Catecholamines and hyperthyroidism

Crash Landing of Thyroid Storm: A Case Report and Review of the Role of Extra-Corporeal Systems

Thyroid storm is a rare but life-threatening endocrinological emergency with significant mortality ranging from 10-30% with multi-organ involvement and failure. In view of the rarity of this condition and efficacy of established first line medical treatment, use of extra-corporeal treatments are uncommon, not well-studied, and its available evidence exists only from case reports and case series. We describe a 28-year-old man who presented with an out-of-hospital cardiac arrest secondary to thyroid storm. Despite conventional first-line pharmacotherapy, he developed cardiogenic shock and circulatory collapse with intravenous esmolol infusion, as well as multi-organ failure. He required therapeutic plasma exchange, concurrent renal replacement therapy, and veno-arterial extra-corporeal membrane oxygenation, one of the few reported cases in the literature. While there was clinical stabilization and improvement in tri-iodothyronine levels on three extra-corporeal systems, he suffered irreversible hypoxic-ischemic brain injury. We reviewed the use of early therapeutic plasma exchange and extra-corporeal membrane oxygenation, as well as the development of other novel extra-corporeal modalities when conventional pharmacotherapy is unsuccessful or contraindicated. This case also highlights the complexities in the management of thyroid storm, calling for caution with beta-blockade use in thyrocardiac disease, with close monitoring and prompt organ support.

Normothermic thyroid storm: an unusual presentation

Thyroid storm is a rare life-threatening emergency due to thyrotoxicosis. A 30-year-old female presented with restlessness, tachycardia and vomiting but with normothermia which is an unusual presentation. There is the need for clinicians to be aware of atypical clinical features that can make the diagnosis of thyroid storm difficult.

Thyroid hormone interacts with the sympathetic nervous system to modulate bone mass and structure in young adult mice

To investigate whether thyroid hormone (TH) interacts with the sympathetic nervous system (SNS) to modulate bone mass and structure, we studied the effects of daily T3 treatment in a supraphysiological dose for 12 wk on the bone of young adult mice with chronic sympathetic hyperactivity owing to double-gene disruption of adrenoceptors that negatively regulate norepinephrine release, α(2A)-AR, and α(2C)-AR (α(2A/2C)-AR(-/-) mice). As expected, T3 treatment caused a generalized decrease in the areal bone mineral density (aBMD) of WT mice (determined by DEXA), followed by deleterious effects on the trabecular and cortical bone microstructural parameters (determined by μCT) of the femur and vertebra and on the biomechanical properties (maximum load, ultimate load, and stiffness) of the femur. Surprisingly, α(2A/2C)-AR(-/-) mice were resistant to most of these T3-induced negative effects. Interestingly, the mRNA expression of osteoprotegerin, a protein that limits osteoclast activity, was upregulated and downregulated by T3 in the bone of α(2A/2C)-AR(-/-) and WT mice, respectively. β1-AR mRNA expression and IGF-I serum levels, which exert bone anabolic effects, were increased by T3 treatment only in α(2A/2C)-AR(-/-) mice. As expected, T3 inhibited the cell growth of calvaria-derived osteoblasts isolated from WT mice, but this effect was abolished or reverted in cells isolated from KO mice. Collectively, these findings support the hypothesis of a TH-SNS interaction to control bone mass and structure of young adult mice and suggests that this interaction may involve α2-AR signaling. Finally, the present findings offer new insights into the mechanisms through which TH regulates bone mass, structure, and physiology.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

Thyroid hormones and the adrenergic nervous system

This paper reviews the various possible relationships between thyroid hormones and the adrenergic nervous system. These concern the sympathetic influence on thyroid hormone secretion, thyroid hormone - induced changes of sympathetic activity or adrenoceptor changes - the effects of catecholamines on thyroid hormone metabolism and, finally, the clinical benefits of beta-adrenoceptor blocking drugs on symptoms and signs of hyperthyroidism.

The role of catecholamines in clinical medicine

The sympathetic nervous system is of major importance for the regulation of several physiological functions. Drugs which inhibit the actions of catecholamines and adrenergic drugs are used in the treatment of many clinical disorders. The potential role of catecholamines in a number of human diseases has, however, until recent years been studied to a limited extent only due to lack of methods for quantitation of sympathetic nervous activity. After the development of enzymatic isotope-derivative assays, reliable measurements of noradrenaline and adrenaline in plasma became available. Studies in man have shown that plasma noradrenaline is an index of sympathetic nervous activity. The present survey deals with sympathetic nervous activity and plasma adrenaline in a number of clinical disorders viz. arterial hypertension, duodenal ulcer, thyrotoxicosis, diabetes mellitus and ketotic hypoglycemia.

A case of hyperglycemic hyperosmolar state associated with Graves' hyperthyroidism: a case report

Hyperglycemic hyperosmolar state (HHS) is an acute complication mostly occurring in elderly type 2 diabetes mellitus (DM). Thyrotoxicosis causes dramatic increase of glycogen degradation and/or gluconeogenesis and enhances breakdown of triglycerides. Thus, in general, it augments glucose intolerance in diabetic patients. A 23-yr-old female patient with Graves' disease and type 2 DM, complying with methimazole and insulin injection, had symptoms of nausea, polyuria and generalized weakness. Her serum glucose and osmolarity were 32.7 mM/L, and 321 mosm/kg, respectively. Thyroid function tests revealed that she had more aggravated hyperthyroid status; 0.01 mU/L TSH and 2.78 pM/L free T3 (reference range, 0.17-4.05, 0.31-0.62, respectively) than when she was discharged two weeks before (0.12 mU/L TSH and 1.41 pM/L free T3). Being diagnosed as HHS and refractory Graves' hyperthyroidism, she was treated successfully with intravenous fluids, insulin and high doses of methimazole (90 mg daily). Here, we described the case of a woman with Graves' disease and type 2 DM developing to HHS.

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.

Thyroid status influences baroreflex function and autonomic contributions to arterial pressure and heart rate

The effect of thyroid status on arterial baroreflex function and autonomic contributions to resting blood pressure and heart rate (HR) were evaluated in conscious rats. Rats were rendered hyperthyroid (Hyper) or hypothyroid (Hypo) with triiodothyronine and propylthiouracil treatments, respectively. Euthyroid (Eut), Hyper, and Hypo rats were chronically instrumented to measure mean arterial pressure (MAP), HR, and lumbar sympathetic nerve activity (LSNA). Baroreflex function was evaluated with the use of a logistic function that relates LSNA or HR to MAP during infusion of phenylephrine and sodium nitroprusside. Contributions of the autonomic nervous system to resting MAP and HR were assessed by blocking autonomic outflow with trimethaphan. In Hypo rats, the arterial baroreflex curve for both LSNA and HR was shifted downward. Hypo animals exhibited blunted sympathoexcitatory and tachycardic responses to decreases in MAP. Furthermore, the data suggest that in Hypo rats, the sympathetic influence on HR was predominant and the autonomic contribution to resting MAP was greater than in Eut rats. In Hyper rats, arterial baroreflex function generally was similar to that in Eut rats. The autonomic contribution to resting MAP was not different between Hyper and Eut rats, but predominant parasympathetic influence on HR was exhibited in Hyper rats. The results demonstrate baroreflex control of LSNA and HR is attenuated in Hypo but not Hyper rats. Thyroid status alters the balance of sympathetic to parasympathetic tone in the heart, and the Hypo state increases the autonomic contributions to resting blood pressure.

Thyroid storm presenting as multiple organ dysfunction syndrome

Thyroid storm is a rare disorder characterized by hypertension, hyperthermia, and multiple systems involvement. Early recognition and treatment of thyroid storm are essential in reducing morbidity and mortality from this disorder. We present the case of a patient with an atypical (normothermic, normotensive) presentation of thyroid storm, accompanied by multiple organ dysfunction syndrome, including lactic acidosis and liver dysfunction, both of which are very rare complications. This case highlights both the multiple organ systems that can be involved in thyroid storm and the importance of recognizing atypical presentations of thyroid storm.

Hyperthyroidism and asthma

Two patients presented with new onset of thyrotoxicosis and they then developed episodic wheezing a few months afterward. The asthmatic attacks improved when the patients were rendered in a euthyroid state with treatment. The present paper discusses the relationship between two diseases, hyperthyroidism and asthma, and the underlying factors potentially contributing to their conditions. The present findings suggest that asthma may develop in a susceptible individual with hyperthyroidism and it is possible that the reactive oxygen species may be a contributory factor in exacerbating wheezing in our hyperthyroid patients.

Atrial flutter: an uncommon pediatric manifestation of hyperthyroidism

OBJECTIVE

Atrial flutter is an uncommon arrhythmia in the pediatric population except for the immediate newborn period or following atrial repair of congenital heart disease. In children the diagnosis of atrial flutter may be difficult, attributable to rapid atrioventricular conduction and superimposition of flutter waves on QRS and T waves. Atrial flutter secondary to hyperthyroidism has been rarely reported in older adults, but there are no reports of children presenting with atrial flutter as the initial manifestation of hyperthyroidism.

CASE REPORT

We report an interesting case of hyperthyroidism in a 3-year-old presenting with congestive heart failure and atrial flutter with 1:1 atrioventricular conduction. The responses to adenosine administration and to cardioversion were unusual and ultimately helpful in suggesting the diagnosis of hyperthyroidism.

CONCLUSION

When atrial flutter is encountered in a pediatric patient in whom there is 1:1 atrioventricular conduction, a lack of a response to adenosine, and persistent sinus tachycardia after cardioversion, the clinician should be alert to the possibility of thyrotoxicosis.

Secondary hypertension: evaluation and treatment

Most patients with hypertension in the United States have essential (primary) hypertension (95%), the cause of which is unknown. The remaining 5% of adults with hypertension have the secondary form of hypertension, the cause and pathophysiologic process of which are known. Internists and other primary care physicians refer to this as treatable or curable hypertension, because the hypertension can be managed or even controlled with medications. Similarly, the condition is called surgical hypertension by surgeons in the belief that once the cause is determined and identified, surgical intervention will result in cure of hypertension. Secondary causes of hypertension include renal parenchymal disease, renovascular diseases, coarctation of the aorta, Cushing's syndrome, primary hyperaldosteronism, pheochromocytoma, hyperthyroidism, and hyperparathyroidism. Occasionally included in this category are alcohol- and oral contraceptive-induced hypertension and hypothyroidism, but these conditions are not discussed herein. The evaluation of secondary hypertension is of interest and can bring together different facets of anatomy, physiology, pharmacology, and radiology in the medical and surgical treatment of these disorders. Despite enthusiasm that can be generated in the evaluation of these conditions, evaluation can be expensive and should not be conducted for all patients with hypertension. Features that aid in the diagnosis of secondary hypertension include the following: 1. Onset of hypertension before the age of 20 or after the age of 50 years. The presence of hypertension at a young age may suggest coarctation of the aorta, fibromuscular dysplasia, or an endocrine disorder. Hypertension found for the first time after the age of 50 years may suggest the presence of renovascular hypertension caused by atherosclerosis. 2. Markedly elevated blood pressure or hypertension with severe end-organ damage, as in grade III or IV retinopathy. These findings suggest the presence of renovascular hypertension or pheochromocytoma. 3. Specific body habitus and ancillary physical findings. For example, truncal obesity and purple striae occur with hypercortisolism, and exophthalmos is associated with hyperthyroidism. 4. Resistant or refractory hypertension (poor response to medical therapy usually necessitating use of more than three antihypertensive medications from three different classes). 5. Specific biochemical test that suggest the existence of certain disorders, such as hypercalcemia in hyperparathyroidism, hyperglycemia in Cushing's syndrome and pheochromocytoma, and unprovoked hypokalemia with renin-producing tumors, primary hyperaldosteronism, or renin-mediated renovascular hypertension. 6. Other characteristics that may suggest secondary hypertension such as abdominal diastolic bruits (renovascular hypertension), decreased femoral pulses (coarctation of the aorta), or bitemporal hemianopias (Cushing's disease). A combination of a good history and physical examination, astute observation, and accurate interpretation of available data usually are helpful in the diagnosis of a specific causation.

Effect of chemical sympathectomy on serum levels of thyroid hormones and the biochemical profile of domestic pigeons

Recent studies have stressed the importance of the cholinergic system on avian metabolism. However, the role of the sympathetic nervous system (SNS) remains unclear. The present study was, therefore, aimed to probe the mechanisms for modulation of avian metabolism by the sympathetic nervous system after inhibition of the adrenergic responses. Activities of serum thyroid hormones (tri-iodothyronine, T3, and thyroxine, T4), body weight, hepatic weight, as well as total lipid and water content in the liver and body temperature were some of the parameters examined after chemical sympathectomy with 6-hydroxydopamine (6-OHDA) and reserpine treatment in 24-h starved pigeons. In addition, glucose was administered to the pigeons to identify the regulatory role played by glucose after disruption of the SNS. A reduction in body weight of the pigeons and an enhancement in the lipogenic machinery along with a corresponding increase in water content were some of the obvious effects in 6-OHDA+reserpine treated, as well as glucose-loaded sympathectomized birds. The cloacal temperature (Tc) and both the thyroid hormones showed a drastic decrease while the T3/T4 ratio was augmented as a result of sympathectomy. However, serum T3 and T4 levels were restored to control values when glucose load was given, indicating that glucose might be reversing some of the detrimental effects of 6-OHDA treatment by activating intrinsic autoregulatory mechanisms of thyroid gland, thereby reviving the levels of thyroid hormones. Thus, the influence of SNS appears to be crucial in the maintenance of serum thyroid hormones and body temperature, as well as metabolic activities of hepatic cells.

Elevated plasma levels of VWF:Ag in hyperthyroidism are mediated through beta-adrenergic receptors

Thyroid dysfunction influences the levels of Von Willebrand factor (VWF:Ag). Plasma VWF:Ag levels in 35 hyperthyroid patients were significantly elevated, which had a positive correlation with both T3 (Triiodothyronine) and T4 (Thyroxine) levels. A follow-up study of seven hyperthyroid patients undergoing anti-thyroid therapy showed that the elevated VWF:Ag levels returned to normal range with the normalization of thyroid function. On the other hand, after administration of 160 mg/day of propranolol, a specific blocker of beta-adrenergic receptors, to five hyperthyroid patients for twenty-eight days, plasma VWF:Ag levels returned to normal range, while T3, T4 levels remained high without significant alteration. These results demonstrated that the elevation of plasma VWF:Ag by thyroid hormone is mediated through beta-adrenergic receptors, which may be useful to further understand the physiological and pathological roles of thyroid hormone in the regulation of VWF:Ag levels.

Beta-adrenergic blockade for the treatment of hyperthyroidism

PURPOSE

To review the clinical and biochemical effects of beta-adrenergic blocking drugs on hyperthyroidism.

MATERIALS AND METHODS

Studies published since 1972 were identified through a computerized search of MEDLINE and extensive searching of the bibliographies of the articles identified. Based on an understanding of the differences in beta-blocker metabolism in euthyroid and hyperthyroid patients, we reviewed the differences in pharmacokinetics and metabolic and clinical outcomes during their use in hyperthyroidism, as reported in the articles reviewed.

RESULTS

beta Blockers have been used to modify the severity of the hyperadrenergic symptoms of hyperthyroidism for the past 20 years. The clinical efficacy of these agents is affected by hyperthyroid-induced alterations in their gastrointestinal absorption, hepatic metabolism, and renal excretion. The mechanisms whereby these clinical changes are effected is unknown. The agents differ in their beta 1 cardioselectivity, membrane-stabilizing activity, intrinsic sympathomimetic activity, and lipid solubility. They do not appear to alter synthesis or secretion of thyroid hormone by the thyroid gland. Their effects on thyroxine metabolism are contradictory. Decreased thyroxine to triiodothyronine conversion is caused by some, but not all, beta blockers, and this appears to correlate with membrane-stabilizing activity. There does not appear to be any alteration in catecholamine sensitivity during beta-adrenergic blockade.

CONCLUSIONS

The principal mechanism of action of beta blockers in hyperthyroidism is to antagonize beta-receptor-mediated effects of catecholamines. beta Blockers are effective in treating hypermetabolic symptoms in a variety of hyperthyroid states. Used alone, they offer significant symptomatic relief. They are also useful adjuvants to antithyroid medications, surgery, and radioactive iodide treatment in patients with Graves' disease and toxic nodular goiters.

The pituitary-thyroid axis effects on ocular and orbital tissues: a histological, histochemical, and morphometric study

This work tests the notion that the effect of thyroid hormone on orbital and ocular tissues is mediated through its action on their lysosomal enzymes. Hyperthyroidism was produced in guinea pigs by thyroxin and TSH; hypothyroidism was induced by thiouracil. After treatment for 10 to 21 days, several ocular and orbital tissues were taken for histological, morphometrical, and histochemical examinations. High acid phosphatase activity was demonstrated in extraocular muscles, optic nerve and the retinal pigment epithelium of thyroxine- and TSH-treated animals. The findings fit the notion that the effects of thyroid hormones are mediated through lysosomes also in ocular and orbital tissues.

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.

Catecholamine-thyroid hormone interactions and the cardiovascular manifestations of hyperthyroidism

The spectrum of classic symptoms of hyperthyroidism suggests that in addition to the effects of increased thyroid hormone, affecting various organ systems, there is also a hyperadrenergic state. Despite this clinical impression, direct measures of serum levels of catecholamines and their urinary metabolites demonstrate values that are equal to or less than normal. In contrast, the hypothyroid patient who clinically manifests signs of decreased adrenergic stimulation can be expected to have increased levels of epinephrine, norepinephrine, and its metabolites. This review discusses possible mechanisms to explain this seeming paradox. Treatment of hyperthyroidism includes the rapid reversal of many of the adrenergic symptoms with use of beta-blocking drugs. Return to a clinically and chemically euthyroid state, however, requires antithyroid therapy accomplished over a longer period of time. A knowledge of the interaction of the cardiovascular and extracardiovascular manifestations of hyperthyroidism and the role of the adrenergic nervous system is important in the rational management of these patients.

The role of thyroid hormones in maturation of the adrenaline-sensitive lung liquid reabsorptive mechanism in fetal sheep

1. Following thyroidectomy at 106-118 days fetal sheep were infused continuously with triiodothyronine (T3) from 110, 118, 125 or 131 days (n = 12) or with thyroxine (T4) from 118 days (n = 4) until the fetuses were delivered. Lung liquid secretion or absorption rates, heart rate, blood pressure and arterial blood gases were measured before and during 45 min periods of fetal infusions of adrenaline (n = 60) at 3-8 day intervals. The effects of T3 or T4 replacement on the response to adrenaline were compared with data previously obtained in groups of euthyroid (control) and thyroidectomized (Tx) fetuses. 2. Fetuses infused with T4 (50 micrograms/day) following thyroidectomy had plasma T4 and T3 concentrations in the normal fetal range. Fetal plasma T3 levels in fetuses infused with T3 (60 micrograms/day) were at or above the high end of the normal range for full-term fetuses. Those receiving 120 micrograms of T3 per day had levels equivalent to those normally seen in the postnatal T3 surge. 3. Normal maturation in the lung of the reabsorptive response of fetal lung liquid to adrenaline was seen in the fetuses infused with T3 or T4 from 118 days. A marginal advance in maturation was seen in fetuses infused with T3 from 110 days and a delay in maturation in those infused with T3 from 125 and 131 days.

Food intake, body and heart composition, and heart rate in T3 plus atenolol-treated rats

Thyroid hormones and beta-blockers both affect energy balance and the heart. The interaction of 3,5,3'-triiodothyronine (T3) and the beta-blocker atenolol on some cardiac and energy balance parameters was therefore investigated. Stock-fed male Wistar rats (approximately 400 g) received 5 micrograms (expt 1) or 1.5 micrograms (expt 2) T3.100 g body wt-1.day-1 for 3 wk, with or without atenolol. In expt 3, rats were overfed with a "cafeteria" diet before and during the experiment and otherwise treated as in experiment 2. Compared with stock-fed (expt 1 and 2) or overfed (expt 3) controls, T3 caused an increase in food intake in experiments 1 and 2 but not in experiment 3. There was a large loss of body fat in all experiments, disproportionately greater than the body weight loss. Protein loss was significant only in experiment 1 and negligible in cafeteria rats. Heart rate and weight were increased, although heart composition remained unchanged. Atenolol, in a dose that abolished T3-induced tachycardia, did not modify any of the other T3 effects investigated, including the hypertrophy of the heart. These results indicate that T3-induced tachycardia can be abolished by concomitant treatment with a beta-blocker without altering parameters connected with energy balance, whereas protein loss caused by T3 can be attenuated by lowering the dose of T3 used and can be further blunted by dietary manipulation (cafeteria overfeeding).

Action of the thyroid hormone at the level of the plasma membrane

In this presentation, I present evidence indicating a direct action of thyroid hormone at the level of the plasma membrane. Characteristically, the plasma membrane-mediated effects of thyroid hormones are prompt in onset, independent of new protein synthesis, and are associated with changes in the transmembrane transport of ions and substrates. The presence of specific binding sites for thyroid hormone in plasma membrane of various tissues and species, although inconclusive in itself, provides additional support for the direct action of thyroid hormone on the plasma membrane. A model for the mechanism of action of thyroid hormone at the plasma membrane level to increase sugar uptake by rat thymocytes is delineated, and the physiological role of the plasma membrane-mediated action of thyroid hormone is discussed.

Reduced cardiac vagal excitability in hyperthyroidism

A great deal of uncertainty persists regarding the exact nature of the interaction between autonomic nervous activity and thyroid hormones in the control of heart rate. In the present work we investigated whether reduced vagal influence could contribute to the tachycardia in hyperthyroidism. Vagal excitability was studied in ten hyperthyroid patients. Prolongation of R-R interval in response to carotid baroreceptor stimulation by neck suction was found to be less in the hyperthyroid state compared to the control state after therapy. The extent of nocturnal bradycardia and the vagal excitatory response to the central effect of low dose atropin was significantly reduced in hyperthyroid patients compared to euthyroid controls. We concluded that in the hyperthyroid state cardiac vagal motoneurones were in low excitability state, and speculated that the inhibition might have resulted from thyroid hormone action on CNS structures integrating autonomic function and behaviour.

Beta-adrenergic blockers

beta-Adrenergic blocking drugs have been available for several years to treat ischemic heart disease and other cardiovascular and noncardiovascular disorders. There are multiple drugs in this class with various pharmacodynamic and pharmacokinetic properties that may be important in specific clinical situations and in avoiding certain adverse reactions. These drugs have been shown to be efficacious in relieving anginal symptoms and prolonging exercise tolerance, in reducing high blood pressure, for treating various arrhythmias, in therapy of hypertrophic cardiomyopathy, and for prolonging life in many survivors of acute myocardial infarction.

Studies on the possible role of thyroid hormone in altered muscle protein turnover during sepsis

Five days after thyroidectomy (Tx) or sham-Tx in young male Sprague-Dawley rats, sepsis was induced by cecal ligation and puncture (CLP). Control animals underwent laparotomy and manipulation of the cecum without ligation or puncture. Sixteen hours after CLP or laparotomy, protein synthesis and degradation were measured in incubated extensor digitorum longus (EDL) and soleus (SOL) muscles by determining rate of 14C-phenylalanine incorporation into protein and tyrosine release into incubation medium, respectively. Triiodothyronine (T3) was measured in serum and muscle tissue. Protein synthesis was reduced by 39% and 22% in EDL and SOL, respectively, 16 hours after CLP in sham-Tx rats. The response to sepsis of protein synthesis was abolished in Tx rats. Protein breakdown was increased by 113% and 68% in EDL and SOL, respectively, 16 hours after CLP in sham-Tx animals. The increase in muscle proteolysis during sepsis was blunted in hypothyroid animals and was 42% and 49% in EDL and SOL, respectively. T3 in serum was reduced by sepsis, both in Tx and sham-Tx rats. T3 in muscle, however, was maintained or increased during sepsis. Abolished or blunted response of muscle protein turnover after CLP in hypothyroid animals may reflect a role of thyroid hormones in altered muscle protein metabolism during sepsis. Reduced serum levels of T3, but maintained or increased muscle concentrations of the hormone, suggests that increased T3 uptake by muscle may be one mechanism of low T3 syndrome in sepsis, further supporting the concept of a role for thyroid hormone in metabolic alterations in muscle during sepsis.

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.

The effect of age on blood pressure in hyperthyroidism

To better define the effect of age on blood pressure in patients with hyperthyroidism, the blood pressures of 321 patients with hyperthyroidism were compared with those of 324 euthyroid controls. Subjects were divided into four age groups by decade from 20 to 59 years. The systolic, but not diastolic, blood pressure in the patients with hyperthyroidism was significantly higher in all age groups. Among the euthyroid controls, the systolic blood pressure increased with age. However, no age-related increase was apparent among the hyperthyroid patients because of the relatively high systolic blood pressure in young patients. Effective antithyroid treatment reduced the systolic blood pressure significantly in hyperthyroid patients. The reduction of systolic blood pressure was greater in the younger hyperthyroid patients than that in the older ones, so that an age-related increase in systolic blood pressure was present after a euthyroid state had been achieved. These results provide evidence against the report that in hyperthyroidism, hypertension tends to occur in older patients, and suggest that the effect of excessive thyroid hormone on the systolic blood pressure may differ according to age.

Enhanced gluconeogenic capacity from glycerol in hyperthyroid man: evidence in favour of a beta-adrenergic mechanism

We have previously shown that gluconeogenesis, assessed by glycerol clearance, is increased in hyperthyroid man. The mechanism underlying this change is uncertain but many of the metabolic changes found in hyperthyroidism are thought to be due to increased catecholamine sensitivity of peripheral tissues. To test the hypothesis that enhanced gluconeogenic capacity from glycerol in hyperthyroidism might be mediated via a beta-adrenergic mechanism we have estimated glycerol clearance in thyrotoxic subjects before and during treatment with a non-selective beta-adrenergic blocking drug propranolol (Inderal LA). Control subjects of similar age and weight were also tested. In hyperthyroid subjects, blood glucose and blood glycerol concentrations were increased but blood lactate, pyruvate and alanine concentrations were normal. Glycerol clearance was increased and followed a double exponential decay with a shortened second component half-time. Endogenous glycerol production was increased three fold. Following beta-blockade blood glucose and blood glycerol concentrations fell although blood glucose concentrations remained above normal values. Glycerol clearance and endogenous glycerol production were also decreased but remained significantly higher than in control subjects. Serum thyroxine and serum triiodothyronine concentrations showed no significant change although mean values fell by 10% and 17% respectively. We conclude that the increase in gluconeogenic capacity from glycerol in hyperthyroid subjects is mediated in part by a beta-adrenergic mechanism.

Hypothyroidism abolishes the hyperdynamic phase and increases susceptibility to sepsis

To evaluate the role of thyroid hormones in sepsis, 250-400 g rats were surgically thyroidectomized and 2-6 weeks later sepsis was produced by cecal ligation and puncture (CLP). In normal rats, total body O2 consumption (VO2) increased by 12.8% (P less than 0.05) in early sepsis (6 hr after CLP) and decreased slightly in late sepsis (16 hr after CLP). In hypothyroid (HT) rats, VO2 was depressed by 19.8% (P less than 0.05) in early sepsis and further decreased to 46.7% (P less than 0.001) of preoperative levels in late sepsis. Hepatic blood flow increased in early sepsis in normal rats but was unchanged in HT rats. The normal hyperglycemic response to early sepsis was also absent in HT rats. The respiratory control ratio (RCR) of isolated mitochondria with succinate was not increased in HT rats in early sepsis. In late sepsis, hypothyroid animals showed further decreases in VO2 and mitochondrial RCR, and, in contrast to normal rats, showed no change in blood glucose levels. Survival (5 days) following late sepsis in normal, HT, and HT rats given daily ip injections of thyroxine (30 micrograms/kg) were 65.2% (15/23), 30% (6/20) (P less than 0.025), and 77.1% (14/18), respectively. Thus, absence of thyroid hormone abolishes the hyperdynamic phase of sepsis and significantly increases mortality in sepsis, and thyroxine replacement following thyroidectomy prevents the increased mortality from sepsis.

Effect of timolol on clinical features and echocardiographic assessment of left ventricular function in hyperthyroidism

The effect of timolol, a beta-adrenoceptor blocking drug on the clinical status, thyroid status and left ventricular function as measured by serial M-mode echocardiographic recordings was assessed in a double-blind randomised study in 18 hyperthyroid patients. A significant clinical improvement was documented after 2 weeks of timolol treatment compared with placebo. There was no evidence that timolol impaired peripheral monodeiodination of thyroxine (T4). There were significant increases in left ventricular fractional shortening (Fr. Sh.) and velocity of circumferential shortening (Vcf) as well as a significant decrease in the left ventricular systolic internal dimension (LVIDs) (all P less than 0.01) in the untreated thyrotoxic patients compared with a normal euthyroid control group. After timolol treatment (2/52) there were significant increases in LVIDs and LVIDd and a significant decrease in Vcf (all P less than 0.05). No further changes occurred after a further 2/52 treatment with carbimazole. The cardiac data suggest that both an augmented sympathetic drive and a direct effect of thyroid hormone on myocardial contractility are mediators of the haemodynamic changes in hyperthyroidism.

Regression of thyroid hormone induced cardiac hypertrophy: effect on cardiac beta receptors and adenyl cyclase activity

Adrenergic mechanisms may be important in the symptomatic manifestations of hyperthyroidism. The chronic administration of thyroid hormone also results in cardiac hypertrophy and increased numbers of beta-adrenergic receptors in cardiac membranes. The roles of adrenergic mechanisms in the initiation and perpetuation of this hypertrophy has been open to speculation. Rats treated chronically with L-thyroxin were sacrificed after 7 days of treatment and 1-4 days after cessation of treatment. Hearts were removed and weighed and norepinephrine measured. In other groups of identically treated rats, membranes were prepared from the left ventricle for in vitro measurements of beta-adrenergic receptor characteristics and adenyl cyclase activity. Regression of cardiac hypertrophy with a decrease in receptor number to control values was seen as early as 2 days after stopping thyroxine. Cardiac norepinephrine concentrations had also returned to control values at this time. Displacement of bound [H3] dihydroalprenolol by isoproterenol was not changed from control. Basal and isoproterenol stimulated adenyl cyclase activity was not changed by thyroxine administration or its cessation. The rapid reversal of the increased beta-adrenergic receptor number and cardiac hypertrophy raises the possibility that thyroid hormone may play a regulatory role in cardiac function. Although the enhancement of myocardial contractility by thyroid hormone may be mediated through cardiac hypertrophy this effect of thyroid hormone is independent of the catecholamine sensitive adenyl cyclase system.

Hypertension in hyperthyroidism: is there an epinephrine connection?

Since hypertension seen in euthyroid subjects with depressed conjugated epinephrine and moderately elevated free epinephrine in plasma is strikingly similar to the beta-hyperadrenergic, high-output hypertension seen in some hyperthyroid patients, the possibility is discussed that thyroid action on sulfoconjugation could affect the biodisposability of endogenous free E and so indirectly the manifestation of sympathetic hyperactivity. There is only circumstantial evidence for such a possibility at present. Differences in the conjugation system may be operative in dealing with endogenous as well as exogenous catecholamines. If the latter are used in evaluating the adrenergic receptor reactivity, this may result in incorrect dose-response curves if the concentrations of catecholamines remaining free following infusion are not monitored. The conjugation of catecholamines, E in particular, is a variable which has to be taken account of in the investigation of catecholamine-like manifestation of T3 and T4 excess and in the evaluation of adrenergic influences on thyroid function.

The effect of thyroid dysfunction on the chronotropic response to noradrenaline

Dose-response curves to noradrenaline in the presence and absence of beta-receptor antagonists were established with isolated atria from euthyroid, hypothyroid and hyperthyroid rats. Baseline atrial rate and Emax were significantly lower than normal in the hypothyroid group and significantly higher than normal in the hyperthyroid group. Differences between the groups were minimal for pD2 and range of response to noradrenaline. The response to beta-receptor antagonists was the same in all 3 groups with the exception of the hypothyroid group which showed an attenuated increase in baseline atrial rate with compounds possessing partial agonist activity. This was particularly marked for practolol. These results do not provide evidence for an altered responsiveness to catecholamines due to altered thyroid status but suggest that thyroid hormones have a direct action on cardiac tissue.

Thyrotoxicosis presenting as orthostatic hypotension

The patient described, whose symptoms were masked initially by the lack of steroids, illustrates an unusual case of orthostatic hypotension due to secondary adrenal insufficiency, caused by thyrotoxicosis.