Abnormal left ventricular function in hyperthyroidism: evidence for a possible reversible cardiomyopathy

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)

Thyrotoxicosis and the heart

Many patients with thyrotoxicosis have clinical features that reflect the effects of excess thyroid hormone on the cardiovascular system. Thyrotoxicosis can aggravate preexisting cardiac disease and can also lead to atrial fibrillation, congestive heart failure, or worsening of angina pectoris. In elderly patients, these cardiac manifestations may dominate the clinical picture and warrant the measurement of the serum thyrotropin concentration. In the absence of preexisting cardiac disease, treatment of thyrotoxicosis usually results in a return of normal cardiac function.

Mechanisms of impaired exercise capacity in short duration experimental hyperthyroidism

To investigate the mechanism of reduced exercise tolerance in hyperthyroidism, we characterized cardiovascular function and determinants of skeletal muscle metabolism in 18 healthy subjects aged 26 +/- 1 yr (mean +/- SE) before and after 2 wk of daily ingestion of 100 micrograms of triiodothyronine (T3). Resting oxygen uptake, heart rate, and cardiac output increased and heart rate and cardiac output at the same submaximal exercise intensity were higher in the hyperthyroid state (P less than 0.05). However, maximal oxygen uptake decreased after T3 administration (3.08 +/- 0.17 vs. 2.94 +/- 0.19 l/min; P less than 0.001) despite increased heart rate and cardiac output at maximal exercise (P less than 0.05). Plasma lactic acid concentration at an equivalent submaximal exercise intensity was elevated 25% (P less than 0.01) and the arteriovenous oxygen difference at maximal effort was reduced (P less than 0.05) in the hyperthyroid state. These effects were associated with a 21-37% decline in activities of oxidative (P less than 0.001) and glycolytic (P less than 0.05) enzymes in skeletal muscle and a 15% decrease in type IIA muscle fiber cross-sectional area (P less than 0.05). Lean body mass was reduced (P less than 0.001) and the rates of whole body leucine oxidation and protein breakdown were enhanced (P less than 0.05). Thus, exercise tolerance is impaired in short duration hyperthyroidism because of decreased skeletal muscle mass and oxidative capacity related to accelerated protein catabolism but cardiac pump function is not reduced.

The role of noncardiac disease in the development and precipitation of heart failure

The varying roles of a widely diverse group of noncardiac disorders on the heart, particularly their ability to induce heart failure, are explored. A general overview of the cardiac effects of volume and pressure overloading is followed by specific discussions of the roles of vascular, endocrine and metabolic, renal, gastrointestinal, central nervous system, hematologic, and other miscellaneous disorders (heat stroke, sepsis, immune-mediated disease, obesity, malnutrition, and pregnancy) in producing cardiac dysfunction and failure in companion animals. Pathogenetic and pathophysiologic mechanisms are emphasized.

Thyroid crises

In the setting of characteristic features of thyrotoxicosis, the timely diagnosis and aggressive management of thyroid storm should result in a successful outcome. However, severe storm may lead to irreversible cardiovascular collapse, especially in the older patient who may have atypical features of thyrotoxicosis. The fundamental approach is prompt and optimal treatment in the emergency department once the presenting clinical features suggest its presence. Delay in the introduction of therapy while awaiting laboratory confirmation may result in further decompensation and death. The prevention of myxedema coma entails paying special attention to certain high-risk patient groups. These groups include older women with a history of Hashimoto's thyroiditis, or previous irradiation or thyroid surgery for hyperthyroidism. Inform such patients of the symptoms and signs of hypothyroidism, and perform annual thyroid function tests, such as a serum TSH, in order to provide early, adequate treatment once the test becomes positive.

O2 uptake in hyperthyroidism during constant work rate and incremental exercise

To investigate the effect of hyperthyroidism on the pattern and time course of O2 uptake (VO2) following the transition from rest to exercise, six patients and six healthy subjects performed cycle exercise at an average work rate (WR) of 18 and 20 W respectively. Cardiorespiratory variables were measured breath-by-breath. The patients also performed a progressively increasing WR test (1-min increments) to the limit of tolerance. Two patients repeated the studies when euthyroid. Resting and exercise steady-state (SS) VO2 (ml.kg-1.min-1) were higher in the patients than control (5.8, SD 0.9 vs 4.0, SD 0.3 and 12.1, SD 1.5 vs 10.2, SD 1.0 respectively). The increase in VO2 during the first 20 s exercise (phase I) was lower in the patients (mean 89 ml.min-1, SD 30) compared to the control (265 ml.min-1, SD 90), while the difference in half time of the subsequent (phase II) increase to the SS VO2 (patient 26 s, SD 8; controls 17 s, SD 8) were not significant (P = 0.06). The O2 cost per WR increment (delta VO2/delta WR) in ml.min-1.w-1, measured during the incremental period (mean 10.9; range 8.3-12.2), was always within two standard deviations of the normal value (10.3, SD 1). In the two patients who repeated the tests, both the increment of VO2 from rest to SS during constant WR exercise and the delta VO2/delta WRs during the progressive exercise were higher in the hyperthyroid state than during the euthyroid state.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid hormone and the cardiovascular system

To understand the pathophysiology of thyroid heart disease, it is necessary to recognize that thyroid hormone has effects on both the peripheral circulation and the myocardium. One of the earliest responses to thyroid hormone administration is a decline in systemic vascular resistance and an increase in cardiac output and cardiac contractility. In many ways, this response is similar to the cardiovascular response to exercise and is associated with increased left ventricular work. The majority of cardiac adaptations to changes in thyroid function are physiologic; however, certain patients do demonstrate clinical evidence of cardiac disease. Atrial arrhythmias, limitations in exercise tolerance, and congestive heart failure are reported to occur as a result of hyperthyroidism and are more common in older patients. Thyroid hormone also plays an important role in the regulation of blood pressure. Diastolic hypertension is a common accompaniment of hypothyroidism. By understanding the mechanisms by which thyroid hormone affects both the peripheral circulation as well as the myocardium, it is possible to predict the clinical response to the treatment of various thyroid disease states.

Recognition and management of cardiovascular disease related to thyroid dysfunction

Hypothyroidism and hyperthyroidism are both associated with clinically significant cardiovascular derangements. In hypothyroidism, these include pericardial effusion, heart failure, and the complex interrelationship between hypothyroidism and ischemic heart disease. Cardiovascular disorders associated with hyperthyroidism include atrial tachyarrhythmias, mitral valve dysfunction, and heart failure. Although these usually occur in individuals with intrinsic heart disease, thyroid dysfunction alone rarely causes serious but reversible cardiovascular dysfunction. Patients with commonly encountered cardiac disorders, e.g., idiopathic cardiomyopathy and atrial fibrillation, should be screened for potentially contributing subclinical thyroid diseases. In patients with heart failure and hypothyroidism, initial management should focus on diagnosis and optimal management of any primary cardiac disease, whereas in hyperthyroidism, aggressive measures to control excess thyroid hormone action should generally have the highest priority.

The hyperthyroid heart. An analysis of systolic and diastolic properties in single rat ventricular myocytes

Single ventricular myocytes were isolated by collagenase digestion from the hearts of 6-8-month-old male Wistar rats in either the control (euthyroid) state or after 7 days of daily injection of 0.64 mg/kg thyroxine (hyperthyroid). Myocytes were field-stimulated from slack length, and contraction was measured with an inverted microscope-photodiode array-computer apparatus. The effect of pacing rate and ouabain administration on systolic and diastolic function was examined. Single myocytes isolated from hyperthyroid hearts maintain the properties of bulk muscle, because maximal twitch velocity is augmented 98% and the time course of contraction as measured by the time to peak shortening, relaxation time, or contraction duration is abbreviated 39%. Spontaneous sarcoplasmic reticulum calcium release, as measured by the occurrence of contractile waves, is increased in the hyperthyroid myocytes. This increased frequency of spontaneous sarcoplasmic reticulum calcium release is most marked under conditions known to be associated with high intracellular calcium, such as low pacing rates or digitalis glycoside administration. It can account for the hypoperformance of the hyperthyroid myocytes noted under these conditions because it is associated with depletion of sarcoplasmic reticulum calcium stores and diminution of subsequent twitch amplitude. These observations may help explain, in part, the cellular basis of the altered cardiac performance in the hyperthyroid state.

Oxidative muscular injury and its relevance to hyperthyroidism

In experimental hyperthyroidism, acceleration of lipid peroxidation occurs in heart and slow-oxidative muscles, suggesting the contribution of reactive oxygen species to the muscular injury caused by thyroid hormones. This article reviews various models of oxidative muscular injury and considers the relevance of the accompanying metabolic derangements to thyrotoxic myopathy and cardiomyopathy, which are the major complications of hyperthyroidism. The muscular injury models in which reactive oxygen species are supposed to play a role are ischemia/reperfusion syndrome, exercise-induced myopathy, heart and skeletal muscle diseases related to the nutritional deficiency of selenium and vitamin E and related disorders, and genetic muscular dystrophies. These models provide evidence that mitochondrial function and the glutathione-dependent antioxidant system are important for the maintenance of the structural and functional integrity of muscular tissues. Thyroid hormones have a profound effect on mitochondrial oxidative activity, synthesis and degradation of proteins and vitamin E, the sensitivity of the tissues to catecholamine, the differentiation of muscle fibers, and the levels of antioxidant enzymes. The large volume of circumstantial evidence presented here indicates that hyperthyroid muscular tissues undergo several biochemical changes that predispose them to free radical-mediated injury.

Thyrotoxicosis complicating pregnancy

During the 12-year period from 1974 through 1985, nearly 120,000 women were delivered of infants at Parkland Hospital, and pregnancy was complicated by overt thyrotoxicosis in 60 of them (1:2000). Initial treatment was based on clinical assessment, and propylthiouracil was usually given in doses of 300 to 800 mg daily. In compliant women seen by midpregnancy, euthyroidism was achieved by a mean of 8 weeks; however, the daily dose was decreased to less than or equal to 150 mg by delivery in only 10%. Metabolic status at delivery correlated directly with pregnancy outcome, and women treated earlier in pregnancy were more likely to be euthyroid at delivery and to have good outcomes. Diagnosis of thyrotoxicosis antecedent to pregnancy was associated with earlier treatment, and 80% of 28 such women were euthyroid by delivery. Conversely, 32 women with a first diagnosis during pregnancy had the preponderance of morbidity, including five of six stillbirths and six of seven cases of heart failure. This group was characterized by a relative delay in gestational age at diagnosis. Preterm delivery, perinatal mortality, and maternal heart failure were more common in women who remained thyrotoxic despite treatment and in those who were never treated. Although we infrequently achieved maintenance doses recommended by most, because there were minimal adverse effects from therapy described here and because uncontrolled thyrotoxicosis caused significant maternal and perinatal morbidity, aggressive medical therapy seems appropriate, especially when pregnancy is advanced.

Higher than conventional doses of carbimazole in the treatment of thyrotoxicosis

In order to ascertain whether higher than conventional doses of carbimazole achieve more rapid control of thyrotoxicosis, 30 thyrotoxic patients were alternately allocated into two groups, group 1 (15 subjects) receiving a conventional starting dose of 45 mg orally daily and group 2 (15 subjects) a dose of 100 mg orally. In addition to weekly estimations of serum T4, T3, free T4, free T3 and TSH, the systolic time intervals ratio (STI), a measure of left ventricular contractility, was calculated as an accurate measure of peripheral thyroid hormone activity, the study end-point being a normal STI (0.26-0.32). None of the individuals studied experienced side-effects during the study period. Mean pre-treatment STI values for the two treatment groups were the same at entry (0.20). The mean recovery times for STI was 4.4 weeks (SE 0.3) in the high dose group and 5.9 weeks (SE 0.4) in the low dose group (P = 0.0037). There was a definite trend towards a shorter recovery time for free T3 in the higher dose group (P = 0.057) but no apparent differences for T4, T3 and free T4. Higher than conventional doses of carbimazole may be advisable in the initial treatment of severe thyrotoxicosis.

Catabolic effects of thyroid hormone excess: the contribution of adrenergic activity to hypermetabolism and protein breakdown

Although patients with thyrotoxicosis improve clinically after treatment with beta-adrenergic blocking drugs, it has never been established whether the hypermetabolism and body protein wasting caused by thyroid hormone excess are actually mediated by adrenergic mechanisms. To evaluate this issue, we measured basal energy expenditure, epinephrine-stimulated calorigenesis, and leucine kinetics (an index of body protein catabolism) in six normal volunteers before and after triiodothyronine (T3) administration (150 micrograms/d for 1 week). Serum T3 rose nearly threefold (P less than 0.001) during T3 administration, producing significant increases in basal metabolic rate (21%, P less than 0.001), nitrogen excretion (45%, P less than 0.001), and leucine flux (45%, P less than 0.01). In response to epinephrine infusion, the absolute rise in metabolic rate above basal was 57% greater in the thyrotoxic condition (P less than 0.02). Although beta-adrenergic blockade with intravenous propranolol totally abolished the calorigenic response to epinephrine, it had no detectable effect on either the accelerated basal metabolic rate or the augmented body protein catabolism caused by thyroid horomone excess. Our data suggest that in the basal, resting state, the increased metabolic rate and accelerated protein breakdown caused by thyroid hormone are not adrenergically mediated. However, under nonbasal conditions (when sympathetic activity is stimulated), enhanced responsiveness to catecholamine calorigenesis may exaggerate the hypermetabolic state and thereby contribute to weight loss and other clinical manifestations of thyrotoxicosis. This mechanism may explain the clinical efficacy of beta-adrenergic blocking agents in the treatment of thyrotoxicosis.

Massive thyroxine intoxication: evaluation of plasma extraction

Six patients were admitted after erroneous massive intake of levothyroxine (70-1200 mg over an interval of 2-12 days). All patients developed classical symptoms of thyrotoxicosis within 3 days of the first dose; five patients presented grade II-III coma and one became stuporous (days 7-10). Two patients developed left ventricular failure and three had arrhythmias (days 8-11). Total thyroid hormone levels in serum on admission ranged 935-7728 nmol/l for T4 (TT4) and 23-399 nmol/l for T3 (TT3). All patients received treatment with hydrocortisone and Propranolol. Propylthiouracil was also given in 3 cases. Extractive techniques (charcoal haemoperfusion and/or plasmapheresis) were initiated 8-14 days after the first dose of L-T4. The plasma disappearance rate (K) of TT4 with plasmapheresis was 30 times higher, on average, than under standard medical treatment (M). Also, K of TT4 under haemoperfusion was about five times higher than K under M. K changes for TT3 were higher under haemoperfusion than under plasmapheresis. Furthermore, extractive procedures shortened the average half life of TT4, (from 106.5 +/- 44.6 to 59.7 +/- 20.2 h, p less than 0.05).

Myocardial mechanics in hyperthyroidism: importance of left ventricular loading conditions, heart rate and contractile state

Hyperthyroidism has been reported to affect all of the major determinants of left ventricular performance in a manner that would augment ventricular shortening characteristics. The hypothesis tested in this study is that reduced afterload in conjunction with increased preload and heart rate, rather than augmented contractility, accounts for much of the increase in left ventricular performance noted previously in these patients. To investigate this hypothesis, 11 hyperthyroid patients were evaluated serially over 4 +/- 2 months. With therapy, serum total thyroxin (T4) decreased significantly (p less than 0.001). Ventricular hemodynamics were assessed by two-dimensional targeted M-mode echocardiograms and calibrated carotid pulse tracings. Ventricular preload was estimated by end-diastolic dimension, whereas afterload was measured as end-systolic wall stress. Overall left ventricular performance was quantitated by the extent and velocity of shortening, whereas myocardial work was assessed by ventricular systolic stress-length relations. With therapy, overall left ventricular performance declined (p less than 0.01). This change was associated with no change in end-diastolic dimension or end-systolic wall stress, and a 24% fall in heart rate (p less than 0.01). This latter finding has been shown previously to have no significant effect on left ventricular contractile state over the range of heart rates encountered in this study. In all cases, the end-systolic stress/rate-corrected shortening velocity relation fell with attainment of normal thyroid status, characteristic of a decline in contractility. There was a strong positive correlation between left ventricular contractility and serum thyroid hormone level (r = 0.83). In addition, ventricular minute work declined with therapy (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Is there a thyrotoxic cardiomyopathy in children?

We measured left ventricular ejection fraction by radionuclide angiocardiography at rest and during graded exercise in eight thyrotoxic children. Five patients were reassessed after return to a clinically euthyroid state. In the thyrotoxic state the left ventricular ejection fraction was 69% +/- 8% at rest, and increased normally by 7% to 10% during exercise in four patients but did not change significantly or decreased by as much as 8% in the other four patients. The change in left ventricular ejection fraction during exercise correlated inversely with serum concentrations of thyroxine (r = -0.82, P less than 0.02) and triiodothyronine (r = -0.88, P less than 0.01). A comparison between patients who had normal left ventricular response to exercise with those who had abnormal response while thyrotoxic failed to uncover any significant differences between their resting ejection fractions or resting and exercise heart rates and systolic blood pressures. Three abnormal and two normal responders were reassessed after return to a clinically euthyroid state. The left ventricular ejection was 64% +/- 6% at rest, and increased during exercise in all patients. Our study suggests that thyrotoxicosis causes diminished left ventricular reserve (compensated functional cardiomyopathy) in some patients, but appears to be reversible. Severity of thyrotoxicosis, as measured by thyroid hormone blood levels, may be a determining factor in the development of functional cardiomyopathy.

Hyperthyroidism. Causes, clinical features, and diagnosis

The usual patient with hyperthyroidism has Graves' disease: If serum levels of thyroid hormone are clearly elevated, the presence of infiltrative ophthalmopathy or pretibial myxedema is probably sufficient for establishing the diagnosis. However, if the ancillary findings of Graves' are not present, the radioactive iodine uptake should be determined to rule out other possible etiologies of hyperthyroidism. Signs of hyperthyroidism may be subtle or misleading, particularly in the elderly; the well-informed clinician keeps the diagnosis in mind, and if the initial thyroid hormone tests are not definitive, employs additional tests that are cost-effective in the individual clinical situation.

Hyperthyroid heart disease

The importance of cardiovascular system involvement in hyperthyroidism has been recognized for many years. In the middle-aged and elderly patient, often with mild but prolonged elevation of plasma thyroid hormones, symptoms and signs of heart failure and complicating atrial fibrillation may dominate the clinical picture and mask the more classical endocrine manifestations of the disease. Pitfalls in diagnosis and the importance of early recognition and treatment are discussed. Despite experimental evidence for a short-term inotropic action of thyroid hormone excess, clinical data support the existence of a reversible cardiomyopathy in hyperthyroidism with impaired contractile reserve. Enhanced myocardial performance at rest primarily reflects the peripheral actions of thyroid hormone excess. Most, if not all, of the cardiac abnormalities return to normal once a euthyroid state has been achieved, although atrial fibrillation may persist in a minority. Optimum treatment requires rapid and definitive antithyroid therapy, usually using a large dose of radio-iodine, and rapid control of heart failure. Systemic anticoagulation is indicated in the presence of atrial fibrillation and should be continued until sinus rhythm has been present for at least three months, either spontaneously or after cardioversion.

Transient ventricular dysfunction associated with cesarean section in a patient with hyperthyroidism

Pulmonary artery catheterization was performed prior to surgery in a severely hyperthyroid patient undergoing cesarean section. A transient but significant decline in left ventricular performance was observed in conjunction with the stress of operation. A parallel is suggested between this phenomenon and the documented exercise-induced reduction of left ventricular function in nonpregnant hyperthyroid patients.

End-organ responses to thyroxine therapy in subclinical hypothyroidism

We studied variables known to change with thyroid hormone status in 18 patients with subclinical hypothyroidism before and during treatment with thyroxine in a dose sufficient to restore the plasma TSH response to TRH to normal. There was an associated increase in both plasma total T4 and free T4 within the normal range but plasma total T3 and free T3 were unchanged. As a result of thyroxine treatment there was a small but significant increase (P less than 0.05) in left ventricular ejection fraction (LVEF) with maximal exercise but no significant changes in LVEF at rest and moderate exercise, continuously monitored mean sleeping heart rate, day/night ratios of urinary sodium excretion, peripheral nerve conduction velocities, fasting serum triglycerides, total cholesterol (TC), high density lipoproteins (HDL) or TC/HDL ratios. On this evidence we do not consider that thyroxine replacement therapy is indicated in patients with subclinical hypothyroidism.

Delayed recovery of left ventricular function after antithyroid treatment. Further evidence for reversible abnormalities of contractility in hyperthyroidism

Sequential measurements of systolic time intervals, left ventricular dimensions, and the derived indices of contractility were undertaken at rest and during isometric exercise in 15 hyperthyroid patients before, during, and after antithyroid treatment. At rest hyperthyroidism was characterised by a shortened pre-ejection period and increased velocity of circumferential shortening of the left ventricle. During isometric exercise, however, the pre-ejection period increased significantly beyond that predicted for normal subjects, and the velocity of circumferential fibre shortening fell by 30%. In contrast, both the pre-ejection period and the velocity of circumferential fibre shortening were unchanged during exercise after a stable euthyroid state had been achieved for at least three months. Comparison between exercise responses and thyroid status during antithyroid treatment showed that a biochemical euthyroid state may be achieved many weeks before normalisation of contractile response to exercise. These findings support the hypothesis of reversible depression of left ventricular function in hyperthyroidism. Responses at rest principally reflect the peripheral actions of thyroid hormone excess.

Cardiac function in hyperthyroidism

It is accepted that thyroid hormones, thyroxine and tri-iodothyronine have direct inotropic and chronotropic effects on the heart. However, the mechanisms of manifestations are still uncertain. The following discusses and evaluates some of the theories regarding the interaction of these hormones with cardiac function.

An etiologic basis for congestive heart failure on the molecular level

It is here proposed that disordered redox balance leads to congestive heart failure in a variety of diverse clinical situations. These conditions include those associated with an excess of reducing agents, such as catecholamines and thyroid hormone, or impaired oxidant defenses, such as in selenium deficiency. The clinical situations include hypertension, hyperthyroidism, progressive congestive heart failure, amphetamine overdose and hemochromatosis. The molecular damage to the cardiac muscle is postulated to be mediated via reaction oxygen radicals.

The effect of minor increments in plasma thyroxine on heart rate and urinary sodium excretion

We studied day/night (D:N) patterns of urinary sodium excretion and the 24 hour ambulatory electrocardiogram in seven normal subjects before and during the administration of T4. Thyroxine increased thyroid hormone levels within the normal range and inhibited the plasma TSH response to TRH. This was associated with a significant decrease in D:N sodium excretion (P less than 0.01) and D:N urine flow (P less than 0.01), a significant increase in mean nocturnal heart rate (P less than 0.01), and a lesser increment in mean daytime heart rate (P less than 0.05). These responses to small changes in thyroid hormone levels suggest that the anterior pituitary is not alone in recognising minor thyroid hormone excess. The clinical implication is that some patients with a normal T3 and T4 but an impaired TSH response to TRH might benefit from antithyroid treatment.