Catecholamine sensitivity in hyperthyroidism and hypothyroidism

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.

ADVERSE DRUG INTERACTIONS IN DENTAL PRACTICE: INTERACTIONS ASSOCIATED WITH VASOCONSTRICTORS

BACKGROUND

Adrenergic vasoconstrictors are commonly used by dentists to enhance the pain-relieving action of local anesthetics and to control local bleeding. Although normally considered safe for these applications, vasoconstrictors can participate in drug interactions that potentially are harmful to patients.

METHODS

The faculty of a March 1998 symposium entitled "Adverse Drug Interactions in Dentistry: Separating the Myths From the Facts" extensively reviewed the literature on drug interactions. They then established a significance rating of alleged adverse drug interactions pertaining to dentistry, based on the quality of documentation and severity of effect. The author of this article focused on the adrenergic vasoconstrictors epinephrine and levonordefrin.

RESULTS

Vasoconstrictor drug interactions involving tricyclic antidepressants, nonselective beta-adrenergic blocking drugs, certain general anesthetics and cocaine are well-documented in both humans and animals as having the potential for causing serious morbidity or death. Evidence for adverse interactions involving adrenergic neuronal blocking drugs, drugs with alpha-adrenergic blocking activity, local anesthetics and thyroid hormones is much less compelling, suggesting for the most part that clinically significant reactions may occur only when both the vasoconstrictor and the interacting drug are used in excessive doses. In the case of monoamine oxidase inhibitors, there is no credible evidence of a significant interaction with epinephrine or levonordefrin.

CONCLUSIONS

Potentially serious adverse drug interactions involving adrenergic vasoconstrictors can occur in dental practice. In most circumstances, careful administration of small doses of vasoconstrictors and avoidance of gingival retraction cord containing epinephrine, coupled with monitoring of vita signs, will permit these drugs to be used with no risk or only minimally increased risk. Only in the case of cocaine intoxication must adrenergic vasoconstrictors be avoided completely.

CLINICAL IMPLICATIONS

For optimal patient safety, dentists must recognize potential drug interactions involving adrenergic vasoconstrictors and modify their use of these agents accordingly.

Short-term variability of blood pressure and heart rate in hyperthyroidism

The effect of hyperthyroidism on the short-term memory variability of blood pressure and heart rate was evaluated in 12 untreated hyperthyroid patients during thyrotoxicosis and after a 6 1/2 month treatment designed to achieve a stable euthyroid state. Beat-by-beat finger blood pressure was measured with a Finapres device. The pulse interval, from which pulse rate was derived, was obtained from the blood pressure signal. Due to the significant change in heart rhythm associated with thyrotoxicosis, both pulse interval (taken as a surrogate of heart period) and pulse rate (taken as a surrogate of heart rate) were computed. Power spectral analysis showed a reduction in the overall heart period variability in the supine position in the hyperthyroid compared to the euthyroid state. This effect was observed in the low-frequency (0.005-0.068 Hz), mid-frequency (0.068-0.127 Hz) and high-frequency (respiratory) domains as well, with a significant reduction of the modulus of these bands of 31%, 35% and 47%, respectively. The heart rate spectral modulus also exhibited a reduction of the high-frequency component (31%) in the supine position in the hyperthyroid subjects. These changes in heart rhythmicity corroborate a vagal deficit in hyperthyroidism. In addition, blood pressure spectral power exhibited a significant deficit in the orthostatism-induced mid-frequency systolic blood pressure rise in the hyperthyroid state (64%) compared with the euthyroid state. This observation may reflect a reduced vascular sympathetic activation with standing. The resulting vasodilatation could well contribute to normalize blood pressure in thyrotoxicosis in which cardiac output is increased.

Thyroid hormone modulates inotropic responses, alpha-adrenoceptor density and catecholamine concentrations in the rat heart

We investigated the influence of hyper- and hypothyroidism on basal parameters of isolated perfused hearts of rats. In addition the effects of different extracellular calcium concentrations ([Ca2+]o), the calcium entry promoter Bay K8644 and the alpha 1-adrenoceptor agonist methoxamine were investigated. Since alterations in alpha-adrenoceptor density could explain the increased sensitivity to methoxamine in hearts from hypothyroid rats, alpha 1-adrenoceptor density in the left ventricle was also established. Different time-schedules of exposure to hyper- and hypothyroidism were used to investigate whether the influence of chronic dysthyroid states on alpha 1-adrenoceptor density is transient and time-dependent. Simultaneously myocardial noradrenaline and adrenaline tissue concentrations were determined, since they might correlate with the observed changes. Hyperthyroidism was induced by feeding rats for 1, 4 and 8 weeks with 5 mg/kg L-thyroxine (T4)-containing rat chow. Hypothyroid rats were obtained by adding 0.05% propylthiouracil (PTU) to the drinking water during 1, 4 and 8 weeks. For the functional experiments animals were treated during 4 weeks, to mimic the clinical situation of a chronic endocrine disease. Langendorff hearts from hyperthyroid hearts showed an increased maximally developed relaxation velocity, whereas Langendorff hearts from hypothyroid rats showed an increased left ventricular pressure (LVP). We observed an increased maximal inotropic response to [Ca2+]o in hearts from both hyperthyroid and hypothyroid rats, indicating that both dysthyroid states interfere with the handling of calcium ions by the contractile apparatus. Unchanged responses to Bay K8644 in hearts from hyperthyroid and depressed responses in hearts from hypothyroid rats suggest that the involvement of L-type calcium channels is rather unlikely. Furthermore, the reflex increase in coronary flow in response to enhanced contractile force appeared to fail in hearts from hypothyroid rats. Sensitivity of the response to methoxamine was increased in hearts from hypothyroid rats, which was accompanied by a decrease in the number of myocardial alpha 1-adrenoceptors. Both T4 and PTU treatment resulted in a non-transient decrease of alpha 1-adrenoceptor density in left ventricular tissue. Furthermore, hypothyroidism increased the percentage of alpha 1A-binding sites, whereas in hyperthyroidism the distribution of the alpha 1-adrenoceptor subtypes was not affected. Myocardial tissue concentrations of noradrenaline and adrenaline were unchanged in hyperthyroid rats and decreased in hypothyroid rats. The present study indicates that thyroid hormones have a direct rather than a sympathetically mediated effect on alpha 1-adrenoceptor mediated myocardial functions.

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

OBJECTIVE

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

PATIENTS

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

DESIGN

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

MEASUREMENTS

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

RESULTS

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

CONCLUSIONS

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

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)

Contraindications to vasoconstrictors in dentistry: Part II. Hyperthyroidism, diabetes, sulfite sensitivity, cortico-dependent asthma, and pheochromocytoma

Dentists are aware of contraindications to the use of vasoconstrictors in patients with cardiovascular diseases. However, there are some other noncardiac conditions we should know. This article discusses the absolute contraindications to the use of vasoconstrictors in patients with a history of hyperthyroidism, diabetes, allergy to sulfites, asthma, and pheochromocytoma.

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.

Decreased adrenergic sensitivity in patients with hypothyroidism

Cardiovascular sensitivity to catecholamines was assessed in 15 patients with hypothyroidism (mean [+/- SEM] thyroxine [T4] index 2.7 +/- 0.5 micrograms/100 ml, thyroid stimulating hormone [TSH] 136.9 +/- 48.3 microU/ml), aged 45 +/- 4 years and in 8 healthy control subjects. The study was repeated in 10 patients with hypothyroidism 4.0 +/- 0.5 months after thyroid replacement therapy (T4 index 9.9 +/- 2.1 micrograms/100 ml, TSH 3.5 +/- 1.3 microU/ml). In addition, basal, average and maximal heart rates were measured using 24 h ambulatory electrocardiographic (ECG) monitoring, and plasma levels of epinephrine and norepinephrine were determined before and after thyroid replacement. Heart rate increased less after bolus injection of 0.8, 1.6 and 3.2 micrograms of isoproterenol in the hypothyroid (10 +/- 2, 15 +/- 2 and 21 +/- 4 beats/min, respectively) than in the euthyroid (16 +/- 3, 22 +/- 3 and 30 +/- 4 beats/min, respectively) state (p less than 0.05). Control subjects reacted similarly to patients receiving thyroid replacement. Basal heart rate (64 +/- 3 versus 68 +/- 3 beats/min, p less than 0.05) and maximal heart rate (116 +/- 5 versus 133 +/- 5 beats/min, p less than 0.05) were lower on 24 h ambulatory ECG monitoring in the hypothyroid than euthyroid state despite higher basal plasma norepinephrine levels (394 +/- 45 versus 315 +/- 45 pg/ml, p less than 0.05). Thus, patients with hypothyroidism display a decreased cardiac chronotropic response to beta-adrenergic stimulation. This may contribute in part to the decreased basal and maximal daily heart rates seen in patients with hypothyroidism, which occurs despite elevated plasma norepinephrine levels.

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.

Plasma noradrenaline and blood pressure in hypothyroid patients: effect of gradual thyroxine treatment

High plasma noradrenaline (PNA) levels have been reported in hypothyroid patients and hypothyroidism has been associated with hypertension. To explore the relationship between PNA and blood pressure (BP) in hypothyroid patients, and the effects of gradual thyroxine replacement, a prospective study was performed comparing BP, heart rate (HR) and PNA in a normotensive and a hypertensive group of hypothyroid patients before and during gradual thyroxine substitution. Thyroxine treatment reduced the BP; the reduction in supine BP was greater in the hypertensive than in the normotensive group. HR increased similarly in both groups during treatment. PNA was elevated in the normotensive group before treatment and decreased gradually during thyroxine treatment. The hypertensive group had normal PNA levels. The present study indicates that normotensive, in contrast to hypertensive, hypothyroid patients have increased sympathetic nervous activity. Although the mechanism is unclear, thyroid replacement therapy can reverse hypertension in hypothyroid patients.

Use of beta-adrenoceptor blocking drugs in hyperthyroidism

There is an increasing use and variety of beta-adrenoceptor blocking agents (beta-blockers) available for the treatment of hyperthyroidism. Recent comparative studies suggest that atenolol (200mg daily), metoprolol (200mg daily); acebutolol (400mg daily), oxprenolol ( 160mg daily), nadolol ( 80mg daily) and timolol (20mg daily) produce a beneficial clinical response equal to that seen with propranolol ( 160mg daily). Most beta-blockers reduce resting heart rate by approximately 25 to 30 beats/min, although a lesser reduction is seen with those possessing intrinsic sympathomimetic activity such as oxprenolol and pindolol. While earlier studies employing large doses of intravenous propranolol concluded that beta-blockade reduced myocardial contractility, more recent non-invasive studies suggest that the predominant cardiac effect is on heart rate. In patients with cardiac failure, beta-blockers may, however, produce a profound fall in cardiac output. Nevertheless, in combination with digoxin they may be useful in controlling the atrial fibrillation of thyrocardiac disease. beta-Blockers improve nervousness and tremor (although to a lesser extent with cardioselective agents) and severe myopathy, and they also reduce the frequency of paralysis in patients with thyrotoxic periodic paralysis. There is often subjective improvement in sweating but usually no major effect on eye signs. Recent studies show a 10% reduction in oxygen consumption/basal metabolic rate with long term oral use of selective or nonselective beta-blockers. In addition, many agents (propranolol, metoprolol, nadolol and sotalol but not acebutolol, atenolol or oxprenolol) reduce circulating tri-iodothyronine (T3) concentration by between 10 and 40%, although the clinical significance of this effect (if any) is not established. beta-Blockers may also have endocrinological effects on gastrin, cyclic AMP, catecholamines and other hormone levels. Given in adequate dosage, propranolol has been shown to control thyrotoxic hypercalcaemia. Minor side effects (nausea, headaches, tiredness, etc.) are quite common but overall beta-blockers are well tolerated by the thyrotoxic patient. The major use of these drugs is in symptomatic control while awaiting definitive diagnosis or treatment. As an adjunct to antithyroid drugs or radioactive iodine, beta-blockers will produce a satisfactory clinical response in the weeks to months before these forms of therapy produce a euthyroid state. beta-Blockers are more convenient than antithyroid drugs in the control of patients receiving therapeutic radioiodine, in that continuous therapy and assessment of biochemical response is possible.(ABSTRACT TRUNCATED AT 400 WORDS)

Airway response to inhaled salbutamol in hyperthyroid and hypothyroid patients before and after treatment

For many years the development of thyrotoxicosis has been known to cause a deterioration in asthma but the mechanism is unknown. We have studied the effect of thyroid function on airway beta adrenergic responsiveness in 10 hyperthyroid and six hypothyroid subjects before and after treatment of their thyroid disease. Airway adrenergic responsiveness was assessed by measuring specific airway conductance (sGaw) after increasing doses of inhaled salbutamol (10-410 micrograms). After treatment there was no difference in resting FEV1, sGaw, or thoracic gas volume. FVC increased in the hyperthyroid subjects but did not change in the hypothyroid subjects. In the hyperthyroid subjects there was a significant increase in delta sGaw after 35, 60, 110, and 41 micrograms salbutamol; in sGaw after 60, 110, and 410 micrograms salbutamol; and in the area under the salbutamol dose response curve (AUC) after treatment of the thyroid disorder. In the hypothyroid subjects there was a significant reduction in sGaw after 10 and 60 micrograms salbutamol and in the AUC after treatment. When all subjects were considered, there was a negative correlation between the AUC and serum thyroxine values. These findings suggest that an inverse relationship exists between the level of thyroid function and airway beta adrenergic responsiveness.

Drug toxicity and hormonal dysfunction

Abnormal levels of hormones can exert profound changes in drug disposition and the number and affinity of receptors. In endocrine diseases a variety of organ functions change. These disease related changes in the function of organs involved with the distribution, metabolism and excretion of drugs can profoundly alter drug disposition. If not taken into account when devising dosage regimens drug toxicity can result.

Quantitative changes in beta-adrenergic responses of isolated atria from hyper- and hypothyroid rats

Concentration-response curves for the chronotropic and inotropic effects of isoprenaline, in the absence and presence of propranolol, were obtained on heart atria isolated from normo- or dysthyroid rats. Hyperthyroidism increased the chronotropic potency and efficacy of the beta-adrenergic agonist. The results are compatible with the view that thyroid hormone increases the density of functional beta-adrenoceptors in cardiac pacemaker tissue.

Pharmacokinetics and pharmacodynamics of propranolol stereoisomers in hyperthyroid patients

The disposition of propranolol stereoisomers after administration of a single oral dose of the racemic drug was investigated in seven hyperthyroid patients before and after antithyroid drug therapy. The possibility of hypersensitivity to propranolol in the patients was evaluated by constructing plasma propranolol concentration -- beta-blocking effect curves. There was no statistically significant difference in elimination half-life (t1/2) between (+/-)- and (-)-propranolol before and after antithyroid drug therapy. However, the plasma clearance (Vp) of (-)-propranolol was smaller than that of (+/-)-propranolol, and the difference was statistically significant after antithyroid drug therapy. Vp decreased or did not change in young patients after therapy. No significant difference was observed in the relationship between the tilt-induced pulse rate response and plasma propranolol concentration when treated patients became euthyroid compared to their response in the hyperthyroid state.

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.

Absence of changes in drug disposition and catecholamine sensitivity in the hyperthyroid dog

1 In order to study the relative contribution of hepatic drug metabolizing enzymes and hepatic blood flow to the clearance of drugs in the hyperthyroid state, the disposition kinetics of two model compounds (antipyrine and propranolol) were examined in thyroid-fed dogs as compared to euthyroid and phenobarbitone-pretreated animals. 2 In hyperthyroid dogs, the possibility of catecholamine hypersensitivity was evaluated by assessing the chronotropic response to isoprenaline and by constructing a drug concentration-effect (beta-blockade) relationship. 3 The plasma propranolol half-life (0.97 +/- 0.12 h) of the hyperthyroid animals did not differ significantly from either the euthyroid group or the phenobarbitone-pretreated group. This was observed with no significant change in the apparent volume of distribution among the three experimental groups. 4 Phenobarbitone pretreatment accelerated significantly the elimination of antipyrine (half-life, 1.09 +/- 0.15 h, P less than 0.01) as compared to the euthyroid (2.84 +/- 0.35 h) and the hyperthyroid groups (2.58 +/- 0.13 h), respectively, without any changes in the apparent volume of distribution in any group. 5 Neither the chronotropic responses to exogenously administered catecholamine, nor the antagonist concentration-effect relationships support the concept that the hyperthyroid state potentiates sensitivity of the receptor-effect system of the heart. 6 The data obtained from the present study fit best with the view that thyroid hormone excess alters neither the disposition of the model compounds used nor the catecholamine-sensitivity examined.

Plasma protein binding of propranolol and isoprenaline in hyperthyroidism and hypothyroidism

1. The possibility that thyroid disease might result in alterations in the plasma proteins binding of drugs has been investigated by studying the binding of propranolol and isoprenaline in patients with hyperthyroidism and hypothyroidism. 2. Plasma protein binding of propranolol and isoprenaline has been measured in seven hyperthyroid patients and ten hypothyroid patients. Plasma binding was estimated by equilibrium dialysis at 37 degree C using triatiated propranolol and isoprenaline, both when the patients had thyroid dysfunction and again when they were euthyroid. 3. In the hyperthyroid group, mean propranolol binding varied from 86 +/- 1.7% when hyperthyroid to 88.4 +/- 0.8% when euthyroid. The comparable isoprenaline figures were 65.1 +/- 3.2% and 68.1 +/- 1.4% respectively. Neither difference was significant. 4. Isoprenaline binding was significantly lower (64.3 +/- 1.6%) when patients were hypothyroid than when they became euthyroid (68.8 +/- 1.2%). Propranolol binding was not altered by hypothyroidism. 5. It is concluded that clinically important alterations in free drug concentrations of propranolol or isoprenaline do not occur in hyperthyroidism or hypothyroidism.