol treatment also led to a significant increase of delta (delta) stroke volume index. Maximal work load and delta heart rate were markedly lower in hyper- vs. euthyroidism. Compared to the control value, systemic vascular resistance was lowered by 36% in hyperthyroidism at rest, but no further decline was noted at maximal exercise. The delta stroke volume index, delta ejection fraction, delta heart rate, and maximal work load were significantly reduced in severe hyperthyroidism. Negative correlations between free T3 and diastolic blood pressure, maximal work load, delta heart rate, and delta ejection fraction were noted. Thus, in hyperthyroidism, stress echocardiography revealed impaired chronotropic, contractile, and vasodilatatory cardiovascular reserves, which were reversible when euthyroidism was restored.

[Cardiopulmonary parameters in hyperthyroidism]

BACKGROUND

Hyperthyroid patients often suffer from impaired exercise capacity with dyspnoea. Two well established, non-invasive methods were used to evaluate the influence of hyperthyroidism on cardiopulmonary function.

PATIENTS AND METHODS

In 42 patients with hyperthyroidism we performed spirometry and cardiopulmonary exercise testing before and after 7 days of propranolol therapy as well as in euthyroidism.

RESULTS

In hyperthyroidism reduced vital capacity and 1-second capacity were observed (95.5 +/- 2.4% vs 102.6 +/- 1.5%; p = 0.0087; 89.4 +/- 2.3% vs 95.2 +/- 2.2%; p = 0.0179). No changes showed during beta-blockade. At the anaerobic threshold reduced tidal volume and enhanced respiratory frequency were noted (1119.8 +/- 48.9 ml vs 1289.3 +/- 62.7 ml; p = 0.0227; 28.3 +/- 0.8 vs 25.4 +/- 0.9; p = 0.0012). A significant tachycardia could be shown. Impaired response to exercise in pulse and respiratory frequency were observed. Work at the anaerobic threshold was impaired in hyperthyroidism (70 +/- 5 watts vs 86.9 +/- 5.7 watts; p = 0.016) and did not change during propranolol therapy. Oxygen pulse at the anaeorbic threshold was reduced in hyperthyroidism (7.7 +/- 0.4 ml O2/beat vs 9.1 +/- 0.4 ml O2/beat; p = 0.0012) and increased with propranolol (8.9 +/- 0.4 ml O2/beat; p = 0.0001).

CONCLUSION

In hyperthyroidism significant changes in cardiopulmonary function were noted at rest and exercise. High resting function and impaired response to exercise suggest a cardiopulmonary work with low efficiency. Propranolol leads to economization and lowers patients complaints.

Ineffective cardiorespiratory function in hyperthyroidism

Dyspnea on exertion is a common complaint in hyperthyroidism, and this thyroid dysfunction has been implicated as a primary cause of impaired effort tolerance. Using spirometry and spiroergometry, 42 patients with untreated hyperthyroidism were examined, and the condition was controlled 7 days later under propranolol monotherapy, as well as after 6 months in euthyroidism. While hyperthyroid, reduced forced vital capacity and tidal volume at the anaerobic threshold (AT) were observed in comparison to euthyroidism. Decreased oxygen (O2) pulse at AT (7 +/- 0.4 vs. 9.1 +/- 0.4 mL/beat, P = 0.0012) and at maximal exercise was noted in hyperthyroidism and was enhanced under propranolol (8.9 +/- 0.4 mL/beat, P = 0.0001). During exercise, the increment of minute ventilation (16.1 +/- 0.7 vs. 20.2 +/- 1.0 L/min, P = 0.0015), O2 uptake (9 +/- 0.5 vs. 11.4 +/- 0.5 mL/min/kg, P = 0.0022), O2 pulse (4.0 +/- 0.3 vs. 5.6 +/- 0.3 mL/beat, P = 0.0001), and heart rate (53 +/- 2 vs. 65 +/- 3 beat/min, P = 0.0004) was markedly lower in hyper- vs. euthyroidism. Work rate at AT and at maximum was reduced in hyper- vs. euthyroidism (107.4 +/- 3 vs. 141.1 +/- 4 watt, P = 0.0001). Negative correlations between free T3 and O2 pulse at AT (r = -0.59, P = 0.0005), delta O2 uptake (r = -0.54, P = 0.0007), delta minute ventilation (r = -0.48, P = 0.0007), and maximal work rate (r = -0.62, P = 0.0001) were noted. In hyperthyroidism, analysis of respiratory gas exchange showed low efficiency of cardiopulmonary function, respiratory muscle weakness, and impaired exercise capacity, which were reversible in euthyroidism.

Contribution of the autonomic nervous system to blood pressure and heart rate variability changes in early experimental hyperthyroidism

A great deal of uncertainty persists regarding the exact nature of the interaction between autonomic nervous system activity and thyroid hormones in the control of heart rate and blood pressure. We now report on thyrotoxicosis produced by daily intraperitoneal (i.p.) injection of L-thyroxine (0.5 mg/kg body wt. in 1 ml of 5 mM NaOH for 5 days). Control rats received i.p. daily injections of the thyroxine solvent. In order to estimate the degree of autonomic activation in hyperthyroidism, specific blockers were administered intravenously: atropine (0.5 mg/kg), prazosin (1 mg/kg), atenolol (1 mg/kg) or the combination of atenolol and atropine. A jet of air was administered in other animals to induce sympathoactivation. Eight animals were studied in each group. The dose and duration of L-thyroxine treatment was sufficient to induce a significant degree of hyperthyroidism with accompanying tachycardia, systolic blood pressure elevation, increased pulse pressure, cardiac hypertrophy, weight loss, tachypnea and hyperthermia. In addition, the intrinsic heart period observed after double blockade (atenolol + atropine) was markedly decreased after treatment with L-thyroxine (121.5+/-3.6 ms vs. 141.2+/-3.7 ms, P < 0.01). Of the autonomic indices, vagal tone (difference between heart period obtained after atenolol and intrinsic heart period) was negatively linearly related to intrinsic heart period (r = 0.71, P < 0.05). Atenolol modified neither the heart period nor blood pressure variability in rats with hyperthyroidism and in these rats the jet of air did not significantly affect the heart period level. The thyrotoxicosis was associated with a reduction of the 0.4 Hz component of blood pressure variability (analyses on 102.4 s segments, modulus 1.10+/-0.07 vs. 1.41+/-0.06 mm Hg, P < 0.01) and prazosin was without effect on this 0.4 Hz component in these animals. These data show a functional diminution of the vascular and cardiac sympathetic tone in early experimental hyperthyroidism. The marked rise in the intrinsic heart rate could be the main determinant of tachycardia. The blood pressure elevation may reflexly induce vagal activation and sympathetic (vascular and cardiac) inhibition.

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.

The prevention and management of iodine-induced hyperthyroidism and its cardiac features

Review of available literature and experience supports a recommended daily iodine intake of 150 microg for adults, 200 microg during pregnancy, 50 microg for the first year of life, 90 microg for ages 1 to 6, and 120 microg for ages 7 to 12. The amount of iodine added to salt in fortification programs should be adjusted to achieve these intakes. Iodine-induced hyperthyroidism (IIH) is an occasional consequence of the correction of iodine deficiency, occurring most frequently in older subjects with multinodular goiter. This complication is usually mild and self-limited, but may be serious and occasionally lethal. The most important clinical manifestations are cardiovascular. Thyrotoxicosis can aggravate pre-existing cardiac disease and can also lead to atrial fibrillation, congestive heart failure, worsening of angina, thromboembolism, and rarely, death. In the absence of pre-existing cardiac disease, treatment of thyrotoxicosis usually returns cardiac function to normal. Heightened awareness on the part of the health sector will promote early detection and prompt treatment of IIH. Monitoring should be an important part of a successful program of iodization, and in addition it offers the best opportunity for recognizing and treating IIH. Further research to improve the characterization and prevention of IIH is strongly encouraged. The most important conclusion is that IIH, while an issue that needs serious address, is not a reason to stop iodine supplementation in deficient regions. The benefits to the community from correcting iodine deficiency and avoiding its associated disorders far outweigh the damage from IIH.

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.

Cardiac hypertrophy as a result of long-term thyroxine therapy and thyrotoxicosis

OBJECTIVES

To define the effects of long-term thyroxine treatment upon heart rate, blood pressure, left ventricular systolic function, and left ventricular size, as well as indices of autonomic function, and to compare findings with those in patients with thyrotoxicosis before and during treatment.

DESIGN

Cross sectional study of patients prescribed thyroxine long term (n = 11), patients with thyrotoxicosis studied at presentation (n = 23), compared with controls (n = 25); longitudinal study of patients with thyrotoxicosis studied at presentation and serially after beginning antithyroid drug treatment (n = 23).

METHODS

24 h ambulatory monitoring of pulse and blood pressure, echocardiography, forearm plethysmography, and autonomic function tests.

RESULTS

Long-term thyroxine treatment in doses that reduced serum thyrotrophin to below normal had no effect on blood pressure, heart rate, left ventricular systolic function or stroke volume index, but was associated with an 18.4% increase in left ventricular mass index (mean (SEM) 101.9 (3.09) g/m2 v controls 86.1 (4.61), P < 0.01). Thryoxine treatment, like thyrotoxicosis, had no effect on tests of autonomic function. Untreated thyrotoxicosis resulted in pronounced changes in systolic and diastolic blood pressure and an increase in heart rate during waking and sleep. Patients with thyrotoxicosis at presentation had an increase in left ventricular systolic function (ejection fraction 70.5 (1.66)% v 65.4 (1.79), P < 0.01; fractional shortening 40.4 (1.54)% v 35.6 (1.46), P < 0.01), increased stroke volume index (45.9 (2.4) ml/m2 v 36.6 (1.7), P < 0.001), and an increase in forearm blood flow, and decrease in vascular resistance. They had a similar degree of left ventricular hypertrophy to that associated with thyroxine treatment (99.3 (4.03) g/m2); all changes were corrected within 2 months by antithyroid drugs.

CONCLUSIONS

The development of left ventricular hypertrophy in patients receiving thyroxine in the absence of significant changes in heart rate, blood pressure, and left ventricular systolic function is consistent with a direct trophic effect of thyroid hormone on the myocardium. The presence of left ventricular hypertrophy determines that further studies are essential to assess cardiovascular risk in patients taking thyroxine long term.

Thyroid disease: effects on cardiovascular function

Thyroid hormones exert direct effects upon the heart and indirect influences mediated via changes in cardiac work. Overt thyroid dysfunction is frequently associated with cardiovascular symptoms and signs and less frequently with significant cardiovascular morbidity due to atrial fibrillation or cardiac failure. Whether subclinical thyroid dysfunction is similarly associated with adverse effects upon the cardiovascular system remains unclear.

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.

Usefulness of L-thyroxine to improve cardiac and exercise performance in idiopathic dilated cardiomyopathy

The short-term effects of L-thyroxine (100 micrograms/day, 10 patients) and placebo (10 patients) on idiopathic dilated cardiomyopathy were compared. Before and at the end of the treatment, a hemodynamic study was performed in the control state and during dobutamine infusion. A cardiopulmonary exercise test was also performed with hemodynamic monitoring. An echocardiogram was recorded in the control state and during acute changes of left ventricular afterload. Plasma levels of triiodothyronine, thyroxine, thyroid-stimulating hormone and norepinephrine were measured. Placebo was ineffective. After administration of L-thyroxine all patients had normal thyroid function. The increase in left ventricular ejection fraction and the rightward shift of the slope of left ventricular ejection fraction/end-systolic stress relation (p < 0.05) indicated an improvement in the cardiac inotropic state. This proved to be independent of adrenergic influences by the unchanged beta 1 response to dobutamine. A decrease in resting systemic vascular resistances and an increase in cardiac output (p < 0.05) were also observed. Cardiopulmonary effort parameters improved (p < 0.05) without hemodynamic changes at peak exercise. It is concluded that L-thyroxine short-term administration improves cardiac and exercise performance in patients with chronic heart failure, without modifying the adrenergic support to the heart and the circulatory parameters at peak exercise.

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)